TOP232-234

TOPSwitch-FX

Family

Design Flexible,

EcoSmart

Integrated

Off-line Switcher

Figure 1. Typical Flyback Application.

PART

ORDER

NUMBER

Adapter

Open

Frame

Open

Frame

OUTPUT POWER TABLE

Table 1. Notes: 1. Typical continuous power in a non-ventilated
enclosed adapter measured at 50 °C ambient. 2. Maximum practical
continuous power in an open frame design with adequate heat sinking,
measured at 50 °C ambient. See key applications section for detailed
conditions. 3. Packages: P: DIP-8B, G: SMD-8B, Y: TO-220-7B.

PI-2503-073099

OUT

TOPSwitch-FX

CONTROL

July 2001

Product Highlights

Lower System Cost, High Design Flexibility

· Features eliminate or reduce cost of external components
· Fully integrated soft-start for minimum stress/overshoot
· Externally settable accurate current limit
· Wider duty cycle for more power, smaller input capacitor
· Line under-voltage (UV) detection: no turn off glitches
· Line overvoltage (OV) shutdown extends line surge limit
· Line feed forward with maximum duty cycle (DC

MAX

)

reduction rejects ripple and limits DC

MAX

at high line

· Single resistor sets OV/UV thresholds, DC

MAX

reduction

· Frequency jittering reduces EMI and EMI filtering costs
· Regulates to zero load without dummy loading
· 132 kHz frequency reduces transformer/power supply size
· Half frequency option for video applications
· Hysteretic thermal shutdown for automatic recovery
· Large thermal hysteresis prevents PC board overheating
· Standard packages with omitted pins for large creepage
· Active-on and active-off remote ON/OFF capability
· Synchronizable to a lower frequency

EcoSmart

- Energy Efficient

· Cycle skipping reduces no-load consumption
· Reduced consumption in remote off mode
· Half frequency option for high efficiency standby
· Allows shutdown/wake-up via LAN/input port

Description

TOPSwitch-FX uses the proven TOPSwitch topology and cost
effectively integrates many new functions that reduce system
cost and, at the same time, improve design flexibility,
performance and energy efficiency. Like TOPSwitch, the high
voltage power MOSFET, PWM control, fault protection and
other control circuitry are all integrated onto a single CMOS
chip, but with two added terminals. The first one is a MULTI-
FUNCTION (M) pin, which implements programmable line
OV/UV shutdown and line feed forward/DC

MAX

reduction with

line voltage. The same pin can be used instead to externally set
an accurate current limit. In either case, this pin can also be used
for remote ON/OFF or to synchronize the oscillator to an
external, lower frequency signal. The second added terminal is
the FREQUENCY (F) pin and is available only in the Y
package. This pin provides the half frequency option when
connected to CONTROL (C) instead of SOURCE (S). The
features on the new pins can be disabled by shorting them to the
SOURCE, which allows the device to operate in a three terminal

TOPSwitch mode, but with the following new transparent
features: soft-start, cycle skipping, 132 kHz switching frequency,
frequency jittering, wider DC

MAX

, hysteretic thermal shutdown

and larger creepage. In addition, all critical parameters such as
frequency, current limit, PWM gain, etc. have tighter temperature
and absolute tolerances compared to the TOPSwitch-II family.
Higher current limit accuracy and larger DC

MAX

, when combined

with other features allow for a 10% to 15% higher power
capability on the TOPSwitch-FX devices compared to equivalent
TOPSwitch-II devices for the same input/output conditions.

TOP232P

TOP232G

TOP232Y

TOP233P

TOP233G

TOP233Y

TOP234P

TOP234G

TOP234Y

230 VAC

15%

Adapter

85-265 VAC

9 W

15 W

6.5 W

10 W

25 W

7 W

15 W

13 W

25 W

9 W

15 W

20 W

50 W

15 W

30 W

16 W

30 W

11 W

20 W

30 W

75 W

20 W

45 W

TOP232-234

B
7/01

Section List

Pin Functional Description ......................................................................................................................................... 3

TOPSwitch-FX Family Functional Description ......................................................................................................... 4

CONTROL (C) Pin Operation ................................................................................................................................. 4
Oscillator and Switching Frequency ....................................................................................................................... 5
Pulse Width Modulator and Maximum Duty Cycle ................................................................................................. 5
Minimum Duty Cycle and Cycle Skipping ............................................................................................................... 6
Error Amplifier ......................................................................................................................................................... 6
On-chip Current Limit with External Programability ................................................................................................ 6
Line Under-Voltage Detection (UV) ........................................................................................................................ 6
Line Overvoltage Shutdown (OV) ........................................................................................................................... 7
Line Feed Forward with DC

MAX

Reduction .............................................................................................................. 7

Remote ON/OFF and Synchronization ................................................................................................................... 7
Soft-Start ................................................................................................................................................................ 8
Shutdown/Auto-Restart .......................................................................................................................................... 8
Hysteretic Over-Temperature Protection ................................................................................................................ 8
Bandgap Reference ................................................................................................................................................ 8
High-Voltage Bias Current Source .......................................................................................................................... 8

Using FREQUENCY and MULTI-FUNCTION Pins ..................................................................................................... 9

FREQUENCY (F) Pin Operation ............................................................................................................................. 9
MULTI-FUNCTION (M) Pin Operation .................................................................................................................... 9

Typical Uses of FREQUENCY (F) Pin ...................................................................................................................... 11

Typical Uses of MULTI-FUNCTION (M) Pin ............................................................................................................. 12

Application Examples ............................................................................................................................................... 14

A High Efficiency, 30 W, Universal Input Power Supply ........................................................................................ 14
35 W Multiple Output Power Supply ..................................................................................................................... 15
17 W PC Standby Power Supply .......................................................................................................................... 16
Processor Controlled Supply Turn On/Off ............................................................................................................ 17

Key Application Considerations .............................................................................................................................. 19

TOPSwitch-FX vs. TOPSwitch-ll ........................................................................................................................... 19
TOPSwitch-FX Design Considerations ................................................................................................................. 20

TOPSwitch-FX Selection ................................................................................................................................ 20
Input Capacitor ............................................................................................................................................... 20
Primary Clamp and Output Reflected Voltage V

......................................................................................... 20

Output Diode .................................................................................................................................................. 21
Soft-Start ........................................................................................................................................................ 21
EMI ................................................................................................................................................................. 21
Transformer Design ........................................................................................................................................ 21
Standby Consumption .................................................................................................................................... 23

TOPSwitch-FX Layout Considerations ................................................................................................................. 23

Primary Side Connections .............................................................................................................................. 23
Y-Capacitor..................................................................................................................................................... 23
Heat Sinking ................................................................................................................................................... 23

Quick Design Checklist ......................................................................................................................................... 23
Design Tools ......................................................................................................................................................... 23

Product Specifications and Test Conditions .......................................................................................................... 24

Typical Performance Characteristics ...................................................................................................................... 30

Package Outlines ...................................................................................................................................................... 34

TOP232-234

7/01

Figure 2. Functional Block Diagram.

Pin Functional Description

DRAIN (D) Pin:
High voltage power MOSFET drain output. The internal start-
up bias current is drawn from this pin through a switched high-
voltage current source. Internal current limit sense point for
drain current.

CONTROL (C) Pin:
Error amplifier and feedback current input pin for duty cycle
control. Internal shunt regulator connection to provide internal
bias current during normal operation. It is also used as the
connection point for the supply bypass and auto-restart/
compensation capacitor.

MULTI-FUNCTION (M) Pin:
Input pin for OV, UV, line feed forward with DC

MAX

reduction,

external set current limit, remote ON/OFF and synchronization.
A connection to SOURCE pin disables all functions on this pin
and makes TOPSwitch-FX operate in simple three terminal
mode (like TOPSwitch-II).

FREQUENCY (F) Pin: (Y package only)
Input pin for selecting switching frequency: 132 kHz if connected
to SOURCE pin and 66 kHz if connected to CONTROL pin.

Figure 3. Pin Configuration.

PI-2535-083099

The switching frequency is internally set for 132 kHz only
operation in P and G packages.

SOURCE (S) Pin:
Output MOSFET source connection for high voltage power
return. Primary side control circuit common and reference point.

SHUTDOWN/

AUTO-RESTART

PWM

COMPARATOR

CLOCK

SAW

HALF
FREQUENCY

OSCILLATOR WITH JITTER

CONTROLLED

TURN-ON

GATE DRIVER

CURRENT LIMIT

COMPARATOR

INTERNAL UV

COMPARATOR

INTERNAL
SUPPLY

5.8 V
4.8 V

SOURCE (S)

DMAX

STOP

SOFT-

START

CONTROL (C)

MULTI-

FUNCTION (M)

FREQUENCY (F)

(Y Package Only)

5.8 V

IFB

LEADING

EDGE

BLANKING

HYSTERETIC

THERMAL

SHUTDOWN

SHUNT REGULATOR/

ERROR AMPLIFIER

DRAIN (D)

ON/OFF

DCMAX

VBG

DCMAX

VBG + VT

OV/UV

VI (LIMIT)

CURRENT

LIMIT

ADJUST

LINE

SENSE

SOFT START

PI-2501-031901

Tab Internally

Connected to SOURCE Pin

Y Package (TO-220-7B)

P Package (DIP-8B)

G Package (SMD-8B)

1 C

3 M

5 F

4 S

7 D

TOP232-234

B
7/01

TOPSwitch-FX

Family Functional Description

Figure 4. Relationship of Duty Cycle to CONTROL Pin Current.

PI-2504-072799

Duty Cycle (%)

(mA)

= 140

= 190

< I

M(DC)

1.9

1.5

5.5 5.9

Slope = PWM Gain

CD1

Auto-restart

1.5

Like TOPSwitch, TOPSwitch-FX is an integrated switched
mode power supply chip that converts a current at the control
input to a duty cycle at the open drain output of a high voltage
power MOSFET. During normal operation the duty cycle of the
power MOSFET decreases linearly with increasing CONTROL
pin current as shown in Figure 4.

In addition to the three terminal TOPSwitch features, such as the
high voltage start-up, the cycle-by-cycle current limiting, loop
compensation circuitry, auto-restart, thermal shutdown, etc.,
the TOPSwitch-FX incorporates many additional functions that
reduce system cost, increase power supply performance and
design flexibility. A patented high voltage CMOS technology
allows both the high voltage power MOSFET and all the low
voltage control circuitry to be cost effectively integrated onto a
single monolithic chip.

Two terminals, FREQUENCY (available only in Y package)
and MULTI-FUNCTION, have been added to implement some
of the new functions. These terminals can be connected to the
SOURCE pin to operate the TOPSwitch-FX in a TOPSwitch-
like three terminal mode. However, even in this three terminal
mode, the TOPSwitch-FX offers many new transparent features
that do not require any external components:

1. A fully integrated 10 ms soft-start reduces peak currents and

voltages during start-up and practically eliminates output
overshoot in most applications.

2. DC

MAX

of 78% allows smaller input storage capacitor, lower

input voltage requirement and/or higher power capability.

3. Cycle skipping at minimum pulse width achieves regulation

and very low power consumption at no load.

4. Higher switching frequency of 132 kHz reduces the

transformer size with no noticeable impact on EMI or on
high line efficiency.

5. Frequency jittering reduces EMI.
6. Hysteretic over-temperature shutdown ensures automatic

recovery from thermal fault. Large hysteresis prevents circuit
board overheating.

7. Packages with omitted pins and lead forming provide large

DRAIN creepage distance.

8. Tighter absolute tolerances and smaller temperature vari-

ations on switching frequency, current limit and PWM gain.

The MULTI-FUNCTION pin is usually used for line sensing by
connecting a resistor from this pin to the rectified DC high
voltage bus to implement line over-voltage (OV)/under-voltage
(UV) and line feed forward with DC

MAX

reduction. In this

mode, the value of the resistor determines the OV/UV thresholds
and the DC

MAX

is reduced linearly starting from a line voltage

above the under-voltage threshold. In high efficiency
applications, this pin can be used in the external current limit
mode instead, to reduce the current limit externally (to a value

close to the operating peak current), by connecting the pin to
SOURCE through a resistor. The same pin can also be used as
a remote ON/OFF and a synchronization input in both modes.
The FREQUENCY pin in the TO-220 package sets the switching
frequency to the default value of 132 kHz when connected to
SOURCE pin. A half frequency option can be chosen by
connecting this pin to CONTROL pin instead. Leaving this pin
open is not recommended.

CONTROL (C) Pin Operation
The CONTROL pin is a low impedance node that is capable of
receiving a combined supply and feedback current. During
normal operation, a shunt regulator is used to separate the
feedback signal from the supply current. CONTROL pin voltage
V

is the supply voltage for the control circuitry including the

MOSFET gate driver. An external bypass capacitor closely
connected between the CONTROL and SOURCE pins is
required to supply the instantaneous gate drive current. The
total amount of capacitance connected to this pin also sets the
auto-restart timing as well as control loop compensation.

When rectified DC high voltage is applied to the DRAIN pin
during start-up, the MOSFET is initially off, and the CONTROL
pin capacitor is charged through a switched high voltage current
source connected internally between the DRAIN and CONTROL
pins. When the CONTROL pin voltage V

reaches

approximately 5.8 V, the control circuitry is activated and the
soft-start begins. The soft-start circuit gradually increases the
duty cycle of the MOSFET from zero to the maximum value
over approximately 10 ms. If no external feedback/supply
current is fed into the CONTROL pin by the end of the soft-start,
the high voltage current source is turned off and the CONTROL
pin will start discharging in response to the supply current
drawn by the control circuitry. If the power supply is designed
properly, and no fault condition such as open loop or shorted
output exists, the feedback loop will close, providing external

TOP232-234

7/01

PI-2545-082299

5.8 V

4.8 V

0 V

LINE

DRAIN

OUT

Note: S0 through S7 are the output states of the auto-restart counter

0 V

~ ~

~ ~
~ ~

~ ~

CONTROL pin current, before the CONTROL pin voltage has
had a chance to discharge to the lower threshold voltage of
approximately 4.8 V (internal supply under-voltage lockout
threshold). When the externally fed current charges the
CONTROL pin to the shunt regulator voltage of 5.8 V, current
in excess of the consumption of the chip is shunted to SOURCE
through resistor R

as shown in Figure 2. This current flowing

through R

controls the duty cycle of the power MOSFET to

provide closed loop regulation. The shunt regulator has a finite
low output impedance Z

that sets the gain of the error amplifier

when used in a primary feedback configuration. The dynamic
impedance Z

of the CONTROL pin together with the external

CONTROL pin capacitance sets the dominant pole for the
control loop.

When a fault condition such as an open loop or shorted output
prevents the flow of an external current into the CONTROL pin,
the capacitor on the CONTROL pin discharges towards 4.8 V.
At 4.8 V auto-restart is activated which turns the output MOSFET
off and puts the control circuitry in a low current standby mode.
The high-voltage current source turns on and charges the
external capacitance again. A hysteretic internal supply under-
voltage comparator keeps V

within a window of typically 4.8

to 5.8 V by turning the high-voltage current source on and off
as shown in Figure 5. The auto-restart circuit has a divide-by-
8 counter which prevents the output MOSFET from turning on
again until eight discharge/charge cycles have elapsed. This is
accomplished by enabling the output MOSFET only when the
divide-by-8 counter reaches full count (S7). The counter
effectively limits TOPSwitch-FX power dissipation by reducing
the auto-restart duty cycle to typically 4%. Auto-restart mode
continues until output voltage regulation is again achieved
through closure of the feedback loop.

Oscillator and Switching Frequency
The internal oscillator linearly charges and discharges an internal
capacitance between two voltage levels to create a sawtooth
waveform for the pulse width modulator. The oscillator sets the
pulse width modulator/current limit latch at the beginning of
each cycle.

The nominal switching frequency of 132 kHz was chosen to
minimize transformer size while keeping the fundamental EMI
frequency below 150 kHz. The FREQUENCY pin (available
only in TO-220 package), when shorted to the CONTROL pin,
lowers the switching frequency to 66 kHz (half frequency)
which may be preferable in some cases such as noise sensitive
video applications or a high efficiency standby mode. Otherwise,
the FREQUENCY pin should be connected to the SOURCE pin
for the default 132 kHz. Trimming of the current reference
improves oscillator frequency accuracy.

To further reduce the EMI level, the switching frequency is
jittered (frequency modulated) by approximately

±4 kHz at

250 Hz (typical) rate as shown in Figure 6. Figure 28 shows the
typical improvement of EMI measurements with frequency
jitter.

Pulse Width Modulator and Maximum Duty Cycle
The pulse width modulator implements voltage mode control
by driving the output MOSFET with a duty cycle inversely
proportional to the current into the CONTROL pin

that is in

excess of the internal supply current of the chip (see Figure 4).
The excess current is the feedback error signal that appears
across R

(see Figure 2). This signal is filtered by an RC

network with a typical corner frequency of 7 kHz to reduce the
effect of switching noise in the chip supply current generated by

Figure 5. Typical Waveforms for (1) Power Up (2) Normal Operation (3) Auto-restart (4) Power Down .

TOP232-234

B
7/01

Figure 6. Switching Frequency Jitter.

the MOSFET gate driver. The filtered error signal is compared
with the internal oscillator sawtooth waveform to generate the
duty cycle waveform. As the control current increases, the duty
cycle decreases. A clock signal from the oscillator sets a latch
which turns on the output MOSFET. The pulse width modulator
resets the latch, turning off the output MOSFET. Note that a
minimum current must be driven into the CONTROL pin
before the duty cycle begins to change.

The maximum duty cycle, DC

MAX

is set at a default maximum

value of 78% (typical). However, by connecting the MULTI-
FUNCTION pin to the rectified DC high voltage bus through a
resistor with appropriate value, the maximum duty cycle can be
made to decrease from 78% to 38% (typical) as shown in
Figure 8 when input line voltage increases (see line feed
forward with DC

MAX

reduction).

Minimum Duty Cycle and Cycle Skipping
To maintain power supply output regulation, the pulse width
modulator reduces duty cycle as the load at the power supply
output decreases. This reduction in duty cycle is proportional
to the current flowing into the CONTROL pin. As the
CONTROL pin current increases, the duty cycle reduces linearly
towards a minimum value specified as minimum duty cycle,
DC

MIN

. After reaching DC

MIN

, if CONTROL pin current is

increased further by approximately 0.4 mA, the pulse width
modulator will force the duty cycle from DC

MIN

to zero in a

discrete step (refer to Figure 4). This feature allows a power
supply to operate in a cycle skipping mode when the load at its
output consumes less power than the power that TOPSwitch-FX
delivers at minimum duty cycle, DC

MIN

. No additional control

is needed for the transition between normal operation and cycle
skipping. As the load increases or decreases, the power supply
automatically switches between normal operation and cycle
skipping mode as necessary.

Cycle skipping may be avoided, if so desired, by connecting a
minimum load at the power supply output such that the duty
cycle remains at a level higher than DC

MIN

at all times.

Error Amplifier
The shunt regulator can also perform the function of an error
amplifier in primary feedback applications. The shunt regulator
voltage is accurately derived from a temperature-compensated
bandgap reference. The gain of the error amplifier is set by the
CONTROL pin dynamic impedance. The CONTROL pin
clamps external circuit signals to the V

voltage level. The

CONTROL pin current in excess of the supply current is
separated by the shunt regulator and flows through R

as a

voltage error signal.

On-chip Current Limit with External Programmability
The cycle-by-cycle peak drain current limit circuit uses the
output MOSFET ON-resistance as a sense resistor. A current

limit comparator compares the output MOSFET on-state drain
to source voltage, V

DS(ON)

with a threshold voltage. High drain

current causes V

DS(ON)

to exceed the threshold voltage and turns

the output MOSFET off until the start of the next clock cycle.
The default current limit of TOPSwitch-FX is preset internally.
However, with a resistor connected between MULTI-
FUNCTION pin and SOURCE pin, current limit can be
programmed externally to a lower level between 40% and
100% of the default current limit. Please refer to the graphs in
the typical performance characteristics section for the selection
of the resistor value. By setting current limit low, a
TOPSwitch-FX that is bigger than necessary for the power
required can be used to take advantage of the lower R

DS(ON)

for

higher efficiency. With a second resistor connected between
the MULTI-FUNCTION pin and the rectified DC high voltage
bus providing a small amount of feed forward current, a true
power limiting operation against line variation can be
implemented. When using an RCD clamp, this feed forward
technique reduces maximum clamp voltage at high line allowing
for higher reflected voltage designs. The current limit
comparator threshold voltage is temperature compensated to
minimize the variation of the current limit due to temperature
related changes in R

DS(ON)

of the output MOSFET.

The leading edge blanking circuit inhibits the current limit
comparator for a short time after the output MOSFET is turned
on. The leading edge blanking time has been set so that, if a
power supply is designed properly, current spikes caused by
primary-side capacitances and secondary-side rectifier reverse
recovery time will not cause premature termination of the
switching pulse.

The current limit can be lower for a short period after the
leading edge blanking time as shown in Figure 33. This is due
to dynamic characteristics of the MOSFET. To avoid triggering
the current limit in normal operation, the drain current waveform
should stay within the envelope shown.

Line Under-Voltage Detection (UV)
At power up, UV keeps TOPSwitch-FX off until the input line

PI-2550-092499

128 kHz

4 ms

Time

Switching

Frequency

DRAIN

136 kHz

TOP232-234

7/01

Figure 7. Synchronization Timing Diagram.

voltage reaches the under-voltage threshold. At power down,
UV prevents auto-restart attempts after the output goes out of
regulation. This eliminates power down glitches caused by the
slow discharge of input storage capacitor present in applications
such as standby supplies. A single resistor connected from the
MULTI-FUNCTION pin to the rectified DC high voltage bus
sets UV threshold during power up. Once the power supply is
successfully turned on, UV is disabled to allow extended input
voltage operating range. Input voltage is not checked again
until the power supply loses regulation and attempts another
turn-on. This is accomplished by enabling the UV comparator
only when the divide-by-8 counter used in auto-restart reaches
full count (S7) which is also the state that the counter is reset to
at power up (see Figure 5). The UV feature can be disabled
independent of OV feature as shown in Figure 16.

Line Overvoltage Shutdown (OV)
The same resistor used for UV also sets an overvoltage threshold
which, once exceeded, will force TOPSwitch-FX output into
off-state. The ratio of OV and UV thresholds is preset at 4.5
as can be seen in Figure 8. This feature turns off the
TOPSwitch-FX power MOSFET when the rectified DC high
voltage exceeds the OV threshold. When the MOSFET is off,
the rectified DC high voltage surge capability is increased to the
voltage rating of the MOSFET (700 V), due to the absence of
the reflected voltage and leakage spikes on the drain. Small
amount of hysteresis is provided on the OV threshold to prevent
noise triggering. The OV feature can be disabled independent
of UV feature as shown in Figure 15.

Line Feed Forward with DC

MAX

Reduction

The same resistor used for UV and OV also implements line
voltage feed forward which minimizes output line ripple and
reduces power supply output sensitivity to line transients. This

feed forward operation is illustrated in Figure 4 by the different
values of I

. Note that for the same CONTROL pin current,

higher line voltage results in smaller operating duty cycle. As
an added safety measure, the maximum duty cycle DC

MAX

also reduced from 78% (typical) at a voltage slightly higher than
the UV threshold to 38% (typical) at the OV threshold (see
Figures 4, 8). DC

MAX

of 38% at the OV threshold was chosen

to ensure that the power capability of the TOPSwitch-FX is not
restricted by this feature under normal operation.

Remote ON/OFF and Synchronization
TOPSwitch-FX can be turned on or off by controlling the
current into or out from the MULTI-FUNCTION pin (see
Figure 8). This allows easy implementation of remote ON/OFF
control of TOPSwitch-FX in several different ways. A transistor
or an optocoupler output connected between the MULTI-
FUNCTION pin and the SOURCE pin implements this function
with "active-on" (Figure 19) while a transistor or an optocoupler
output connected between the MULTI-FUNCTION pin and the
CONTROL pin implements the function with "active-off"
(Figure 20).

When a signal is received at the MULTI-FUNCTION pin to
disable the output through any of the MULTI-FUNCTION pin
functions such as OV, UV and remote ON/OFF,
TOPSwitch-FX always completes its current switching cycle as
illustrated in Figure 7 before the output is forced off. The
internal oscillator is stopped slightly before the end of the
current cycle and stays there as long as the disable signal exists.
When the signal at the MULTI-FUNCTION pin changes state
from disable to enable, the internal oscillator starts the next
switching cycle. This approach allows the use of this pin to
synchronize TOPSwitch-FX to any external signal with a
frequency lower than its internal switching frequency.

PI-2558-092999

Oscillator

(SAW)

DMAX

Enable from

M Pin (STOP)

Time

TOP232-234

B
7/01

As seen above, the remote ON/OFF feature allows the
TOPSwitch-FX to be turned on and off instantly, on a cycle-by-
cycle basis, with very little delay. However, remote ON/OFF
can also be used as a standby or power switch to turn off the
TOPSwitch-FX and keep it in a very low power consumption
state for indefinitely long periods. If the TOPSwitch-FX is held
in remote off state for long enough time to allow the CONTROL
pin to dishcharge to the internal supply under-voltage threshold
of 4.8 V (approximately 32 ms for a 47

µF CONTROL pin

capacitance), the CONTROL pin goes into the hysteretic mode
of regulation. In this mode, the CONTROL pin goes through
alternate charge and discharge cycles between 4.8 V and 5.8 V
(see CONTROL pin operation section above) and runs entirely
off the high voltage DC input, but with very low power
consumption (160 mW typical at 230 VAC with M pin open).
When the TOPSwitch-FX is remotely turned on after entering
this mode, it will initiate a normal start-up sequence with soft-
start the next time the CONTROL pin reaches 5.8 V. In the
worst case, the delay from remote on to start-up can be equal to
the full discharge/charge cycle time of the CONTROL pin,
which is approximately 125 ms for a 47

µF CONTROL pin

capacitor. This reduced consumption remote off mode can
eliminate expensive and unreliable in-line mechanical switches.
It also allows for microprocessor controlled turn-on and turn-
off sequences that may be required in certain applications such
as inkjet and laser printers. See Figure 27 under application
examples for more information.

Soft-Start
An on-chip soft-start function is activated at start-up with a
duration of 10 ms (typical). Maximum duty cycle starts from
zero and linearly increases to the default maximum of 78% at
the end of the 10 ms duration. In addition to start-up, soft-start
is also activated at each restart attempt during auto-restart and
when restarting after being in hysteretic regulation of CONTROL
pin voltage (V

), due to remote off or thermal shutdown

conditions. This effectively minimizes current and voltage
stresses on the output MOSFET, the clamp circuit and the
output rectifier, during start-up. This feature also helps minimize
output overshoot and prevents saturation of the transformer
during start-up.

Shutdown/Auto-Restart
To minimize TOPSwitch-FX power dissipation under fault
conditions, the shutdown/auto-restart circuit turns the power

supply on and off at an auto-restart duty cycle of typically 4%
if an out of regulation condition persists. Loss of regulation
interrupts the external current into the CONTROL pin. V

regulation changes from shunt mode to the hysteretic auto-
restart mode described above. When the fault condition is
removed, the power supply output becomes regulated, V

regulation returns to shunt mode, and normal operation of the
power supply resumes.

Hysteretic Over-Temperature Protection
Temperature protection is provided by a precision analog
circuit that turns the output MOSFET off when the junction
temperature exceeds the thermal shutdown temperature (135 °C
typical). When the junction temperature cools to below the
hysteretic temperature, normal operation resumes. A large
hysteresis of 70 °C (typical) is provided to prevent overheating
of the PC board due to a repeating fault condition. V

regulated in hysteretic mode and a 4.8 V to 5.8 V (typical)
sawtooth waveform is present on the CONTROL pin when the
power supply is turned off.

Bandgap Reference
All critical TOPSwitch-FX internal voltages are derived from a
temperature-compensated bandgap reference. This reference is
also used to generate a temperature-compensated current
reference which is trimmed to accurately set the switching
frequency, MOSFET gate drive current, current limit, and the
line OV/UV thresholds. TOPSwitch-FX has improved circuitry
to maintain all of the above critical parameters within very tight
absolute and temperature tolerances.

High-Voltage Bias Current Source
This current source biases TOPSwitch-FX from the DRAIN pin
and charges the CONTROL pin external capacitance during
start-up or hysteretic operation. Hysteretic operation occurs
during auto-restart, remote off and over-temperature shutdown.
In this mode of operation, the current source is switched on and
off with an effective duty cycle of approximately 35%. This
duty cycle is determined by the ratio of CONTROL pin charge
(I

) and discharge currents (I

CD1

and I

CD2

). This current source

is turned off during normal operation when the output MOSFET
is switching.

TOP232-234

7/01

for line sensing by connecting a resistor from this pin to the
rectified DC high voltage bus to implement OV, UV and DC

MAX

reduction with line voltage functions. In this mode, the value
of the resistor determines the line OV/UV thresholds, and the
DC

MAX

is reduced linearly with rectified DC high voltage

starting from just above the UV threshold. In high efficiency
applications this pin can be used in the external current limit
mode instead, to reduce the current limit externally to a value
close to the operating peak current, by connecting the pin to the
SOURCE pin through a resistor. The same pin can also be used
as a remote on/off and a synchronization input in both modes.
Please refer to Table 2 for possible combinations of the functions
with example circuits shown in Figure 13 through Figure 23. A
description of specific functions in terms of the MULTI-
FUNCTION pin I/V characteristic is shown in Figure 8. The
horizontal axis represents MULTI-FUNCTION pin current
with positive polarity indicating currents flowing into the pin.
The meaning of the vertical axes varies with functions. For
those that control the on/off states of the output such as UV, OV
and remote ON/OFF, the vertical axis represents the enable/
disable states of the output. UV triggers at I

(+50

µA typical)

and OV triggers at I

(+225

µA typical). Between +50 µA and

+225

µA, the output is enabled. For external current limit and

line feed forward with DC

MAX

reduction, the vertical axis

represents the magnitude of the I

LIMIT

and DC

MAX

. Line feed

forward with DC

MAX

reduction lowers maximum duty cycle from

78% at I

M(DC)

(+90

µA typical) to 38% at I

(+225

µA). External

current limit is available only with negative MULTI-FUNCTION
pin current. Please see graphs in the typical performance
characteristics section for the current limit programming range
and the selection of appropriate resistor value.

FREQUENCY (F) Pin Operation
The FREQUENCY pin is a digital input pin available in
TO-220 package only. Shorting the FREQUENCY pin to
SOURCE pin selects the nominal switching frequency of
132 kHz (Figure 10) which is suited for most applications. For
other cases that may benefit from lower switching frequency
such as noise sensitive video applications, a 66 kHz switching
frequency (half frequency) can be selected by shorting the
FREQUENCY pin to the CONTROL pin (Figure 11). In
addition, an example circuit shown in Figure 12 may be used to
lower the switching frequency from 132 kHz in normal operation
to 66 kHz in standby mode for very low standby power
consumption.

MULTI-FUNCTION (M) Pin Operation
When current is fed into the MULTI-FUNCTION pin, it works
as a voltage source of approximately 2.6 V up to a maximum
current of +400

µA (typical). At +400 µA, this pin turns into

a constant current sink. When current is drawn out of the
MULTI-FUNCTION pin, it works as a voltage source of
approximately 1.32 V up to a maximum current of 240

µA

(typical). At 240

µA, it turns into a constant current source.

Refer to Figure 9.

There are a total of five functions available through the use of
the MULTI-FUNCTION pin: OV, UV, line feed forward with
DC

MAX

reduction, external current limit and remote ON/OFF.

A short circuit between the MULTI-FUNCTION pin and
SOURCE pin disables all five functions and forces
TOPSwitch-FX to operate in a simple three terminal mode like
TOPSwitch-II. The MULTI-FUNCTION pin is typically used

Table 2. Typical MULTI-FUNCTION Pin Configurations.

MULTI-FUNCTION PIN TABLE*

*This table is only a partial list of many MULTI-FUNCTION pin configurations that are possible.

Using FREQUENCY and MULTI-
FUNCTION Pins

Figure Number

Three Terminal Operation

Under-Voltage

Overvoltage

Line Feed Forward (DC

MAX

)

Line Feed Forward (I

LIMIT

)

External Current Limit

Remote ON/OFF

TOP232-234

B
7/01

Figure 15. Line Sensing for Under-Voltage Only (Overvoltage

Disabled).

PI-2510-040401

Input

Voltage

= R

x I

For Value Shown
V

= 100 VDC

6.2 V

2 M

22 k

CONTROL

Figure 16. Line Sensing for Overvoltage Only (Under-Voltage

Disabled).

Figure 17. Externally Set Current Limit.

PI-2517-040401

Input

Voltage

For R

= 12 k

LIMIT

= 67%

CONTROL

See graph for other
resistor values (R

)

For R

= 25 k

LIMIT

= 40%

PI-2516-040401

Input

Voltage

= I

For Values Shown
V

= 450 VDC

CONTROL

IN4148

2 M

30 k

Figure 13. Three Terminal Operation (MULTI-FUNCTION

Features Disabled. FREQUENCY Pin Tied to SOURCE
or CONTROL Pin).

Figure 14. Line Sensing for Under-Voltage, Overvoltage and

Maximum Duty Cycle Reduction.

PI-2508-081199

Input

Voltage

CONTROL

PI-2509-040401

Input

Voltage

= I

x R

For R

= 2 M

= 100 VDC

450 VDC

MAX

@100 VDC = 78%

MAX

@375 VDC = 47%

CONTROL

2 M

Typical Uses of MULTI-FUNCTION (M) Pin

Figure 18. Current Limit Reduction with Line Voltage.

PI-2518-040401

Input

Voltage

CONTROL

2.5 M

6 k

90% @ 100 VDC
55% @ 300 VDC

LIMIT

TOP232-234

B
7/01

Figure 24. 30 W Power Supply using External Current Limit.

Application Examples

A High Efficiency, 30 W, Universal Input Power Supply
The circuit shown in Figure 24 takes advantage of several of the
TOPSwitch-FX features to reduce system cost and power supply
size and to improve efficiency. This design delivers 30 W at
12 V, from an 85 to 265 VAC input, at an ambient of 50 °C, in
an open frame configuration. A nominal efficiency of 80% at
full load is achieved using TOP234.

The current limit is externally set by resistors R1 and R2 to a
value just above the low line operating peak current of
approximately 70% of the default current limit. This allows use
of a smaller transformer core size and/or higher transformer
primary inductance for a given output power, reducing
TOPSwitch-FX power dissipation, while at the same time
avoiding transformer core saturation during startup and output
transient conditions. The resistor R1 provides a feed forward
signal that reduces the current limit with increasing line voltage,
which, in turn, limits the maximum overload power at high
input line voltage. The feed forward function in combination
with the built-in soft-start feature of TOPSwitch-FX, allows the
use of a low cost RCD clamp (R3, C3 and D1) with a higher

reflected voltage, by safely limiting the TOPSwitch-FX drain
voltage, with adequate margin, under worst case conditions.
The extended maximum duty cycle feature of TOPSwitch-FX
(guaranteed minimum value of 75% vs. 64% for TOPSwitch-II)
allows the use of a smaller input capacitor (C1). The extended
maximum duty cycle and the higher reflected voltage possible
with the RCD clamp also permit the use of a higher primary to
secondary turns ratio for T1 which reduces the peak reverse
voltage experienced by the secondary rectifier D8. As a result,
a 60 V Schottky rectifier can be used for up to 15 V outputs,
which greatly improves power supply efficiency. The cycle
skipping feature of the TOPSwitch-FX eliminates the need for
any dummy loading for regulation at no load and reduces the no
load/standby consumption of the power supply. Frequency
jitter provides improved margin for conducted EMI meeting the
CISPR 22 (FCC B) specification.

A simple Zener sense circuit is used for low cost. The output
voltage is determined by the Zener diode (VR2) voltage and the
voltage drops across the optocoupler (U2) LED and resistor R6.
Resistor R8 provides bias current to Zener VR2 for typical
regulation of

±5% at the 12 V output level, over line and load

and component variations.

12 V
@ 2.5 A

1N4148

10 V

LTV817A

VR2
1N5240C
10 V, 2%

150

R15

150

C14

1 nF

UF4005

68 k

4.7 nF

1KV

CY1

2.2 nF

TOP234Y

TOPSwitch-FX

150

400 V

100 nF

MBR1060

C10

560

35 V

C12

220

35 V

C11

560

35 V

CONTROL

RTN

6.8

4.7 M

1/2 W

9.09 k

PI-2525-040401

3.3

BR1

600 V

3.15 A

20 mH

CX1

100 nF

250 VAC

TOP232-234

7/01

Figure 25. 35 W Set-Top Box Power Supply.

35 W Multiple Output Power Supply
Figure 25 shows a five output, 35 W, secondary regulated
power supply utilizing a TOP233 for multiple output applications
such as set-top box, VCR, DVD, etc. The circuit shown is
designed for a 230 VAC input but can be used over the universal
range at a derated output power of 25 W. Alternatively, a
doubler input stage can be used at 100 or 115 VAC for the full
power rating of 35 W. TOPSwitch-FX provides several
advantages in the above mentioned applications.

A single line sense resistor R1 (2 M

) implements an under-

voltage detect (at 100 V), over-voltage shutdown (at 450 V) and
line feed forward with DC

MAX

reduction features. Under-

voltage detect ensures that the outputs are glitch free on power
down. The over-voltage shutdown turns off the TOPSwitch-FX
MOSFET above 450 V on the DC input rail, eliminating
reflected voltage and leakage inductance spikes, and hence,
extending the surge withstand to the 700 VDC rating of the
MOSFET. This feature prevents field failures in countries
where prolonged line voltage surges are common.

This design also takes advantage of soft-start and higher operating
frequency to reduce transformer size. A snubber circuit (R4,
C4) is used to slowdown dv/dt of the switching waveform
minimizing radiated video noise that could interfere with TV
reception. The half frequency option of the TOPSwitch-FX can
be used by connecting the FREQUENCY pin to the CONTROL

pin instead of the SOURCE pin in video noise sensitive
applications to allow for heavier snubbing without significant
impact on efficiency.

This design achieves

±5% load regulation on 3.3 V and 5 V

outputs using dual sensed optocoupler feedback through resistors
R9, R10 and R11. Other output voltages are set by the transformer
turns ratio. Output voltage on the low power -5 V output is shunt
regulated by resistor R12 and Zener diode VR2. Dummy load
resistor R13 is required to maintain regulation of the 30 V
output under light load conditions. Compensation of the TL431
(U3) is achieved with resistor R8 and capacitor C7. Primary
side compensation and auto-restart frequency are determined
by resistor R5 and capacitor C5. Second stage LC post-filtering
is used on the 3.3 V, 5 V and 18 V high power outputs (L2, L3,
L4 and C13, C15, C17) for low ripple. Full load operating
efficiency exceeds 75% across the AC input range. Primary
clamp components VR1 and D1 limit peak drain voltage to a
safe value.

The frequency jittering in TOPSwitch-FX helps reduce EMI,
maintaining emissions below CISPR 22 (FCC B) levels through
proper choice of Y1 capacitor (CY1) and input filtering elements
(CX1, L1). To minimize coupling of common mode transients
to the TOP233, Y1 capacitor is tied to the positive input DC rail.
Lightning strike immunity to 3 kV is attained with the addition
of a 275 V MOV (RV1).

5 V
@ 2.5 A

3.3 V
@ 3 A

RTN

1N4148

LTV817

U3
TL431CLP

R11

10.0 k

0.1

9.53 k

R10

15.0 k

510

MUR120

UF5402

C12

220

25 V

CY1

2.2 nF

30 V
@ 100 mA

18 V
@ 550 mA

UF4007

VR1

P6KE200

BR1

400 V

3.15 A

RV1

275 V

20 mH

6.8

47 pF

TOP233Y

TOPSwitch-FX

2 M

1/2 W

R13

24 k

2 k

400 V

100 nF

C10

100

50 V

D10

MBR1045

C14

1000

25 V

3.3

C15

100

10 V

C13

100

25 V

C11

50 V

3.3

D11

BYW29-

100

C16

1000

25 V

C18

330

10 V

R12

D12

1N5819

-5 V
@ 100 mA

CX1

0.1

250 VAC

C17

100

10 V

C19

100

10 V

VR2

1N5231

PI-2536-040401

CONTROL

TOP232-234

B
7/01

17 W PC Standby Power Supply
Figure 26 shows a 17 W PC standby application with 3.3 V and
5 V secondary outputs and a 15 V primary output. The supply
uses the TOP232 operating from 230 VAC or 100/115 VAC
with doubler input. This design takes advantage of the soft-
start, line under-voltage detect, tighter current limit variation
and higher switching frequency features of TOPSwitch-FX. For
example, the higher switching frequency with tighter current
limit variation allows use of an EE19 transformer core.
Furthermore, the spacing between high voltage DRAIN pin and
low voltage pins of the TOPSwitch-FX packages provides large
creepage distance which is a significant advantage in high
pollution environments such as fan cooled PC power supplies.

Capacitor C1 provides high frequency decoupling of the high
voltage DC supply, and is necessary only if there is a long trace
length from the source of the DC supply to the inputs of this
standby circuit. The line sense resistor R1 senses the DC input
voltage for line under-voltage. When AC is turned off, the
under-voltage detect feature of the TOPSwitch-FX prevents
auto-restart glitches at the output caused by the slow discharge
of large storage capacitance in the main converter. This is

achieved by turning the power supply off when the input
voltage goes below a level needed to maintain output regulation
and keeping it off until the input voltage goes above the
under-voltage threshold (V

), when the AC is turned on again.

The under voltage threshold is set at 200VDC, slightly below
the required lowest operating DC input voltage, for start-up at
170VAC. This feature saves several components needed to
implement the glitch free turn off with discrete or
TOPSwitch-II based designs.

The bias winding is rectified and filtered by D2 and C6 to create
a bias voltage for the TOP232 and to provide a 15V primary
bias output voltage for the main power supply primary control
circuitry. Both 3.3V and 5V output voltages are sensed by R9,
R10 and R11 using a TL431 (U3) circuit shown. Resistor R6
limits current through optocoupler U2 and sets overall AC
control loop gain. Resistor R7 assures that there is sufficient
bias current for the TL431 when the optocoupler is at a minimum
current. Capacitor C8 provides a soft-finish function to eliminate
turn-on overshoot. The no load regulation (cycle-skipping) of
TOPSwitch-FX permits the circuit to meet the low standby
power requirement of the Blue Angel specification for PCs.

Figure 26. 17 W PC Standby Supply.

3.3 V
@ 2 A

200-375

VDC

BAV21

35 V

C11

100

10 V

SFH615-2

U3
TL431CLP

0.1

301

16.2 k

UF4005

VR1

BZY97C-200

TOP232Y

TOPSwitch-FX

510

0.01

1 kV

(optional)

(Primary

Referenced)

35 V

SB540

5 V
@ 2 A

C12

1000

10 V

C10

1000

10 V

CY1
1 nF

PI-2537-040401

RTN

15 V
@ 30 mA

6.8

3.9 M

3.3

R10

12.1 k

R11

10 k

C13

100

10 V

CONTROL

TOP232-234

7/01

Processor Controlled Supply Turn On/Off
A low cost momentary contact switch can be used to turn the
TOPSwitch-FX power on and off under microprocessor control
that may be required in some applications such as printers. The
low power remote off feature allows an elegant implementation
of this function with very few external components as shown in
Figure 27. Whenever the push button momentary contact switch
P1 is closed by the user, the optocoupler U3 is activated to
inform the microprocessor of this action. Initially, when the
power supply is off (M pin is floating), closing of P1 turns the
power supply on by shorting the M pin of the TOPSwitch-FX to
SOURCE through a diode (remote on). When the secondary
output voltage VCC is established, the microprocessor comes
alive and recognizes that the switch P1 is closed through the
switch status input that is driven by the optocoupler U3 output.
The microprocessor then sends a power supply control signal to
hold the power supply in the on-state through the optocoupler
U4. If the user presses the switch P1 again to command a turn
off, the microprocessor detects this through the optocoupler U3
and initiates a shutdown procedure that is product specific. For
example, in the case of the inkjet printer, the shutdown procedure

may include safely parking the print heads in the storage
position. In the case of products with a disk drive, the shutdown
procedure may include saving data or settings to the disk. After
the shutdown procedure is complete, when it is safe to turn off
the power supply, the microprocessor releases the M pin by
turning the optocoupler U4 off. If the manual switch and the
optocouplers U3 and U4 are not located close to the M pin, a
capacitor C

may be needed to prevent noise coupling to the pin

when it is open.

The power supply could also be turned on remotely through a
local area network or a parallel or serial port by driving the
optocoupler U4 input LED with a logic signal. Sometimes it is
easier to send a train of logic pulses through a cable (due to AC
coupling of cable, for example) instead of a DC logic level as
a wake-up signal. In this case, a simple RC filter can be used to
generate a DC level to drive U4 (not shown in Figure 27). This
remote on feature can be used to wake-up peripherals such as
printers, scanners, external modems, disk drives, etc., as needed
from a computer. Peripherals are usually designed to turn off
automatically if they are not being used for a period of time, to
save power.

Figure 27. Remote ON/OFF Using Microcontroller.

PI-2561-040401

(+5 V)

RETURN

CONTROL

High Voltage

DC Input

TOPSwitch-FX

1N4148

LTV817A

6.8 k

1 nF

100 k

6.8 k

LTV817A

27 k

1N4148

P1 Switch

Status

Power

Supply

ON/OFF

Control

External

Wake-up

Signal

LOGIC

INPUT

LOGIC

OUTPUT

MICRO

PROCESSOR/
CONTROLLER

TOP232-234

B
7/01

In addition to using a minimum number of components,
TOPSwitch-FX provides many technical advantages in this
type of application:

1. Extremely low power consumption in the off mode: 80 mW

typical at 110 VAC and 160 mW typical at 230 VAC. This
is because in the remote/off mode the TOPSwitch-FX
consumes very little power, and the external circuitry does
not consume any current (M pin is open) from the high
voltage DC input.

2. A very low cost, low voltage/current, momentary contact

switch can be used.

3. No debouncing circuitry for the momentary switch is required.

During turn-on, the start-up time of the power supply
(typically 10 to 20 ms) plus the microprocessor initiation
time act as a debouncing filter, allowing a turn-on only if the
switch is depressed firmly for at least the above delay time.
During turn-off, the microprocessor initiates the shutdown

sequence when it detects the first closure of the switch, and
subsequent bouncing of the switch has no effect. If necessary,
the microprocessor could implement the switch debouncing
in software during turn-off, or a filter capacitor can be used
at the switch status input.

4. No external current limiting circuitry is needed for the

operation of the U4 optocoupler output due to internal
limiting of M pin current.

5. No high voltage resistors to the input DC voltage rail are

required to power the external circuitry in the primary. Even
the LED current for U3 can be derived from the CONTROL
pin. This not only saves components and simplifies layout,
but also eliminates the power loss associated with the high
voltage resistors in both on and off states.

6. Robust design: There is no on/off latch that can be accidentally

triggered by transients. Instead, the power supply is held in
the on-state through the secondary side microprocessor.

TOP232-234

7/01

Key Application Considerations

TOPSwitch-FX

vs.

TOPSwitch-ll

Table 3 compares the features and performance differences
between TOPSwitch-FX and TOPSwitch-II. Many of the new
features eliminate the need for costly discrete component.

Other features increase the robustness of design allowing cost
savings in the transformer and other power components.

*Not available
Table 3. Comparison Between TOPSwitch-II and TOPSwitch-FX. (continued on next page)

Function

TOPSwitch-II

TOPSwitch-FX

Figures Advantages

Soft-Start

N/A*

10 ms

· Limits peak current and voltage

component stresses during start-up

· Eliminates external components

used for soft-start in most
applications

· Minimizes output overshoot

External Current Limit

N/A*

Programmable

8, 17,

· Smaller transformer

100% to 40% of

18, 21,

· Higher efficiency

default current

· Allows tighter power limit

limit

during output overload conditions

MAX

67%

78%

· Smaller input cap (wider dynamic

range)

· Higher power capability (when used

with RCD clamp for large V

)

· Allows use of Schottky secondary

rectifier diode for up to 15 V output
for high efficiency

Line Feed Forward with

N/A*

78% to 38%

4, 8, 14, · Rejects line ripple

MAX

Reduction

· Increases transient and surge

voltage withstand capability

Line OV Shutdown

N/A*

Single resistor

8, 14,

· Increases voltage withstand cap-

programmable

16, 23

ability against line surge

Line UV Detection

N/A*

Single resistor

5, 8, 14, · Prevents auto-restart glitches

programmable

15, 23

during power down

Switching Frequency

100 kHz

10%

132 kHz

· Smaller transformer
· Fundamental below 150 kHz for

conducted EMI

Switching Frequency

N/A*

66 kHz

11, 12

· Lower losses when using RC and

Option (TO-220 only)

RCD snubber for noise reduction in
video applications

· Allows for higher efficiency in

standby mode

· Lower EMI (second harmonic below

150 kHz)

Frequency Jitter

N/A*

4 kHz@132 kHz

6, 28

· Reduces conducted EMI

2 kHz@66 kHz

Cycle Skipping

N/A*

At DC

MIN

(1.5%)

· Zero load regulation without dummy

load

· Low power consumption at no load

TOP232-234

B
7/01

TOPSwitch-FX

Design Considerations

TOPSwitch-FX Selection
Selecting the optimum TOPSwitch-FX depends upon required
maximum output power, efficiency, heat sinking constraints
and cost goals. With the option to externally reduce current
limit, a larger TOPSwitch-FX may be used for lower power
applications where higher efficiency is needed or minimal heat
sinking is available.

Input Capacitor
The input capacitor must be chosen to provide the minimum
DC voltage required for the TOPSwitch-FX converter to maintain
regulation at the lowest specified input voltage and maximum
output power. Since TOPSwitch-FX has a higher DC

MAX

than

TOPSwitch-II, it is possible to use a smaller input capacitor. For
TOPSwitch-FX, a capacitance of 2

µF per watt is usually

sufficient for universal input with an appropriately designed
transformer.

Primary Clamp and Output Reflected Voltage V

A primary clamp is necessary to limit the peak TOPSwitch-FX
drain to source voltage. A Zener clamp (see Figure 26, VR1)
requires few parts and takes up little board space. For good
efficiency, the clamp Zener should be selected to be at least 1.5
times the output reflected voltage V

as this keeps the leakage

spike conduction time short. When using a Zener clamp in a
universal input application, a V

of less than 135 V is

recommended to allow for the absolute tolerances and
temperature variations of the Zener. This will ensure efficient
operation of the clamp circuit and will also keep the maximum
drain voltage below the rated breakdown voltage of the
TOPSwitch-FX MOSFET.

A high V

is required to take full advantage of the wider DC

MAX

of TOPSwitch-FX. An RCD clamp provides tighter clamp
voltage tolerance than a Zener clamp and allows a V

as high

as 165 V. The V

can be further increased in continuous mode

designs up to 185 V by reducing the external current limit as a
function of input line voltage (see Figure 18). The RCD clamp

*Not available
Table 3 (cont). Comparison Between TOPSwitch-II and TOPSwitch-FX.

Function

TOPSwitch-II

TOPSwitch-FX

Figures Advantages

Remote ON/OFF

N/A*

Single transistor

8, 19,

· Fast on/off (cycle by cycle)

or optocoupler

20, 21,

· Active-on or active-off control

interface or manual 22, 23,

· Low consumption in remote off state

switch

· Active-on control for fail-safe
· Eliminates expensive in-line on/off

switch

· Allows processor controlled turn on/

off

· Permits shutdown/wake-up of

peripherals via LAN or parallel port

Synchronization

N/A*

Single transistor

· Synchronization to external lower

or optocoupler

frequency signal

interface

· Starts new switching cycle on

demand

Thermal Shutdown

Latched

Hysteretic (with

· Automatic recovery from thermal

C hysteresis)

fault

· Large hysteresis prevents circuit

board overheating

Current Limit Tolerance

10% (@25

7% (@25

· 10% higher power capability due to

-8% (0

C to100

C) -4% (0

C to 100

tighter tolerance

DRAIN

DIP

0.037" / 0.94 mm

0.137" / 3.48 mm

· Greater immunity to arcing as a

Creepage at

SMD

0.037" / 0.94 mm

0.137" / 3.48 mm

result of build-up of dust, debris and

Package

TO-220 0.046" / 1.17 mm

0.068" / 1.73 mm

other contaminants

DRAIN Creepage at

0.045" / 1.14 mm

0.113" / 2.87 mm

· Preformed leads accommodate

PCB for TO-220

(preformed leads)

large creepage for PCB layout

· Easier to meet Safety (UL/VDE)

TOP232-234

7/01

is more cost effective than the Zener clamp but requires more
careful design (see quick design checklist).

Output Diode
The output diode is selected for peak inverse voltage, output
current, and thermal conditions in the application (including
heat sinking, air circulation, etc.). The higher DC

MAX

TOPSwitch-FX along with an appropriate transformer turns
ratio can allow the use of a 60 V Schoktty diode for higher
efficiency on output voltages as high as 15 V (See Figure 24. A
12 V, 30 W design using a 60 V Schottky for the output diode).

Soft-Start
Generally a power supply experiences maximum stress at start-
up before the feedback loop achieves regulation. For a period
of 10 ms the on-chip soft-start linearly increases the duty cycle
from zero to the default DC

MAX

at turn on, which causes the

primary current and output voltage to rise in an orderly manner
allowing time for the feedback loop to take control of the duty
cycle. This reduces the stress on the TOPSwitch-FX MOSFET,
clamp circuit and output diode(s), and helps prevent transformer
saturation during start-up. Also, soft-start limits the amount of
output voltage overshoot, and in many applications eliminates
the need for a soft-finish capacitor.

EMI
The frequency jitter feature modulates the switching frequency
over a narrow band as a means to reduce conducted EMI peaks
associated with the harmonics of the fundamental switching
frequency. This is particularly beneficial for average detection
mode. As can be seen in Figure 28, the benefits of jitter increase
with the order of the switching harmonic due to an increase in
frequency deviation.

The FREQUENCY pin of TOPSwitch-FX offers a switching
frequency option of 132 kHz or 66 kHz. In applications that
require heavy snubbers on the drain node for reducing high
frequency radiated noise (for example, video noise sensitive
applications such as VCR, DVD, monitor, TV, etc.), operating
at 66 kHz will reduce snubber loss resulting in better efficiency.
Also, in applications where transformer size is not a concern,
use of the 66 kHz option will provide lower EMI and higher
efficiency. Note that the second harmonic of 66 kHz is still
below 150 kHz, above which the conducted EMI specifications
get much tighter.

For 10 W or below, it is possible to use a simple inductor in place
of a more costly AC input common mode choke to meet
worldwide conducted EMI limits.

Transformer Design
It is recommended that the transformer be designed for maximum
operating flux density of 3000 gauss and a peak flux density of
4200 gauss at maximum current limit. The turns ratio should be
chosen for a reflected voltage (V

) no greater than 135 V when

2nd

3rd

4th

5th

Switching Harmonic

PI-2559-093099

Noise Reduction (dB)

Quasi-Peak

Average

-20

-10

0.15

Frequency (MHz)

Amplitude (dB

PI-2576-010600

VFG243B (QP)
VF646B (AV)

TOPSwitch-II (no jitter)

VFG243B (QP)
VF646B (AV)

-20

-10

0.15

Frequency (MHz)

Amplitude (dB

PI-2577-010600

TOPSwitch-FX (with jitter)

(a)

(b)

(c)

Figure 28. (a) Conducted noise improvement for low frequency

harmonics due to jitter, (b) TOPSwitch-II full range EMI
scan (100kHz, no jitter), (c) TOPSwitch-FX full range
EMI scan (132 kHz, with jitter) with identical circuitry
and conditions.

TOP232-234

7/01

using a Zener clamp, 165 V when using an RCD clamp and
185 V when using RCD clamp with current limit feed forward.

For designs where operating current is significantly lower than
the default current limit, it is recommended to use an externally
set current limit close to the operating peak current to reduce
peak flux density and peak power (see Figure 17). In most
applications, the tighter current limit tolerance, higher switching
frequency and soft-start features of TOPSwitch-FX contribute
to a smaller transformer when compared to TOPSwitch-II.

Standby Consumption
Cycle skipping can significantly reduce power loss at zero load,
especially when a Zener clamp is used. For very low secondary
power consumption use a TL431 regulator for feedback control.
Alternately, switching losses can be significantly reduced by
switching from 132 kHz in normal operation to 66 kHz under
light load conditions.

TOPSwitch-FX

Layout Considerations

Primary Side Connections
Use a single point (Kelvin) connection at the negative terminal
of the input filter capacitor for TOPSwitch-FX SOURCE pin
and bias winding return. This improves surge capabilities by
returning surge currents from the bias winding directly to the
input filter capacitor.

The CONTROL pin bypass capacitor should be located as close
as possible to the SOURCE and CONTROL pins and its
SOURCE connection trace should not be shared by the main
MOSFET switching currents.

All SOURCE pin referenced components connected to the
MULTI-FUNCTION pin should also be located close to
SOURCE and MULTI-FUNCTION pins with dedicated SOURCE
pin connection. The MULTI-FUNCTION pin's trace should be
kept as short as possible and away from the DRAIN trace to
prevent noise coupling. Line sense resistor (R1 in Figures 29 and
30) should be located close to the MULTI-FUNCTION pin to
minimize the trace length on the MULTI-FUNCTION pin side.

In addition to the 47

µF CONTROL pin capacitor, a high frequency

bypass capacitor in parallel may be used for better noise immunity.
The feedback optocoupler output should also be located close to
the CONTROL and SOURCE pins of TOPSwitch-FX.

Y-Capacitor
The Y-capacitor should be connected close to the secondary
output return pin(s) and the primary DC input pin of the
transformer (see Figures 29 and 30).

Heat Sinking
The tab of the Y package (TO-220) is internally electrically
tied to the SOURCE pin. To avoid circulating currents, a heat

sink attached to the tab should not be electrically tied to any
nodes on the PC board.

When using P (DIP-8) or G (SMD-8) packages, a copper area
underneath the package connected to the SOURCE pins will act
as an effective heat sink.

In addition, sufficient copper area should be provided at the
anode and cathode leads of the output diode(s) for heat sinking.

Quick Design Checklist

As with any power supply design, all TOPSwitch-FX designs
should be verified on the bench to make sure that components
specifications are not exceeded under worst case conditions.
The following minimum set of tests is strongly recommended:

1. Maximum drain voltage Verify that peak V

does not

exceed 675 V at highest input voltage and maximum overload
output power. Maximum overload output power occurs
when the ouput is overloaded to a level just before the power
supply goes into auto-restart (loss of regulation).

2. Maximum drain current At maximum ambient temperature,

maximum input voltage and maximum output load, verify
drain current waveforms at start-up for any signs of
transformer saturation and excessive leading edge current
spikes. TOPSwitch-FX has a leading edge blanking time of
200 ns to prevent premature termination of the on-cycle.
Verify that the leading edge current spike is below the
allowed current limit envelope (see Figure 33) for the drain
current waveform at the end of the 200 ns blanking period.

3. Thermal check At maximum output power, minimum

input voltage and maximum ambient temperature, verify
that temperature specifications are not exceeded for
TOPSwitch-FX, transformer, output diodes and output
capacitors. Enough thermal margin should be allowed for
the part-to-part variation of the R

DS(ON)

of TOPSwitch-FX as

specified in the data sheet. The margin required can either
be calculated from the tolerances or it can be accounted for
by connecting an external resistance in series with the
DRAIN pin and attached to the same heatsink, having a
resistance value that is equal to the difference between the
measured R

DS(ON)

of the device under test and the worst case

maximum specification.

Design Tools

1. Technical literature: Data Sheet, Application Notes,

Design Ideas, etc.

2. Transformer design spreadsheet.
3. Engineering prototype boards.

Up to date information on design tools can be found at Power
Integrations Web site: www.powerint.com

TOP232-234

B
7/01

ABSOLUTE MAXIMUM RATINGS

(1)

DRAIN Voltage ............................................ -0.3 to 700 V
DRAIN Peak Current: TOP232 ................................. 0.8 A

TOP233 ................................. 1.6 A
TOP234 ................................. 2.4 A

CONTROL Voltage .......................................... -0.3 to 9 V
CONTROL Current ...............................................100 mA
MULTI-FUNCTION Pin Voltage .................... -0.3 to 9 V
FREQUENCY Pin Voltage ............................... -0.3 to 9 V

= 4 mA;

= 25

= I

CD1

OSC

MAX

MIN

SOFT

CONTROL FUNCTIONS

Conditions

(Unless Otherwise Specified)

See Figure 34

SOURCE = 0 V

;

= -40 to 125

Min

Typ

Max

Parameter

Symbol

Units

THERMAL IMPEDANCE

kHz

%/mA

Thermal Impedance: Y Package (

)

(1)

............... 70

°C/W

(

)

(2)

................. 2

°C/W

P/G Package:
(

) ........ 45

°C/W

(3)

; 35

°C/W

(4)

(

)

(5)

.......................... 11

°C/W

Notes:

1. Free standing with no heatsink.
2. Measured at the back surface of tab.
3. Soldered to 0.36 sq. inch (232 mm

), 2oz. (610 gm/m

) copper clad.

4. Soldered to 1 sq. inch (645 mm

), 2oz. (610 gm/m

) copper clad.

5. Measured on the SOURCE pin close to plastic interface.

124

132

140

61.5

70.5

250

75.0

78.0

82.0

35.0

47.0

57.0

0.8

1.5

2.7

-27

-22

-17

= 4 mA; T

= 25

Switching
Frequency
(average)

Frequency Jitter
Deviation

Frequency Jitter
Modulation Rate

Maximum Duty
Cycle

Minimum Duty
Cycle (Prior to
Cycle Skipping)

Soft Start Time

PWM
Gain

FREQUENCY Pin

Connected to SOURCE

FREQUENCY Pin

Connected to CONTROL

132 kHz Operation

66 kHz Operation

M(DC)

= 190

= 25

C; DC

MIN

MAX

Storage Temperature ..................................... -65 to 150

°C

Operating Junction Temperature

(2)

................ -40 to 150

°C

Lead Temperature

(3)

................................................ 260

°C

Notes:

1. All voltages referenced to SOURCE, T

= 25

°C.

2. Normally limited by internal circuitry.
3. 1/16" from case for 5 seconds.

TOP232-234

B
7/01

100

5.60

5.85

6.10

1.0

1.5

2.0

0.3

0.6

1.0

ppm/

Conditions

(Unless Otherwise Specified)

See Figure 34

SOURCE = 0 V

;

= -40 to 125

Min

Typ

Max

Parameter

Symbol

Units

C(SHUNT)

CD1

CD2

Measured in a Typical

Flyback Converter Application

See Note D

= 4 mA

Output

MOSFET Enabled

= 0 V

Output

MOSFET Disabled

= 0 V

SUPPLY VOLTAGE CHARACTERISTICS

OUTPUT (cont.)

Off-State
Current

Breakdown
Voltage

DSS

150

700

Rise
Time

Fall
Time

DRAIN Supply
Voltage

Shunt Regulator
Voltage

Shunt Regulator
Temperature Drift

Control Supply/
Discharge Current

= Floating; I

= 4mA

= 560 V; T

= 125

= Floating; I

= 4mA

= 100

A; T

= 25

NOTES:

A. For specifications with negative values, a negative temperature coefficient corresponds to an increase in

magnitude with increasing temperature, and a positive temperature coefficient corresponds to a decrease in
magnitude with increasing temperature.

B. Guaranteed by characterization. Not tested in production.

C. For externally adjusted current limit values, please refer to the graph (Current Limit vs. External Current Limit

Resistance) in the Typical Performance Characteristics section.

D. It is possible to start up and operate TOPSwitch-FX at DRAIN voltages well below 36 V. However, the CONTROL

pin charging current is reduced, which affects start-up time, auto-restart frequency, and auto-restart duty cycle.
Refer to the characteristic graph on CONTROL pin charge current (I

) vs. DRAIN voltage for low voltage operation

characteristics.

TOP232-234

B
7/01

The following precautions should be followed when testing
TOPSwitch-FX by itself outside of a power supply. The schematic
shown in Figure 34 is suggested for laboratory testing of
TOPSwitch-FX.

When the DRAIN pin supply is turned on, the part will be in the
auto-restart mode. The CONTROL pin voltage will be oscillating
at a low frequency between 4.8 and 5.8 V and the drain is turned
on every eighth cycle of the CONTROL pin oscillation. If the
CONTROL pin power supply is turned on while in this auto-

BENCH TEST PRECAUTIONS FOR EVALUATION OF ELECTRICAL CHARACTERISTICS

Typical Performance Characteristics

restart mode, there is only a 12.5% chance that the CONTROL
pin oscillation will be in the correct state (drain active state) so
that the continuous drain voltage waveform may be observed.
It is recommended that the V

power supply be turned on first

and the DRAIN pin power supply second if continuous drain
voltage waveforms are to be observed. The 12.5% chance of
being in the correct state is due to the divide-by-8 counter.
Temporarily shorting the CONTROL pin to the SOURCE pin
will reset TOPSwitch-FX, which then will come up in the
correct state.

1.0

-250

-200

-150

-100

-50

(

Current Limit (A)

CURRENT LIMIT vs. MULTI-FUNCTION

PIN CURRENT

PI-2540-033001

100

120

140

160

180

200

di/dt (mA/

TOP234 1.50
TOP233 1.00
TOP232 0.50

Scaling Factors:

1.0

1.1

100

120

140

160

180

200

10K

15K

20K

25K

External Current Limit Resistor R

(

)

Current Limit (A)

di/dt (mA/

CURRENT LIMIT vs. EXTERNAL

CURRENT LIMIT RESISTANCE

PI-2539-033001

30K

Maximum and minimum levels
are based on characterization.

Maximum

Minimum

Typical

TOP234 1.50
TOP233 1.00
TOP232 0.50

Scaling Factors:

TOP232-234

7/01

Notes:
1. Package dimensions conform to JEDEC specification
MS-001-AB (Issue B 7/85) for standard dual-in-line (DIP)
package with .300 inch row spacing.
2. Controlling dimensions are inches. Millimeter sizes are
shown in parentheses.
3. Dimensions shown do not include mold flash or other
protrusions. Mold flash or protrusions shall not exceed
.006 (.15) on any side.
4. Pin locations start with Pin 1, and continue counter-clock-
wise to Pin 8 when viewed from the top. The notch and/or
dimple are aids in locating Pin 1. Pin 6 is omitted.
5. Minimum metal to metal spacing at the package body for
the omitted lead location is .137 inch (3.48 mm).
6. Lead width measured at package body.
7. Lead spacing measured with the leads constrained to be
perpendicular to plane T.

.010 (.25)
.015 (.38)

.300 (7.62) BSC

(NOTE 7)

.300 (7.62)
.390 (9.91)

.375 (9.53)
.385 (9.78)

.245 (6.22)
.255 (6.48)

.128 (3.25)
.132 (3.35)

.057 (1.45)
.063 (1.60)

.125 (3.18)
.135 (3.43)

0.15 (.38)

MINIMUM

.048 (1.22)
.053 (1.35)

.100 (2.54) BSC

.014 (.36)
.022 (.56)

-E-

Pin 1

SEATING
PLANE

-D-

-T-

P08B

DIP-8B

PI-2551-033001

D S .004 (.10)

T E D S .010 (.25) M

(NOTE 6)

SMD-8B

PI-2546-040501

.004 (.10)
.012 (.30)

.036 (0.91)

.044 (1.12)

.004 (.10)

0 -

.375 (9.53)
.385 (9.78)

.048 (1.22)

.009 (.23)

.053 (1.35)

.032 (.81)

.037 (.94)

.128 (3.25)
.132 (3.35)

-D-

Notes:
1. Controlling dimensions are
inches. Millimeter sizes are
shown in parentheses.
2. Dimensions shown do not
include mold flash or other
protrusions. Mold flash or
protrusions shall not exceed
.006 (.15) on any side.
3. Pin locations start with Pin 1,
and continue counter-clock
Pin 8 when viewed from the
top. Pin 6 is omitted.
4. Minimum metal to metal
spacing at the package body
for the omitted lead location
is .137 inch (3.48 mm).
5. Lead width measured at
package body.
6. D and E are referenced
datums on the package
body.

.057 (1.45)
.063 (1.60)

(NOTE 5)

E S

.100 (2.54) (BSC)

.372 (9.45)

.245 (6.22)

.388 (9.86)

.255 (6.48)

.010 (.25)

-E-

Pin 1

D S .004 (.10)

G08B

Heat Sink is 2 oz. Copper

As Big As Possible

.420

.046 .060

.060 .046

.080

Pin 1

.086

.186

.286

Solder Pad Dimensions

TOP232-234

B
7/01

Notes

1) Corrected rounding of operating frequency (132/66 kHz).

2) Corrected spelling.

3) Corrected Storage Temperature

and updated nomenclature in parameter table.

Date

1/00

7/01

Revision

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