LM2588
SIMPLE SWITCHER

5A Flyback Regulator with

Shutdown

General Description

The LM2588 series of regulators are monolithic integrated
circuits specifically designed for flyback, step-up (boost), and
forward converter applications. The device is available in 4
different output voltage versions: 3.3V, 5.0V, 12V, and adjust-
able.

Requiring a minimum number of external components, these
regulators are cost effective, and simple to use. Included in
the datasheet are typical circuits of boost and flyback regula-
tors. Also listed are selector guides for diodes and capacitors
and a family of standard inductors and flyback transformers
designed to work with these switching regulators.

The power switch is a 5.0A NPN device that can stand-off
65V. Protecting the power switch are current and thermal
limiting circuits, and an undervoltage lockout circuit. This IC
contains an adjustable frequency oscillator that can be pro-
grammed up to 200 kHz. The oscillator can also be synchro-
nized with other devices, so that multiple devices can oper-
ate at the same switching frequency.

Other features include soft start mode to reduce in-rush cur-
rent during start up, and current mode control for improved
rejection of input voltage and output load transients and
cycle-by-cycle current limiting. The device also has a shut-
down pin, so that it can be turned off externally. An output
voltage tolerance of

4%, within specified input voltages and

output load conditions, is guaranteed for the power supply
system.

Features

Requires few external components

Family of standard inductors and transformers

NPN output switches 5.0A, can stand off 65V

Wide input voltage range: 4V to 40V

Adjustable switching frequency: 100 kHz to 200 kHz

External shutdown capability

Draws less than 60 µA when shut down

Frequency synchronization

Current-mode operation for improved transient
response, line regulation, and current limit

Internal soft-start function reduces in-rush current during
start-up

Output transistor protected by current limit, under
voltage lockout, and thermal shutdown

System output voltage tolerance of

4% max over line

and load conditions

Typical Applications

Flyback regulator

Forward converter

Multiple-output regulator

Simple boost regulator

Flyback Regulator

SIMPLE SWITCHER

and

Switchers Made Simple

are registered trademarks of National Semiconductor Corporation.

DS012420-1

April 1998

LM2588

SIMPLE

SWITCHER

Flyback

Regulator

with

Shutdown

DS012420

www.national.com

Absolute Maximum Ratings

(Note 1)

If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.

Input Voltage

-0.4V

45V

Switch Voltage

-0.4V

65V

Switch Current (Note 2)

Internally Limited

Compensation Pin Voltage

-0.4V

COMP

2.4V

Feedback Pin Voltage

-0.4V

2 V

OUT

ON /OFF Pin Voltage

-0.4V

Sync Pin Voltage

-0.4V

SYNC

Power Dissipation (Note 3)

Internally Limited

Storage Temperature Range

-65°C to +150°C

Lead Temperature

(Soldering, 10 sec.)

260°C

Maximum Junction Temperature

(Note 3)

150°C

Minimum ESD Rating

(C = 100 pF, R = 1.5 k

)

2 kV

Operating Ratings

Supply Voltage

40V

Output Switch Voltage

60V

Output Switch Current

5.0A

Junction Temperature Range

-40°C

+125°C

LM2588-3.3
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, V

= 5V.

Symbol

Parameters

Conditions

Typical

Min

Max

Units

SYSTEM PARAMETERS Test Circuit of

Figure 1 (Note 4)

OUT

Output Voltage

= 4V to 12V

3.3

3.17/3.14

3.43/3.46

LOAD

= 400 mA to 1.75A

OUT

Line Regulation

= 4V to 12V

50/100

LOAD

= 400 mA

OUT

Load Regulation

= 12V

50/100

LOAD

= 400 mA to 1.75A

Efficiency

= 12V, I

LOAD

= 1A

UNIQUE DEVICE PARAMETERS (Note 5)

REF

Output Reference

Measured at Feedback Pin

3.3

3.242/3.234

3.358/3.366

Voltage

COMP

= 1.0V

REF

Reference Voltage

= 4V to 40V

2.0

Line Regulation

Error Amp

COMP

= -30 µA to +30 µA

1.193

0.678

2.259

mmho

Transconductance

COMP

= 1.0V

VOL

Error Amp

COMP

= 0.5V to 1.6V

260

151/75

V/V

Voltage Gain

COMP

= 1.0 M

(Note 6)

LM2588-5.0
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, V

= 5V.

Symbol

Parameters

Conditions

Typical

Min

Max

Units

SYSTEM PARAMETERS Test Circuit of

Figure 1 (Note 4)

OUT

Output Voltage

= 4V to 12V

5.0

4.80/4.75

5.20/5.25

LOAD

= 500 mA to 1.45A

OUT

Line Regulation

= 4V to 12V

50/100

LOAD

= 500 mA

OUT

Load Regulation

= 12V

50/100

LOAD

= 500 mA to 1.45A

Efficiency

= 12V, I

LOAD

= 750 mA

UNIQUE DEVICE PARAMETERS (Note 5)

REF

Output Reference

Measured at Feedback Pin

5.0

4.913/4.900

5.088/5.100

Voltage

COMP

= 1.0V

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LM2588-5.0
Electrical Characteristics

(Continued)

Specifications with standard type face are for T

= 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, V

= 5V.

Symbol

Parameters

Conditions

Typical

Min

Max

Units

UNIQUE DEVICE PARAMETERS (Note 5)

REF

Reference Voltage

= 4V to 40V

3.3

Line Regulation

Error Amp

COMP

= -30 µA to +30 µA

0.750

0.447

1.491

mmho

Transconductance

COMP

= 1.0V

VOL

Error Amp

COMP

= 0.5V to 1.6V

165

99/49

V/V

Voltage Gain

COMP

= 1.0 M

(Note 6)

LM2588-12
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, V

= 5V.

Symbol

Parameters

Conditions

Typical

Min

Max

Units

SYSTEM PARAMETERS Test Circuit of

Figure 2 (Note 4)

OUT

Output Voltage

= 4V to 10V

12.0

11.52/11.40

12.48/12.60

LOAD

= 300 mA to 1.2A

OUT

Line Regulation

= 4V to 10V

100/200

LOAD

= 300 mA

OUT

Load Regulation

= 10V

100/200

LOAD

= 300 mA to 1.2A

Efficiency

= 10V, I

LOAD

= 1A

UNIQUE DEVICE PARAMETERS (Note 5)

REF

Output Reference

Measured at Feedback Pin

12.0

11.79/11.76

12.21/12.24

Voltage

COMP

= 1.0V

REF

Reference Voltage

= 4V to 40V

7.8

Line Regulation

Error Amp

COMP

= -30 µA to +30 µA

0.328

0.186

0.621

mmho

Transconductance

COMP

= 1.0V

VOL

Error Amp

COMP

= 0.5V to 1.6V

41/21

V/V

Voltage Gain

COMP

= 1.0 M

(Note 6)

LM2588-ADJ
Electrical Characteristics

Specifications with standard type face are for T

= 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, V

= 5V.

Symbol

Parameters

Conditions

Typical

Min

Max

Units

SYSTEM PARAMETERS Test Circuit of

Figure 2 (Note 4)

OUT

Output Voltage

= 4V to 10V

12.0

11.52/11.40

12.48/12.60

LOAD

= 300 mA to 1.2A

OUT

Line Regulation

= 4V to 10V

100/200

LOAD

= 300 mA

OUT

Load Regulation

= 10V

100/200

LOAD

= 300 mA to 1.2A

Efficiency

= 10V, I

LOAD

= 1A

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LM2588-ADJ
Electrical Characteristics

(Continued)

Specifications with standard type face are for T

= 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, V

= 5V.

Symbol

Parameters

Conditions

Typical

Min

Max

Units

UNIQUE DEVICE PARAMETERS (Note 5)

REF

Output Reference

Measured at Feedback Pin

1.230

1.208/1.205

1.252/1.255

Voltage

COMP

= 1.0V

REF

Reference Voltage

= 4V to 40V

1.5

Line Regulation

Error Amp

COMP

= -30 µA to +30 µA

3.200

1.800

6.000

mmho

Transconductance

COMP

= 1.0V

VOL

Error Amp

COMP

= 0.5V to 1.6V

670

400/200

V/V

Voltage Gain

COMP

= 1.0 M

(Note 6)

Error Amp

COMP

= 1.0V

125

425/600

Input Bias Current

All Output Voltage Versions
Electrical Characteristics

(Note 5)

Specifications with standard type face are for T

= 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, V

= 5V.

Symbol

Parameters

Conditions

Typical

Min

Max

Units

Input Supply Current

Switch Off

15.5/16.5

(Note 8)

SWITCH

= 3.0A

140/165

S/D

Shutdown Input

= 3V

100/300

µA

Supply Current

Input Supply

LOAD

= 100

3.30

3.05

3.75

Undervoltage
Lockout

Oscillator Frequency

Measured at Switch Pin

LOAD

= 100

, V

COMP

= 1.0V

100

85/75

115/125

kHz

Freq. Adj. Pin Open (Pin 1)

SET

= 22 k

200

kHz

Short-Circuit

Measured at Switch Pin

Frequency

LOAD

= 100

kHz

FEEDBACK

= 1.15V

EAO

Error Amplifier

Upper Limit

2.8

2.6/2.4

Output Swing

(Note 7)

Lower Limit

0.25

0.40/0.55

(Note 8)

EAO

Error Amp

(Note 9)

Output Current

165

110/70

260/320

µA

(Source or Sink)

Soft Start Current

FEEDBACK

= 0.92V

11.0

8.0/7.0

17.0/19.0

µA

COMP

= 1.0V

MAX

Maximum Duty
Cycle

LOAD

= 100

93/90

(Note 7)

Switch Leakage

Switch Off

300/600

µA

Current

SWITCH

= 60V

SUS

Switch Sustaining
Voltage

dV/dT = 1.5V/ns

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All Output Voltage Versions
Electrical Characteristics

(Note 5) (Continued)

Specifications with standard type face are for T

= 25°C, and those in bold type face apply over full Operating Temperature

Range. Unless otherwise specified, V

= 5V.

Symbol

Parameters

Conditions

Typical

Min

Max

Units

SAT

Switch Saturation
Voltage

SWITCH

= 5.0A

0.7

1.1/1.4

NPN Switch Current
Limit

6.5

5.0

9.5

STH

Synchronization

SYNC

= 200 kHz

0.75

0.625/0.40

0.875/1.00

Threshold Voltage

COMP

= 1V, V

= 5V

SYNC

Synchronization

= 5V

100

200

µA

Pin Current

COMP

= 1V, V

SYNC

= V

STH

SHTH

ON /OFF Pin (Pin 1)

COMP

= 1V

1.6

1.0/0.8

2.2/2.4

Threshold Voltage

(Note 10)

ON /OFF Pin (Pin 1)

COMP

= 1V

15/10

65/75

µA

Current

= V

SHTH

Thermal Resistance

T Package, Junction to Ambient

(Note 11)

T Package, Junction to Ambient

(Note 12)

T Package, Junction to Case

S Package, Junction to Ambient

°C/W

(Note 13)

S Package, Junction to Ambient

(Note 14)

S Package, Junction to Ambient

(Note 15)

S Package, Junction to Case

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. These ratings apply when the current is limited to less than 1.2 mA
for pins 1, 2, 3, and 6. Operating ratings indicate conditions for which the device is intended to be functional, but device parameter specifications may not be guar-
anteed under these conditions. For guaranteed specifications and test conditions, see the Electrical Characteristics.

Note 2: Note that switch current and output current are not identical in a step-up regulator. Output current cannot be internally limited when the LM2588 is used as
a step-up regulator. To prevent damage to the switch, the output current must be externally limited to 5A. However, output current is internally limited when the
LM2588 is used as a flyback regulator (see the Application Hints section for more information).

Note 3: The junction temperature of the device (T

) is a function of the ambient temperature (T

), the junction-to-ambient thermal resistance (

), and the power

dissipation of the device (P

). A thermal shutdown will occur if the temperature exceeds the maximum junction temperature of the device: P

+ T

A(MAX)

J-

(MAX)

. For a safe thermal design, check that the maximum power dissipated by the device is less than: P

J(MAX)

- T

A(MAX)

. When calculating the maximum

allowable power dissipation, derate the maximum junction temperature -- this ensures a margin of safety in the thermal design.

Note 4: External components such as the diode, inductor, input and output capacitors can affect switching regulator performance. When the LM2588 is used as
shown in

Figure 1 and Figure 2, system performance will be as specified by the system parameters.

Note 5: All room temperature limits are 100% production tested, and all limits at temperature extremes are guaranteed via correlation using standard Statistical Qual-
ity Control (SQC) methods.

Note 6: A 1.0 M

resistor is connected to the compensation pin (which is the error amplifier output) to ensure accuracy in measuring A

VOL

Note 7: To measure this parameter, the feedback voltage is set to a low value, depending on the output version of the device, to force the error amplifier output high
and the switch on.

Note 8: To measure this parameter, the feedback voltage is set to a high value, depending on the output version of the device, to force the error amplifier output low
and the switch off.

Note 9: To measure the worst-case error amplifier output current, the LM2588 is tested with the feedback voltage set to its low value (specified in (Note 7)) and at
its high value (specified in (Note 8)).

Note 10: When testing the minimum value, do not sink current from this pin -- isolate it with a diode. If current is drawn from this pin, the frequency adjust circuit will
begin operation (see

Figure 41).

Note 11: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with

inch leads in a socket, or on a PC

board with minimum copper area.

Note 12: Junction to ambient thermal resistance (no external heat sink) for the 7 lead TO-220 package mounted vertically, with

inch leads soldered to a PC board

containing approximately 4 square inches of (1 oz.) copper area surrounding the leads.

Note 13: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.136 square inches (the same size as the
TO-263 package) of 1 oz. (0.0014 in. thick) copper.

Note 14: Junction to ambient thermal resistance for the 7 lead TO-263 mounted horizontally against a PC board area of 0.4896 square inches (3.6 times the area
of the TO-263 package) of 1 oz. (0.0014 in. thick) copper.

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Block Diagram

Flyback Regulator Operation

The LM2588 is ideally suited for use in the flyback regulator
topology. The flyback regulator can produce a single output
voltage, such as the one shown in

Figure 4, or multiple out-

put voltages. In

Figure 4, the flyback regulator generates an

output voltage that is inside the range of the input voltage.
This feature is unique to flyback regulators and cannot be
duplicated with buck or boost regulators.

The operation of a flyback regulator is as follows (refer to
Figure 4): when the switch is on, current flows through the
primary winding of the transformer, T1, storing energy in the
magnetic field of the transformer. Note that the primary and
secondary windings are out of phase, so no current flows
through the secondary when current flows through the pri-
mary. When the switch turns off, the magnetic field col-

lapses, reversing the voltage polarity of the primary and sec-
ondary windings. Now rectifier D1 is forward biased and
current flows through it, releasing the energy stored in the
transformer. This produces voltage at the output.

The output voltage is controlled by modulating the peak
switch current. This is done by feeding back a portion of the
output voltage to the error amp, which amplifies the differ-
ence between the feedback voltage and a 1.230V reference.
The error amp output voltage is compared to a ramp voltage
proportional to the switch current (i.e., inductor current dur-
ing the switch on time). The comparator terminates the
switch on time when the two voltages are equal, thereby
controlling the peak switch current to maintain a constant
output voltage.

DS012420-23

For Fixed Versions
3.3V, R1 = 3.4k, R2 = 2k
5.0V, R1 = 6.15k, R2 = 2k
12V, R1 = 8.73k, R2 = 1k
For Adj. Version
R1 = Short (0

), R2 = Open

FIGURE 3.

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Typical Flyback Regulator Applications

(Continued)

Transformer Selection (T)

Figure 13 lists the standard transformers available for flyback regulator applications. Included in the table are the turns ratio(s) for
each transformer, as well as the output voltages, input voltage ranges, and the maximum load currents for each circuit.

Transformer Footprints

Figure 15 through 32 show the footprints of each transformer, listed in Figure 14.

Applications

Figure 7

Figure 8

Figure 9

Figure 10

Figure 11

Figure 12

Transformers

4V6V

8V16V

4V6V

18V36V

OUT1

3.3V

12V

OUT1

(Max)

1.8A

1.4A

1.2A

0.3A

2.5A

2.5

0.8

0.35

OUT2

-12V

12V

OUT2

(Max)

0.3A

0.5A

2.5

0.8

OUT3

-12V

OUT3

(Max)

0.5A

0.8

FIGURE 13. Transformer Selection Table

Transformer

Type

Manufacturers' Part Numbers

Coilcraft

Coilcraft (Note 16)

Pulse (Note 17)

Renco

Schott

(Note 16)

Surface Mount

(Note 18)

(Note 19)

Q4434-B

Q4435-B

PE-68411

RL-5530

67141450

Q4337-B

Q4436-B

PE-68412

RL-5531

67140860

Q4343-B

PE-68421

RL-5534

67140920

Q4344-B

PE-68422

RL-5535

67140930

Note 16: Coilcraft Inc.,:

Phone: (800) 322-2645

1102 Silver Lake Road, Cary, IL 60013:

Fax: (708) 639-1469

European Headquarters, 21 Napier Place:

Phone: +44 1236 730 595

Wardpark North, Cumbernauld, Scotland G68 0LL:

Fax: +44 1236 730 627

Note 17: Pulse Engineering Inc.,:

Phone: (619) 674-8100

12220 World Trade Drive, San Diego, CA 92128:

Fax: (619) 674-8262

European Headquarters, Dunmore Road:

Phone: +353 93 24 107

Tuam, Co. Galway, Ireland:

Fax: +353 93 24 459

Note 18: Renco Electronics Inc.,:

Phone: (800) 645-5828

60 Jeffryn Blvd. East, Deer Park, NY 11729:

Fax: (516) 586-5562

Note 19: Schott Corp.,:

Phone: (612) 475-1173

1000 Parkers Lane Road, Wayzata, MN 55391:

Fax: (612) 475-1786

FIGURE 14. Transformer Manufacturer Guide

DS012420-31

Top View

FIGURE 15. Coilcraft Q4434-B

DS012420-32

Top View

FIGURE 16. Coilcraft Q4337-B

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Step-Up (Boost) Regulator Operation

Figure 33 shows the LM2588 used as a step-up (boost)
regulator. This is a switching regulator that produces an out-
put voltage greater than the input supply voltage.

A brief explanation of how the LM2588 Boost Regulator
works is as follows (refer to

Figure 33). When the NPN

switch turns on, the inductor current ramps up at the rate of
V

/L, storing energy in the inductor. When the switch turns

off, the lower end of the inductor flies above V

, discharging

its current through diode (D) into the output capacitor (C

OUT

)

at a rate of (V

OUT

- V

)/L. Thus, energy stored in the induc-

tor during the switch on time is transferred to the output dur-
ing the switch off time. The output voltage is controlled by
adjusting the peak switch current, as described in the flyback
regulator section.

By adding a small number of external components (as shown in

Figure 33), the LM2588 can be used to produce a regulated out-

put voltage that is greater than the applied input voltage. The switching waveforms observed during the operation of this circuit
are shown in

Figure 34. Typical performance of this regulator is shown in Figure 35.

Typical Performance Characteristics

DS012420-49

FIGURE 33. 12V Boost Regulator

DS012420-62

A: Switch Voltage,10V/div
B: Switch Current, 5A/div
C: Inductor Current, 5A/div
D: Output Ripple Voltage,
100 mV/div, AC-Coupled

FIGURE 34. Switching Waveforms

DS012420-63

FIGURE 35. V

OUT

Response to Load Current Step

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Typical Boost Regulator Applications

Figure 36 and 38 through 40 show four typical boost
applications -- one fixed and three using the adjustable ver-
sion of the LM2588. Each drawing contains the part num-
ber(s) and manufacturer(s) for every component. For the

fixed 12V output application, the part numbers and manufac-
turers' names for the inductor are listed in a table in

Figure

37. For applications with different output voltages, refer to
the

Switchers Made Simplesoftware.

Figure 37 contains a table of standard inductors, by part number and corresponding manufacturer, for the fixed output regulator
of

Figure 36.

DS012420-50

FIGURE 36. +5V to +12V Boost Regulator

Coilcraft (Note 20)

Pulse (Note 21)

Renco (Note 22)

Schott (Note 23)

R4793-A

PE-53900

RL-5472-5

67146520

Note 20: Coilcraft Inc.,:

Phone: (800) 322-2645

1102 Silver Lake Road, Cary, IL 60013:

Fax: (708) 639-1469

European Headquarters, 21 Napier Place:

Phone: +44 1236 730 595

Wardpark North, Cumbernauld, Scotland G68 0LL:

Fax: +44 1236 730 627

Note 21: Pulse Engineering Inc.,:

Phone: (619) 674-8100

12220 World Trade Drive, San Diego, CA 92128:

Fax: (619) 674-8262

European Headquarters, Dunmore Road:

Phone: +353 93 24 107

Tuam, Co. Galway, Ireland:

Fax: +353 93 24 459

Note 22: Renco Electronics Inc.,:

Phone: (800) 645-5828

60 Jeffryn Blvd. East, Deer Park, NY 11729:

Fax: (516) 586-5562

Note 23: Schott Corp.,:

Phone: (612) 475-1173

1000 Parkers Lane Road, Wayzata, MN 55391:

Fax: (612) 475-1786

FIGURE 37. Inductor Selection Table

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Application Hints

LM2588 SPECIAL FEATURES

SHUTDOWN CONTROL

A feature of the LM2588 is its ability to be shut down using
the ON /OFF pin (pin 1). This feature conserves input power
by turning off the device when it is not in use. For proper op-
eration, an isolation diode is required (as shown in

Figure

41).

The device will shut down when 3V or greater is applied on
the ON /OFF pin, sourcing current into pin 1. In shut down
mode, the device will draw typically 56 µA of supply current
(16 µA to V

and 40 µA to the ON /OFF pin). To turn the de-

vice back on, leave pin 1 floating, using an (isolation) diode,
as shown in

Figure 41 (for normal operation, do not source

or sink current to or from this pin -- see the next section).

FREQUENCY ADJUSTMENT

The switching frequency of the LM2588 can be adjusted with
the use of an external resistor. This feature allows the user to
optimize the size of the magnetics and the output capaci-
tor(s) by tailoring the operating frequency. A resistor con-
nected from pin 1 (the Freq. Adj. pin) to ground will set the
switching frequency from 100 kHz to 200 kHz (maximum).
As shown in

Figure 41, the pin can be used to adjust the fre-

quency while still providing the shut down function. A curve in
the Performance Characteristics Section graphs the resistor
value to the corresponding switching frequency. The table in
Figure 42 shows resistor values corresponding to commonly
used frequencies.

However, changing the LM2588's operating frequency from
its nominal value of 100 kHz will change the magnetics se-
lection and compensation component values.

FREQUENCY SYNCHRONIZATION

Another feature of the LM2588 is the ability to synchronize
the switching frequency to an external source, using the
sync pin (pin 6). This feature allows the user to parallel mul-
tiple devices to deliver more output power.

A negative falling pulse applied to the sync pin will synchro-
nize the LM2588 to an external oscillator (see

Figure 43 and

44).

Use of this feature enables the LM2588 to be synchronized
to an external oscillator, such as a system clock. This opera-
tion allows multiple power supplies to operate at the same
frequency,

thus

eliminating

frequency-related

noise

problems.

The scope photo in

Figure 44 shows a LM2588 12V Boost

Regulator synchronized to a 200 kHz signal. There is a 700
ns delay between the falling edge of the sync signal and the
turning on of the switch.

PROGRAMMING OUTPUT VOLTAGE
(SELECTING R1 AND R2)

Referring to the adjustable regulator in

Figure 45, the output

voltage is programmed by the resistors R1 and R2 by the fol-
lowing formula:

OUT

= V

REF

(1 + R1/R2)

where V

REF

= 1.23V

Resistors R1 and R2 divide the output voltage down so that
it can be compared with the 1.23V internal reference. With
R2 between 1k and 5k, R1 is:

R1 = R2 (V

OUT

REF

- 1)

where V

REF

= 1.23V

For best temperature coefficient and stability with time, use
1% metal film resistors.

SHORT CIRCUIT CONDITION

Due to the inherent nature of boost regulators, when the out-
put is shorted (see

Figure 45 ), current flows directly from the

input, through the inductor and the diode, to the output, by-
passing the switch. The current limit of the switch

does not

limit the output current for the entire circuit. To protect the

DS012420-54

FIGURE 41. Shutdown Operation

SET

)

Frequency (kHz)

Open

100

200

125

150

175

200

FIGURE 42. Frequency Setting Resistor Guide

DS012420-55

FIGURE 43. Frequency Synchronization

DS012420-64

FIGURE 44. Waveforms of a Synchronized

12V Boost Regulator

www.national.com

Application Hints

(Continued)

load and prevent damage to the switch, the current must be
externally limited, either by the input supply or at the output
with an external current limit circuit. The external limit should
be set to the maximum switch current of the device, which is
5A.

In a flyback regulator application (

Figure 46 ), using the stan-

dard transformers, the LM2588 will survive a short circuit to

the main output. When the output voltage drops to 80% of its
nominal value, the frequency will drop to 25 kHz. With a
lower frequency, off times are larger. With the longer off
times, the transformer can release all of its stored energy be-
fore the switch turns back on. Hence, the switch turns on ini-
tially with zero current at its collector. In this condition, the
switch current limit will limit the peak current, saving the
device.

FLYBACK REGULATOR INPUT CAPACITORS

A flyback regulator draws discontinuous pulses of current
from the input supply. Therefore, there are two input capaci-
tors needed in a flyback regulator -- one for energy storage
and one for filtering (see

Figure 46). Both are required due to

the inherent operation of a flyback regulator. To keep a
stable or constant voltage supply to the LM2588, a storage
capacitor (

100 µF) is required. If the input source is a reciti-

fied DC supply and/or the application has a wide tempera-
ture range, the required rms current rating of the capacitor
might be very large. This means a larger value of capaci-
tance or a higher voltage rating will be needed for the input

capacitor. The storage capacitor will also attenuate noise
which may interfere with other circuits connected to the
same input supply voltage.

In addition, a small bypass capacitor is required due to the
noise generated by the input current pulses. To eliminate the
noise, insert a 1.0 µF ceramic capacitor between V

and

ground as close as possible to the device.

DS012420-56

FIGURE 45. Boost Regulator

DS012420-57

FIGURE 46. Flyback Regulator

www.national.com

Application Hints

(Continued)

SWITCH VOLTAGE LIMITS

In a flyback regulator, the maximum steady-state voltage ap-
pearing at the switch, when it is off, is set by the transformer
turns ratio, N, the output voltage, V

OUT

, and the maximum in-

put voltage, V

(Max):

SW(OFF)

= V

(Max) + (V

OUT

)/N

where V

is the forward biased voltage of the output diode,

and is typically 0.5V for Schottky diodes and 0.8V for
ultra-fast recovery diodes. In certain circuits, there exists a
voltage spike, V

, superimposed on top of the steady-state

voltage (see

Figure 5, waveform A). Usually, this voltage

spike is caused by the transformer leakage inductance
and/or the output rectifier recovery time. To "clamp" the volt-
age at the switch from exceeding its maximum value, a tran-
sient suppressor in series with a diode is inserted across the
transformer primary (as shown in the circuit in

Figure 4 and

other flyback regulator circuits throughout the datasheet).
The schematic in

Figure 46 shows another method of clamp-

ing the switch voltage. A single voltage transient suppressor
(the SA51A) is inserted at the switch pin. This method
clamps the total voltage across the switch, not just the volt-
age across the primary.

If poor circuit layout techniques are used (see the "Circuit
Layout Guideline" section), negative voltage transients may
appear on the Switch pin (pin 5). Applying a negative voltage
(with respect to the IC's ground) to any monolithic IC pin
causes erratic and unpredictable operation of that IC. This
holds true for the LM2588 IC as well. When used in a flyback
regulator, the voltage at the Switch pin (pin 5) can go nega-
tive when the switch turns on. The "ringing" voltage at the
switch pin is caused by the output diode capacitance and the
transformer leakage inductance forming a resonant circuit at
the secondary(ies). The resonant circuit generates the "ring-
ing" voltage, which gets reflected back through the trans-
former to the switch pin. There are two common methods to
avoid this problem. One is to add an RC snubber around the
output rectifier(s), as in

Figure 46. The values of the resistor

and the capacitor must be chosen so that the voltage at the
Switch pin does not drop below -0.4V. The resistor may
range in value between 10

and 1 k

, and the capacitor will

vary from 0.001 µF to 0.1 µF. Adding a snubber will (slightly)
reduce the efficiency of the overall circuit.

The other method to reduce or eliminate the "ringing" is to in-
sert a Schottky diode clamp between pins 5 and 4 (ground),
also shown in

Figure 46. This prevents the voltage at pin 5

from dropping below -0.4V. The reverse voltage rating of the
diode must be greater than the switch off voltage.

OUTPUT VOLTAGE LIMITATIONS

The maximum output voltage of a boost regulator is the
maximum switch voltage minus a diode drop. In a flyback
regulator, the maximum output voltage is determined by the
turns ratio, N, and the duty cycle, D, by the equation:

OUT

N x V

x D/(1 - D)

The duty cycle of a flyback regulator is determined by the fol-
lowing equation:

Theoretically, the maximum output voltage can be as large
as desired -- just keep increasing the turns ratio of the trans-
former. However, there exists some physical limitations that
prevent the turns ratio, and thus the output voltage, from in-
creasing to infinity. The physical limitations are capacitances
and inductances in the LM2588 switch, the output diode(s),
and the transformer -- such as reverse recovery time of the
output diode (mentioned above).

NOISY INPUT LINE CONDITION

A small, low-pass RC filter should be used at the input pin of
the LM2588 if the input voltage has an unusually large
amount of transient noise, such as with an input switch that
bounces. The circuit in

Figure 47 demonstrates the layout of

the filter, with the capacitor placed from the input pin to
ground and the resistor placed between the input supply and
the input pin. Note that the values of R

and C

shown in

the schematic are good enough for most applications, but
some readjusting might be required for a particular applica-
tion. If efficiency is a major concern, replace the resistor with
a small inductor (say 10 µH and rated at 200 mA).

STABILITY

All current-mode controlled regulators can suffer from an in-
stability, known as subharmonic oscillation, if they operate
with a duty cycle above 50%. To eliminate subharmonic os-
cillations, a minimum value of inductance is required to en-
sure stability for all boost and flyback regulators. The mini-
mum inductance is given by:

where V

SAT

is the switch saturation voltage and can be

found in the Characteristic Curves.

DS012420-58

FIGURE 47. Input Line Filter

www.national.com

Application Hints

(Continued)

CIRCUIT LAYOUT GUIDELINES

As in any switching regulator, layout is very important. Rap-
idly switching currents associated with wiring inductance
generate voltage transients which can cause problems. For
minimal inductance and ground loops, keep the length of the
leads and traces as short as possible. Use single point
grounding or ground plane construction for best results.
Separate the signal grounds from the power grounds (as in-
dicated in

Figure 48). When using the Adjustable version,

physically locate the programming resistors as near the
regulator IC as possible, to keep the sensitive feedback wir-
ing short.

HEAT SINK/THERMAL CONSIDERATIONS

In many cases, a heat sink is not required to keep the
LM2588 junction temperature within the allowed operating
temperature range. For each application, to determine
whether or not a heat sink will be required, the following must
be identified:

1) Maximum ambient temperature (in the application).

2) Maximum regulator power dissipation (in the application).

3) Maximum allowed junction temperature (125°C for the
LM2588). For a safe, conservative design, a temperature ap-
proximately 15°C cooler than the maximum junction tem-
perature should be selected (110°C).

4) LM2588 package thermal resistances

and

(given

in the Electrical Characteristics).

Total power dissipated (P

) by the LM2588 can be estimated

as follows:

Boost:

is the minimum input voltage, V

OUT

is the output voltage,

N is the transformer turns ratio, D is the duty cycle, and I

LOAD

is the maximum load current (and

LOAD

is the sum of the

maximum load currents for multiple-output flyback regula-
tors). The duty cycle is given by:

Boost:

where V

is the forward biased voltage of the diode and is

typically 0.5V for Schottky diodes and 0.8V for fast recovery
diodes. V

SAT

is the switch saturation voltage and can be

found in the Characteristic Curves.

When no heat sink is used, the junction temperature rise is:

= P

Adding the junction temperature rise to the maximum ambi-
ent temperature gives the actual operating junction tempera-
ture:

+ T

If the operating junction temperature exceeds the maximum
junction temperatue in item 3 above, then a heat sink is re-
quired. When using a heat sink, the junction temperature rise
can be determined by the following:

= P

· (

Interface

Heat Sink

)

Again, the operating junction temperature will be:

+ T

As before, if the maximum junction temperature is exceeded,
a larger heat sink is required (one that has a lower thermal
resistance).

Included in the

Switchers Made Simple design software is a

more precise (non-linear) thermal model that can be used to
determine junction temperature with different input-output
parameters or different component values. It can also calcu-
late the heat sink thermal resistance required to maintain the
regulator junction temperature below the maximum operat-
ing temperature.

DS012420-59

FIGURE 48. Circuit Board Layout

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Physical Dimensions

inches (millimeters) unless otherwise noted (Continued)

LIFE SUPPORT POLICY

NATIONAL'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT
DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT AND GENERAL
COUNSEL OF NATIONAL SEMICONDUCTOR CORPORATION. As used herein:

1. Life support devices or systems are devices or

systems which, (a) are intended for surgical implant
into the body, or (b) support or sustain life, and
whose failure to perform when properly used in
accordance with instructions for use provided in the
labeling, can be reasonably expected to result in a
significant injury to the user.

2. A critical component is any component of a life

support device or system whose failure to perform
can be reasonably expected to cause the failure of
the life support device or system, or to affect its
safety or effectiveness.

National Semiconductor
Corporation
Americas
Tel: 1-800-272-9959
Fax: 1-800-737-7018
Email: support@nsc.com

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Fax: +49 (0) 1 80-530 85 86

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Tel: 81-3-5639-7560
Fax: 81-3-5639-7507

www.national.com

Order Number LM2588S-3.3, LM2588S-5.0,

LM2588S-12 or LM2588S-ADJ

Tape and Reel Order Number LM2588SX-3.3,

LM2588SX-5.0, LM2588SX-12 or LM2588SX-ADJ

NS Package Number TS7B

LM2588

SIMPLE

SWITCHER

Flyback

Regulator

with

Shutdown

National does not assume any responsibility for use of any circuitry described, no circuit patent licenses are implied and National reserves the right at any time without notice to change said circuitry and specifications.

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