April 1999
TOP412/414
TOPSwitch
Family
Three-terminal DC to DC PWM Switch
Product Highlights
Low Cost Replacement for Discrete Switchers
Up to 15 fewer components - cuts cost, increases reliability
Allows for a smaller and lighter solution under 12 mm
height, all surface mount components
Over 80% Efficiency in Flyback Topology
Built-in start-up and current limit reduce DC losses
Low capacitance MOSFET cuts switching losses
CMOS controller/gate driver consumes only 7 mW
70% maximum duty cycle minimizes conduction losses
Simplifies Design - Reduces Time to Market
Integrated PWM Controller and high power MOSFET
Only one external capacitor needed for compensation,
bypass and start-up/auto-restart functions
System Level Fault Protection Features
Auto-restart and cycle by cycle current limiting functions
handle both primary and secondary faults
On-chip latching thermal shutdown protects the entire
system against overload
Highly Versatile
Implements Buck, Boost, Flyback or Forward topology
Easily interfaces with both opto and primary feedback
Supports continuous or discontinuous mode of operation
Specified for operation down to 16 V DC input
Description
The TOPSwitch family implements, with only three terminals,
all functions necessary for a DC to DC, converter: high voltage
N-channel power MOSFET with controlled turn-on gate driver,
voltage mode PWM controller with integrated 120 kHz oscillator,
high voltage start-up bias circuit, bandgap derived reference,
bias shunt regulator/error amplifier for loop compensation and
fault protection circuitry. Compared to discrete MOSFET and
controller or self oscillating (RCC) switching converter solutions,
a TOPSwitch integrated circuit can reduce total cost, component
count, size, weight and at the same time increase efficiency and
system reliability. This device is well suited for Telecom,
Cablecom and other DC to DC converter applications up to
Figure 1. Typical Application.
MINIMUM
INPUT
VOLTAGE
ORDER PART NUMBER
Output Power Capability
1-4
18 VDC
3 W
9 W
TOP412G
TOP414G
24 VDC
36 VDC
48 VDC
60 VDC
72 VDC
90 VDC
12 W
Table 1. TOP412/414 Output Power.
4 W
5 W
6 W
7 W
9 W
18 W
21 W
12 W
15 W
15 W
18 W
Notes: 1. Assumes maximum junction temperature of 100
C
2. Assumes output of 5 V and K
RP
of 0.4 3. Soldered to 1 sq. inch
(645 mm
2
), 2 oz. copper clad (610 gm/mm
2
) 4. The continuous
power capability in a given application depends on thermal
environment, transformer design, efficiency required, input storage
capacity, etc.
PI-2371-120198
D
S
C
CONTROL
TOP414
VIN
VO
21W of output power. Internally, the lead frame of the
SMD-8 package uses six of its pins to transfer heat from the chip
directly to the board, eliminating the cost of a heat sink.
TOP412/414
A
4/99
2
PI-1746-011796
SHUTDOWN/
AUTO-RESTART
PWM
COMPARATOR
CLOCK
SAW
OSCILLATOR
CONTROLLED
TURN-ON
GATE
DRIVER
INTERNAL
SUPPLY
5.7 V
4.7 V
SOURCE
S
R
Q
Q
DMAX
-
+
CONTROL
-
+
5.7 V
IFB
RE
ZC
VC
MINIMUM
ON-TIME
DELAY
+
-
VI
LIMIT
LEADING
EDGE
BLANKING
POWER-UP
RESET
R
S
Q
Q
8
0
1
THERMAL
SHUTDOWN
EXTERNALLY
TRIGGERED
SHUTDOWN
SHUNT REGULATOR/
ERROR AMPLIFIER
+
-
DRAIN
Figure 2. Functional Block Diagram.
Pin Functional Description
DRAIN Pin:
Output MOSFET drain connection. Provides internal bias
current during start-up operation via an internal switched high-
voltage current source. Internal current sense point.
CONTROL Pin:
Error amplifier and feedback current input pin for duty cycle
control. Internal shunt regulator connection to provide internal
bias current during normal operation. Trigger input for latching
shutdown. It is also used as the supply bypass and auto-restart/
compensation capacitor connection point.
SOURCE Pin:
Output MOSFET source connection. Primary-side circuit
common, power return, and reference point.
Figure 3. Pin Configuration.
PI-2208-120798
CONTROL
8
5
7
6
DRAIN
SOURCE (HV RTN)
SOURCE
SOURCE
1
4
2
3
SOURCE (HV RTN)
SOURCE (HV RTN)
SOURCE
G Package (SMD-8)
A
4/99
TOP412/414
3
TOPSwitch
Family Functional Description
TOPSwitch is a self biased and protected linear control current-
to-duty cycle converter with an open drain output. High
efficiency is achieved through the use of CMOS and integration
of the maximum number of functions possible. CMOS
significantly reduces bias currents as compared to bipolar or
discrete solutions. Integration eliminates external power resistors
used for current sensing and/or supplying initial start-up bias
current.
During normal operation, the internal output MOSFET duty
cycle linearly decreases with increasing CONTROL pin current
as shown in Figure 4. To implement all the required control,
bias, and protection functions, the DRAIN and CONTROL pins
each perform several functions as described below. Refer to
Figure 2 for a block diagram and Figure 6 for timing and voltage
waveforms of the TOPSwitch integrated circuit.
Control Voltage Supply
CONTROL pin voltage V
C
is the supply or bias voltage for the
controller and driver circuitry. An external bypass capacitor
closely connected between the CONTROL and SOURCE pins
is required to supply the gate drive current. The total amount of
capacitance connected to this pin (C
T
) also sets the auto-restart
timing as well as control loop compensation. V
C
is regulated in
either of two modes of operation. Hysteretic regulation is used
for initial start-up and overload operation. Shunt regulation is
used to separate the duty cycle error signal from the control
circuit supply current. During start-up, V
C
current is supplied
from a high-voltage switched current source connected internally
between the DRAIN and CONTROL pins. The current source
provides sufficient current to supply the control circuitry as
well as charge the total external capacitance (C
T
).
PI-1691-112895
DMAX
DMIN
ICD1
Duty Cycle (%)
IC (mA)
2.5
6.5
45
Slope = PWM Gain
-16%/mA
IB
Auto-restart
Figure 4. Relationship of Duty Cycle to CONTROL Pin Current.
Figure 5. Start-up Waveforms for (a) Normal Operation and (b)
Auto-restart.
DRAIN
0
VIN
VC
0
4.7 V
5.7 V
8 Cycles
95%
5%
Off
Switching
Switching
Off
IC
Charging CT
ICD1
Discharging CT
ICD2
Discharging CT
IC
Charging CT
Off
PI-1124A-060694
DRAIN
0
VIN
VC
0
4.7 V
5.7 V
Off
Switching
(b)
(a)
C
T
is the total external capacitance
connected to the CONTROL pin
TOP412/414
A
4/99
4
TOPSwitch
Family Functional Description (cont.)
The first time V
C
reaches the upper threshold, the high-voltage
current source is turned off and the PWM modulator and output
transistor are activated, as shown in Figure 5(a). During normal
operation (when the output voltage is regulated) feedback
control current supplies the V
C
supply current. The shunt
regulator keeps V
C
at typically 5.7 V by shunting CONTROL
pin feedback current exceeding the required DC supply current
through the PWM error signal sense resistor R
E
. The low
dynamic impedance of this pin (Z
C
) sets the gain of the error
amplifier when used in a primary feedback configuration. The
dynamic impedance of the CONTROL pin together with the
external resistance and capacitance determines the control loop
compensation of the power system.
If the CONTROL pins total external capacitance (C
T
) should
discharge to the lower threshold, the output MOSFET is turned
off and the control circuit is placed in a low-current standby
mode. The high-voltage current source is turned on and charges
the external capacitance again. Charging current is shown with
a negative polarity and discharging current is shown with a
positive polarity in Figure 6. The hysteretic auto-restart
comparator keeps V
C
within a window of typically 4.7 to 5.7 V
by turning the high-voltage current source on and off as shown
in Figure 5(b). 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. The
counter effectively limits TOPSwitch power dissipation by
reducing the auto-restart duty cycle to typically 5%. Auto-
restart continues to cycle until output voltage regulation is again
achieved.
Bandgap Reference
All critical TOPSwitch internal voltages are derived from a
temperature-compensated bandgap reference. This reference
is also used to generate a temperature-compensated current
source which is trimmed to accurately set the oscillator frequency
and MOSFET gate drive current.
Oscillator
The internal oscillator linearly charges and discharges the
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 frequency of 120 kHz was chosen
to minimize EMI and maximize efficiency in power supply
applications. Trimming of the current reference improves the
frequency accuracy.
Pulse Width Modulator
The pulse width modulator implements a voltage-mode control
loop by driving the output MOSFET with a duty cycle inversely
proportional to the current flowing into the CONTROL pin.
The error signal across R
E
is filtered by an RC network with a
typical corner frequency of 7 kHz to reduce the effect of
switching noise. 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. The maximum
duty cycle is set by the symmetry of the internal oscillator. The
modulator has a minimum ON-time to keep the current
consumption of the TOPSwitch independent of the error signal.
Note that a minimum current must be driven into the CONTROL
pin before the duty cycle begins to change.
Gate Driver
The gate driver is designed to turn the output MOSFET on at a
controlled rate to minimize common-mode EMI. The gate
drive current is trimmed for improved accuracy.
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 the 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
C
voltage level. The
CONTROL pin current in excess of the supply current is
separated by the shunt regulator and flows through R
E
as the
error signal.
Cycle-By-Cycle Current Limit
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-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 current limit comparator threshold voltage is
temperature compensated to minimize variation of the effective
peak current limit due to temperature related changes in output
MOSFET R
DS(ON)
.
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 current
spikes caused by primary-side capacitances and secondary-side
rectifier reverse recovery time will not cause premature
termination of the switching pulse.
Shutdown/Auto-restart
To minimize TOPSwitch power dissipation, the shutdown/
auto-restart circuit turns the power supply on and off at a duty
cycle of typically 5% if an out of regulation condition persists.
Loss of regulation interrupts the external current into the
A
4/99
TOP412/414
5
CONTROL pin. V
C
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
C
regulation returns to shunt mode, and normal
operation of the power supply resumes.
Latching Shutdown
The output overvoltage protection latch is activated by a high-
current pulse into the CONTROL pin. When set, the latch
turns off the TOPSwitch output. Activating the power-up
reset circuit by removing and restoring input power, or
momentarily pulling the CONTROL pin below the power-up
reset threshold resets the latch and allows TOPSwitch to
resume normal power supply operation. V
C
is regulated in
hysteretic mode when the power supply is latched off.
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
(typically 145
C). Activating the power-up reset circuit by
removing and restoring input power or momentarily pulling the
CONTROL pin below the power-up reset threshold resets the
latch and allows TOPSwitch to resume normal power supply
operation. V
C
is regulated in hysteretic mode when the power
supply is latched off.
High-voltage Bias Current Source
This current source biases TOPSwitch from the DRAIN pin and
charges the CONTROL pin external capacitance (C
T
) during
start-up or hysteretic operation. Hysteretic operation occurs
during auto-restart and latched shutdown. 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
C
) and discharge currents (I
CD1
and
I
CD2
). This current source is turned off during normal operation
when the output MOSFET is switching.
PI-1119-110194
VIN
VOUT
0
IOUT
0
1
2
1
4
3
DRAIN
0
VIN
VC
0
1
2
1
2
8
1
0
IC
1
2
8
8
1
2
8
1
VC(reset)
45 mA
Figure 6. Typical Waveforms for (1) Normal Operation, (2) Auto-restart, (3) Latching Shutdown, and (4) Power Down Reset.
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