LM1577/LM2577 Series
SIMPLE SWITCHER

Step-Up Voltage Regulator

General Description

The LM1577/LM2577 are monolithic integrated circuits that
provide all of the power and control functions for step-up
(boost), flyback, and forward converter switching regulators.
The device is available in three different output voltage ver-
sions: 12V, 15V, and adjustable.

Requiring a minimum number of external components, these
regulators are cost effective, and simple to use. Listed in this
data sheet are a family of standard inductors and flyback
transformers designed to work with these switching regula-
tors.

Included on the chip is a 3.0A NPN switch and its associated
protection circuitry, consisting of current and thermal limiting,
and undervoltage lockout. Other features include a 52 kHz
fixed-frequency oscillator that requires no external compo-
nents, a soft start mode to reduce in-rush current during
start-up, and current mode control for improved rejection of
input voltage and output load transients.

Features

Requires few external components

NPN output switches 3.0A, can stand off 65V

Wide input voltage range: 3.5V to 40V

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

52 kHz internal oscillator

Soft-start function reduces in-rush current during start-up

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

Typical Applications

Simple boost regulator

Flyback and forward regulators

Multiple-output regulator

Typical Application

Ordering Information

Temperature

Range

Package

Type

Output Voltage

NSC

12V

15V

ADJ

Package Package

Drawing

-40°C

+125°C

24-Pin Surface Mount

LM2577M-12

LM2577M-15

LM2577M-ADJ

M24B

16-Pin Molded DIP

LM2577N-12

LM2577N-15

LM2577N-ADJ

N16A

5-Lead Surface Mount

LM2577S-12

LM2577S-15

LM2577S-ADJ

TS5B

TO-263

5-Straight Leads

LM2577T-12

LM2577T-15

LM2577T-ADJ

T05A

TO-220

5-Bent Staggered

LM2577T-12

LM2577T-15

LM2577T-ADJ

T05D

TO-220

Leads

Flow LB03

-55°C

+150°C

4-Pin TO-3

LM1577K-12/883 LM1577K-15/883

LM1577K-

ADJ/883

K04A

TO-3

SIMPLE SWITCHER

is a registered trademark of National Semiconductor Corporation.

DS011468-1

Note: Pin numbers shown are for TO-220 (T) package.

June 1999

LM1577/LM2577

Series

SIMPLE

SWITCHER

Step-Up

oltage

Regulator

DS011468

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Absolute Maximum Ratings

(Note 1)

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

Supply Voltage

45V

Output Switch Voltage

65V

Output Switch Current (Note 2)

6.0A

Power Dissipation

Internally Limited

Storage Temperature Range

-65°C to +150°C

Lead Temperature

(Soldering, 10 sec.)

260°C

Maximum Junction Temperature

150°C

Minimum ESD Rating

(C = 100 pF, R = 1.5 k

)

2 kV

Operating Ratings

Supply Voltage

3.5V

40V

Output Switch Voltage

SWITCH

60V

Output Switch Current

SWITCH

3.0A

Junction Temperature Range

LM1577

-55°C

+150°C

LM2577

-40°C

+125°C

Electrical Characteristics -- LM1577-12, LM2577-12

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, and I

SWITCH

= 0.

LM1577-12

LM2577-12

Units

Symbol

Parameter

Conditions

Typical

Limit

(Limits)

(Notes 3, 4)

(Note 5)

SYSTEM PARAMETERS Circuit of

Figure 1 (Note 6)

OUT

Output Voltage

= 5V to 10V

12.0

LOAD

= 100 mA to 800 mA

11.60/11.40

V(min)

(Note 3)

12.40/12.60

V(max)

Line Regulation

= 3.5V to 10V

LOAD

= 300 mA

50/100

mV(max)

Load Regulation

= 5V

LOAD

= 100 mA to 800 mA

50/100

mV(max)

Efficiency

= 5V, I

LOAD

= 800 mA

DEVICE PARAMETERS

Input Supply Current

FEEDBACK

= 14V (Switch Off)

7.5

10.0/14.0

mA(max)

SWITCH

= 2.0A

COMP

= 2.0V (Max Duty Cycle)

50/85

mA(max)

Input Supply

SWITCH

= 100 mA

2.90

Undervoltage Lockout

2.70/2.65

V(min)

3.10/3.15

V(max)

Oscillator Frequency

Measured at Switch Pin

kHz

SWITCH

= 100 mA

48/42

kHz(min)

56/62

kHz(max)

REF

Output Reference

Measured at Feedback Pin

Voltage

= 3.5V to 40V

11.76/11.64

V(min)

COMP

= 1.0V

12.24/12.36

V(max)

Output Reference

= 3.5V to 40V

Voltage Line Regulator

Feedback Pin Input

9.7

Resistance

Error Amp

COMP

= -30 µA to +30 µA

370

µmho

Transconductance

COMP

= 1.0V

225/145

µmho(min)

515/615

µmho(max)

VOL

Error Amp

COMP

= 1.1V to 1.9V

V/V

Voltage Gain

COMP

= 1.0 M

50/25

V/V(min)

(Note 7)

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Electrical Characteristics -- LM1577-12, LM2577-12

(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, and I

SWITCH

= 0.

LM1577-12

LM2577-12

Units

Symbol

Parameter

Conditions

Typical

Limit

(Limits)

(Notes 3, 4)

(Note 5)

DEVICE PARAMETERS

Error Amplifier

Upper Limit

2.4

Output Swing

FEEDBACK

= 10.0V

2.2/2.0

V(min)

Lower Limit

0.3

FEEDBACK

= 15.0V

0.40/0.55

V(max)

Error Amplifier

FEEDBACK

= 10.0V to 15.0V

200

µA

Output Current

COMP

= 1.0V

130/

µA(min)

300/

400

300/

400

µA(max)

Soft Start Current

FEEDBACK

= 10.0V

5.0

µA

COMP

= 0V

2.5/1.5

µA(min)

7.5/9.5

µA(max)

Maximum Duty Cycle

COMP

= 1.5V

SWITCH

= 100 mA

93/90

%(min)

Switch
Transconductance

12.5

A/V

Switch Leakage

SWITCH

= 65V

µA

Current

FEEDBACK

= 15V (Switch Off)

300/600

µA(max)

SAT

Switch Saturation

SWITCH

= 2.0A

0.5

Voltage

COMP

= 2.0V (Max Duty Cycle)

0.7/0.9

V(max)

NPN Switch

4.5

Current Limit

3.7/3.0

A(min)

5.3/6.0

A(max)

Electrical Characteristics -- LM1577-15, LM2577-15

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, and I

SWITCH

= 0.

LM1577-15

LM2577-15

Units

Symbol

Parameter

Conditions

Typical

Limit

(Limits)

(Notes 3, 4)

(Note 5)

SYSTEM PARAMETERS Circuit of

Figure 2 (Note 6)

OUT

Output Voltage

= 5V to 12V

15.0

LOAD

= 100 mA to 600 mA

14.50/14.25

V(min)

(Note 3)

15.50/15.75

V(max)

Line Regulation

= 3.5V to 12V

50/100

LOAD

= 300 mA

mV(max)

Load Regulation

= 5V

50/100

LOAD

= 100 mA to 600 mA

mV(max)

Efficiency

= 5V, I

LOAD

= 600 mA

DEVICE PARAMETERS

Input Supply Current

FEEDBACK

= 18.0V

7.5

(Switch Off)

10.0/14.0

mA(max)

SWITCH

= 2.0A

COMP

= 2.0V

50/85

mA(max)

(Max Duty Cycle)

Input Supply

SWITCH

= 100 mA

2.90

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Electrical Characteristics -- LM1577-15, LM2577-15

(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, and I

SWITCH

= 0.

LM1577-15

LM2577-15

Units

Symbol

Parameter

Conditions

Typical

Limit

(Limits)

(Notes 3, 4)

(Note 5)

DEVICE PARAMETERS

Undervoltage

2.70/2.65

V(min)

Lockout

3.10/3.15

V(max)

Oscillator Frequency

Measured at Switch Pin

kHz

SWITCH

= 100 mA

48/42

kHz(min)

56/62

kHz(max)

REF

Output Reference

Measured at Feedback Pin

Voltage

= 3.5V to 40V

14.70/14.55

V(min)

COMP

= 1.0V

15.30/15.45

V(max)

Output Reference

= 3.5V to 40V

Voltage Line Regulation

Feedback Pin Input

12.2

Voltage Line Regulator

Error Amp

COMP

= -30 µA to +30 µA

300

µmho

Transconductance

COMP

= 1.0V

170/110

µmho(min)

420/500

µmho(max)

VOL

Error Amp

COMP

= 1.1V to 1.9V

V/V

Voltage Gain

COMP

= 1.0 M

40/20

V/V(min)

(Note 7)

Error Amplifier

Upper Limit

2.4

Output Swing

FEEDBACK

= 12.0V

2.2/2.0

V(min)

Lower Limit

0.3

FEEDBACK

= 18.0V

0.4/0.55

0.40/0.55

V(max)

Error Amp

FEEDBACK

= 12.0V to 18.0V

200

µA

Output Current

COMP

= 1.0V

130/

µA(min)

300/

400

300/

400

µA(max)

Soft Start Current

FEEDBACK

= 12.0V

5.0

µA

COMP

= 0V

2.5/1.5

µA(min)

7.5/9.5

µA(max)

Maximum Duty

COMP

= 1.5V

Cycle

SWITCH

= 100 mA

93/90

%(min)

Switch
Transconductance

12.5

A/V

Switch Leakage

SWITCH

= 65V

µA

Current

FEEDBACK

= 18.0V

300/600

µA(max)

(Switch Off)

SAT

Switch Saturation

SWITCH

= 2.0A

0.5

Voltage

COMP

= 2.0V

0.7/0.9

V(max)

(Max Duty Cycle)

NPN Switch

COMP

= 2.0V

4.3

Current Limit

3.7/3.0

A(min)

5.3/6.0

A(max)

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Electrical Characteristics -- LM1577-ADJ, LM2577-ADJ

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, V

FEEDBACK

= V

REF

, and I

SWITCH

= 0.

LM1577-ADJ LM2577-ADJ

Units

Symbol

Parameter

Conditions

Typical

Limit

(Limits)

(Notes 3, 4)

(Note 5)

SYSTEM PARAMETERS Circuit of

Figure 3 (Note 6)

OUT

Output Voltage

= 5V to 10V

12.0

LOAD

= 100 mA to 800 mA

11.60/11.40

V(min)

(Note 3)

12.40/12.60

V(max)

OUT

Line Regulation

= 3.5V to 10V

LOAD

= 300 mA

50/100

mV(max)

OUT

Load Regulation

= 5V

LOAD

= 100 mA to 800 mA

50/100

mV(max)

Efficiency

= 5V, I

LOAD

= 800 mA

DEVICE PARAMETERS

Input Supply Current

FEEDBACK

= 1.5V (Switch Off)

7.5

10.0/14.0

mA(max)

SWITCH

= 2.0A

COMP

= 2.0V (Max Duty Cycle)

50/85

mA(max)

Input Supply

SWITCH

= 100 mA

2.90

Undervoltage Lockout

2.70/2.65

V(min)

3.10/3.15

V(max)

Oscillator Frequency

Measured at Switch Pin

kHz

SWITCH

= 100 mA

48/42

kHz(min)

56/62

kHz(max)

REF

Reference

Measured at Feedback Pin

Voltage

= 3.5V to 40V

1.230

1.214/1.206

V(min)

COMP

= 1.0V

1.246/1.254

V(max)

REF

Reference Voltage

= 3.5V to 40V

0.5

Line Regulation

Error Amp

COMP

= 1.0V

100

Input Bias Current

300/800

nA(max)

Error Amp

COMP

= -30 µA to +30 µA

3700

µmho

Transconductance

COMP

= 1.0V

2400/1600

µmho(min)

4800/5800

µmho(max)

VOL

Error Amp

COMP

= 1.1V to 1.9V

800

V/V

Voltage Gain

COMP

= 1.0 M

(Note 7)

500/250

V/V(min)

Error Amplifier

Upper Limit

2.4

Output Swing

FEEDBACK

= 1.0V

2.2/2.0

V(min)

Lower Limit

0.3

FEEDBACK

= 1.5V

0.40/0.55

V(max)

Error Amp

FEEDBACK

= 1.0V to 1.5V

200

µA

Output Current

COMP

= 1.0V

130/

µA(min)

300/

400

300/

400

µA(max)

Soft Start Current

FEEDBACK

= 1.0V

5.0

µA

COMP

= 0V

2.5/1.5

µA(min)

7.5/9.5

µA(max)

Maximum Duty Cycle

COMP

= 1.5V

SWITCH

= 100 mA

93/90

%(min)

SWITCH

Switch

12.5

A/V

COMP

Transconductance

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Electrical Characteristics -- LM1577-ADJ, LM2577-ADJ

(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, V

FEEDBACK

= V

REF

, and I

SWITCH

= 0.

LM1577-ADJ LM2577-ADJ

Units

Symbol

Parameter

Conditions

Typical

Limit

(Limits)

(Notes 3, 4)

(Note 5)

DEVICE PARAMETERS

Switch Leakage

SWITCH

= 65V

µA

Current

FEEDBACK

= 1.5V (Switch Off)

300/600

µA(max)

SAT

Switch Saturation

SWITCH

= 2.0A

0.5

Voltage

COMP

= 2.0V (Max Duty Cycle)

0.7/0.9

V(max)

NPN Switch

COMP

= 2.0V

4.3

Current Limit

3.7/3.0

A(min)

5.3/6.0

A(max)

THERMAL PARAMETERS (All Versions)

Thermal Resistance

K Package, Junction to Ambient

°C/W

K Package, Junction to Case

1.5

T Package, Junction to Ambient

T Package, Junction to Case

N Package, Junction to

Ambient (Note 8)

M Package, Junction

100

to Ambient (Note 8)

S Package, Junction to

Ambient (Note 9)

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating ratings indicate conditions the device is intended to
be functional, but device parameter specifications may not be guaranteed under these conditions. For guaranteed specifications and test conditions, see the Electrical
Characteristics.

Note 2: Due to timing considerations of the LM1577/LM2577 current limit circuit, output current cannot be internally limited when the LM1577/LM2577 is used as a
step-up regulator. To prevent damage to the switch, its current must be externally limited to 6.0A. However, output current is internally limited when the LM1577/
LM2577 is used as a flyback or forward converter regulator in accordance to the Application Hints.

Note 3: All limits guaranteed at room temperature (standard type face) and at temperature extremes (boldface type). All limits are used to calculate Outgoing Quality
Level, and are 100% production tested.

Note 4: A military RETS electrical test specification is available on request. At the time of printing, the LM1577K-12/883, LM1577K-15/883, and LM1577K-ADJ/883
RETS specifications complied fully with the boldface limits in these columns. The LM1577K-12/883, LM1577K-15/883, and LM1577K-ADJ/883 may also be procured
to Standard Military Drawing specifications.

Note 5: All limits guaranteed at room temperature (standard type face) and at temperature extremes (boldface type). All room temperature limits are 100% produc-
tion tested. All limits at temperature extremes are guaranteed via correlation using standard Statistical Quality Control (SQC) methods.

Note 6: External components such as the diode, inductor, input and output capacitors can affect switching regulator performance. When the LM1577/LM2577 is used
as shown in the Test Circuit, system performance will be as specified by the system parameters.

Note 7: A 1.0 M

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

VOL

. In actual applications,

this pin's load resistance should be

10 M

, resulting in A

VOL

that is typically twice the guaranteed minimum limit.

Note 8: Junction to ambient thermal resistance with approximately 1 square inch of pc board copper surrounding the leads. Additional copper area will lower thermal
resistance further. See thermal model in "Switchers Made Simple" software.

Note 9: If the TO-263 package is used, the thermal resistance can be reduced by increasing the PC board copper area thermally connected to the package. Using
0.5 square inches of copper area,

is 50°C/W; with 1 square inch of copper area,

is 37°C/W; and with 1.6 or more square inches of copper area,

is 32°C/W.

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LM1577-12, LM2577-12 Test Circuit

LM1577-15, LM2577-15 Test Circuit

LM1577-ADJ, LM2577-ADJ Test Circuit

DS011468-30

L = 415-0930 (AIE)
D = any manufacturer
C

OUT

= Sprague Type 673D

Electrolytic 680 µF, 20V

Note: Pin numbers shown are for TO-220 (T) package

FIGURE 1. Circuit Used to Specify System Parameters for 12V Versions

DS011468-26

L = 415-0930 (AIE)
D = any manufacturer
C

OUT

= Sprague Type 673D

Electrolytic 680 µF, 20V

Note: Pin numbers shown are for TO-220 (T) package

FIGURE 2. Circuit Used to Specify System Parameters for 15V Versions

DS011468-9

L = 415-0930 (AIE)
D = any manufacturer
C

OUT

= Sprague Type 673D

Electrolytic 680 µF, 20V

R1 = 48.7k in series with 511

(1%)

R2 = 5.62k (1%)
Note: Pin numbers shown are for TO-220 (T) package

FIGURE 3. Circuit Used to Specify System Parameters for ADJ Versions

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Application Hints

(Continued)

STEP-UP (BOOST) REGULATOR

Figure 4 shows the LM1577-ADJ/LM2577-ADJ used as a
Step-Up Regulator. This is a switching regulator used for
producing an output voltage greater than the input supply
voltage.

The

LM1577-12/LM2577-12

and

LM1577-15/

LM2577-15 can also be used for step-up regulators with 12V
or 15V outputs (respectively), by tying the feedback pin di-
rectly to the regulator output.

A basic explanation of how it works is as follows. The
LM1577/LM2577 turns its output switch on and off at a fre-
quency of 52 kHz, and this creates energy in the inductor (L).
When the NPN switch turns on, the inductor current charges
up at a rate of V

/L, storing current 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 ca-
pacitor (C

OUT

) at a rate of (V

OUT

- V

)/L. Thus, energy

stored in the inductor during the switch on time is transferred
to the output during the switch off time. The output voltage is
controlled by the amount of energy transferred which, in turn,
is controlled by modulating the peak inductor current. This is
done by feeding back a portion of the output voltage to the
error amp, which amplifies the difference between the feed-
back voltage and a 1.230V reference. The error amp output
voltage is compared to a voltage proportional to the switch
current (i.e., inductor current during the switch on time).

The comparator terminates the switch on time when the two
voltages are equal, thereby controlling the peak switch cur-
rent to maintain a constant output voltage.

Voltage and current waveforms for this circuit are shown in
Figure 5, and formulas for calculating them are given in Fig-
ure 6.

STEP-UP REGULATOR DESIGN PROCEDURE

The following design procedure can be used to select the ap-
propriate external components for the circuit in

Figure 4,

based on these system requirements.

Given:

IN (min)

= Minimum input supply voltage

OUT

= Regulated output voltage

LOAD(max)

= Maximum output load current

Before proceeding any further, determine if the LM1577/
LM2577 can provide these values of V

OUT

and I

LOAD(max)

when operating with the minimum value of V

. The upper

limits for V

OUT

and I

LOAD(max)

are given by the following

equations.

OUT

60V

and

OUT

10 x V

IN(min)

These limits must be greater than or equal to the values
specified in this application.

1. Inductor Selection (L)

Voltage Options:

1. For 12V or 15V output

DS011468-11

FIGURE 5. Step-Up Regulator Waveforms

Duty Cycle

Average
Inductor
Current

IND(AVE)

Inductor
Current
Ripple

IND

Peak
Inductor
Current

IND(PK)

Peak Switch
Current

SW(PK)

Switch
Voltage
When Off

SW(OFF)

OUT

+ V

Diode
Reverse
Voltage

OUT

- V

SAT

Average
Diode
Current

D(AVE)

LOAD

Peak Diode
Current

D(PK)

Power
Dissipation
of
LM1577/2577

= Forward Biased Diode Voltage

LOAD

= Output Load Current

FIGURE 6. Step-Up Regulator Formulas

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Application Hints

(Continued)

From

Figure 7 (for 12V output) or Figure 8 (for 15V

output), identify inductor code for region indicated by
V

IN (min)

and I

LOAD (max)

. The shaded region indicates

conditions for which the LM1577/LM2577 output switch
would be operating beyond its switch current rating. The
minimum operating voltage for the LM1577/LM2577 is
3.5V.

From here,

proceed to step C.

2. For Adjustable version

Preliminary calculations:

The inductor selection is based on the calculation of the
following three parameters:

(max)

, the maximum switch duty cycle (0

0.9):

where V

= 0.5V for Schottky diodes and 0.8V for fast recov-

ery diodes (typically);

·T, the product of volts x time that charges the inductor:

IND,DC

, the average inductor current under full load;

Identify Inductor Value:

1. From

Figure 9, identify the inductor code for the re-

gion indicated by the intersection of E

·T and I

IND,DC

This code gives the inductor value in microhenries. The
L or H prefix signifies whether the inductor is rated for a
maximum E

·T of 90 V·µs (L) or 250 V·µs (H).

2. If D

0.85, go on to step C. If D

0.85, then calcu-

late the minimum inductance needed to ensure the
switching regulator's stability:

If L

MIN

is smaller than the inductor value found in step B1, go

on to step C. Otherwise, the inductor value found in step B1
is too low; an appropriate inductor code should be obtained
from the graph as follows:

1. Find the lowest value inductor that is greater than L

MIN

2. Find where E

·T intersects this inductor value to determine

if it has an L or H prefix. If E

·T intersects both the L and H re-

gions, select the inductor with an H prefix.

DS011468-27

FIGURE 7. LM2577-12 Inductor Selection Guide

DS011468-28

FIGURE 8. LM2577-15 Inductor Selection Guide

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Application Hints

(Continued)

Select an inductor from the table of Figure 10 which
cross-references the inductor codes to the part numbers
of three different manufacturers. Complete specifica-
tions for these inductors are available from the respec-
tive manufacturers. The inductors listed in this table
have the following characteristics:

AIE: ferrite, pot-core inductors; Benefits of this type are
low electro-magnetic interference (EMI), small physical
size, and very low power dissipation (core loss). Be
careful not to operate these inductors too far beyond
their maximum ratings for E

·T and peak current, as this

will saturate the core.

Pulse: powdered iron, toroid core inductors; Benefits are
low EMI and ability to withstand E

·T and peak current

above rated value better than ferrite cores.

Renco: ferrite, bobbin-core inductors; Benefits are low
cost and best ability to withstand E

·T and peak current

above rated value. Be aware that these inductors gener-
ate more EMI than the other types, and this may inter-
fere with signals sensitive to noise.

DS011468-12

Note: These charts assume that the inductor ripple current inductor is approximately 20% to 30% of the average inductor current (when the regulator is under
full load). Greater ripple current causes higher peak switch currents and greater output ripple voltage; lower ripple current is achieved with larger-value
inductors. The factor of 20 to 30% is chosen as a convenient balance between the two extremes.

FIGURE 9. LM1577-ADJ/LM2577-ADJ Inductor Selection Graph

www.national.com

Application Hints

(Continued)

2. Compensation Network (R

, C

) and Output Capacitor

OUT

) Selection

and C

form a pole-zero compensation network that sta-

bilizes the regulator. The values of R

and C

are mainly de-

pendant on the regulator voltage gain, I

LOAD(max)

, L and

OUT

. The following procedure calculates values for R

, C

and C

OUT

that ensure regulator stability. Be aware that this

procedure doesn't necessarily result in R

and C

that pro-

vide optimum compensation. In order to guarantee optimum
compensation, one of the standard procedures for testing
loop stability must be used, such as measuring V

OUT

tran-

sient response when pulsing I

LOAD

(see

Figure 15).

First, calculate the maximum value for R

Select a resistor less than or equal to this value, and it
should also be no greater than 3 k

Calculate the minimum value for C

OUT

using the following

two equations.

The larger of these two values is the minimum value that en-
sures stability.

Calculate the minimum value of C

The compensation capacitor is also part of the soft start cir-
cuitry. When power to the regulator is turned on, the switch
duty cycle is allowed to rise at a rate controlled by this ca-
pacitor (with no control on the duty cycle, it would immedi-
ately rise to 90%, drawing huge currents from the input
power supply). In order to operate properly, the soft start cir-
cuit requires C

0.22 µF.

The value of the output filter capacitor is normally large
enough to require the use of aluminum electrolytic capaci-
tors.

Figure 11 lists several different types that are recom-

mended for switching regulators, and the following param-
eters are used to select the proper capacitor.

Working Voltage (WVDC): Choose a capacitor with a work-
ing voltage at least 20% higher than the regulator output volt-
age.

Ripple Current: This is the maximum RMS value of current
that charges the capacitor during each switching cycle. For
step-up and flyback regulators, the formula for ripple current
is

Choose a capacitor that is rated at least 50% higher than this
value at 52 kHz.

Equivalent Series Resistance (ESR) : This is the primary
cause of output ripple voltage, and it also affects the values
of R

and C

needed to stabilize the regulator. As a result,

the preceding calculations for C

and R

are only valid if

ESR doesn't exceed the maximum value specified by the fol-
lowing equations.

Select a capacitor with ESR, at 52 kHz, that is less than or
equal to the lower value calculated. Most electrolytic capaci-
tors specify ESR at 120 Hz which is 15% to 30% higher than
at 52 kHz. Also, be aware that ESR increases by a factor of
2 when operating at -20°C.

In general, low values of ESR are achieved by using large
value capacitors (C

470 µF), and capacitors with high

WVDC, or by paralleling smaller-value capacitors.

Inductor

Manufacturer's Part Number

Code

Schott

Pulse

Renco

L47

67126980

PE - 53112

RL2442

L68

67126990

PE - 92114

RL2443

L100

67127000

PE - 92108

RL2444

L150

67127010

PE - 53113

RL1954

L220

67127020

PE - 52626

RL1953

L330

67127030

PE - 52627

RL1952

L470

67127040

PE - 53114

RL1951

L680

67127050

PE - 52629

RL1950

H150

67127060

PE - 53115

RL2445

H220

67127070

PE - 53116

RL2446

H330

67127080

PE - 53117

RL2447

H470

67127090

PE - 53118

RL1961

H680

67127100

PE - 53119

RL1960

H1000

67127110

PE - 53120

RL1959

H1500

67127120

PE - 53121

RL1958

H2200

67127130

PE - 53122

RL2448

Schott Corp., (612) 475-1173
1000 Parkers Lake Rd., Wayzata, MN 55391
Pulse Engineering, (619) 268-2400
P.O. Box 12235, San Diego, CA 92112
Renco Electronics Inc., (516) 586-5566
60 Jeffryn Blvd. East, Deer Park, NY 11729

FIGURE 10. Table of Standardized Inductors and

Manufacturer's Part Numbers

www.national.com

Application Hints

(Continued)

3. Output Voltage Selection (R1 and R2)

This section is for applications using the LM1577-ADJ/
LM2577-ADJ. Skip this section if the LM1577-12/LM2577-12
or LM1577-15/LM2577-15 is being used.

With the LM1577-ADJ/LM2577-ADJ, the output voltage is
given by

OUT

= 1.23V (1 + R1/R2)

Resistors R1 and R2 divide the output down so it can be
compared

with

the

LM1577-ADJ/LM2577-ADJ

internal

1.23V reference. For a given desired output voltage V

OUT

select R1 and R2 so that

4. Input Capacitor Selection (C

)

The switching action in the step-up regulator causes a trian-
gular ripple current to be drawn from the supply source. This
in turn causes noise to appear on the supply voltage. For
proper operation of the LM1577, the input voltage should be
decoupled. Bypassing the Input Voltage pin directly to
ground with a good quality, low ESR, 0.1 µF capacitor (leads
as short as possible) is normally sufficient.

If the LM1577 is located far from the supply source filter ca-
pacitors, an additional large electrolytic capacitor (e.g.
47 µF) is often required.

5. Diode Selection (D)

The switching diode used in the boost regulator must with-
stand a reverse voltage equal to the circuit output voltage,
and must conduct the peak output current of the LM2577. A
suitable diode must have a minimum reverse breakdown
voltage greater than the circuit output voltage, and should be
rated for average and peak current greater than I

LOAD(max)

and I

D(PK)

. Schottky barrier diodes are often favored for use

in switching regulators. Their low forward voltage drop allows
higher regulator efficiency than if a (less expensive) fast re-
covery diode was used. See

Figure 12 for recommended

part numbers and voltage ratings of 1A and 3A diodes.

BOOST REGULATOR CIRCUIT EXAMPLE

By adding a few external components (as shown in

Figure

13), the LM2577 can be used to produce a regulated output
voltage that is greater than the applied input voltage. Typical
performance of this regulator is shown in

Figure 14 and Fig-

ure 15. The switching waveforms observed during the opera-
tion of this circuit are shown in

Figure 16.

Cornell Dublier -- Types 239, 250, 251, UFT,
300, or 350

P.O. Box 128, Pickens, SC 29671
(803) 878-6311

Nichicon -- Types PF, PX, or PZ

927 East Parkway,
Schaumburg, IL 60173
(708) 843-7500

Sprague -- Types 672D, 673D, or 674D

Box 1, Sprague Road,
Lansing, NC 28643
(919) 384-2551

United Chemi-Con -- Types LX, SXF, or SXJ

9801 West Higgins Road,
Rosemont, IL 60018
(708) 696-2000

FIGURE 11. Aluminum Electrolytic Capacitors

Recommended for Switching Regulators

OUT

Schottky

Fast Recovery

(max)

20V

1N5817

1N5820

MBR120P

MBR320P

1N5818

1N5821

30V

MBR130P

MBR330P

11DQ03

31DQ03

1N5819

1N5822

40V

MBR140P

MBR340P

11DQ04

31DQ04

MBR150

MBR350

1N4933

50V

11DQ05

31DQ05

MUR105

1N4934

MR851

100V

HER102

30DL1

MUR110

MR831

10DL1

HER302

FIGURE 12. Diode Selection Chart

www.national.com

Application Hints

(Continued)

FLYBACK REGULATOR

A Flyback regulator can produce single or multiple output
voltages that are lower or greater than the input supply volt-
age.

Figure 18 shows the LM1577/LM2577 used as a fly-

back regulator with positive and negative regulated outputs.
Its operation is similar to a step-up regulator, except the out-
put switch contols the primary current of a flyback trans-
former. Note that the primary and secondary windings are
out of phase, so no current flows through secondary when
current flows through the primary. This allows the primary to
charge up the transformer core when the switch is on. When
the switch turns off, the core discharges by sending current
through the secondary, and this produces voltage at the out-
puts. The output voltages are controlled by adjusting the
peak primary current, as described in the step-up regulator
section.

Voltage and current waveforms for this circuit are shown in
Figure 17, and formulas for calculating them are given in Fig-
ure 19.

FLYBACK REGULATOR DESIGN PROCEDURE

1. Transformer Selection

A family of standardized flyback transformers is available for
creating flyback regulators that produce dual output volt-
ages, from

10V to

15V, as shown in

Figure 18. Figure

20lists these transformers with the input voltage, output volt-
ages and maximum load current they are designed for.

2. Compensation Network (C

, R

) and

Output Capacitor (C

OUT

) Selection

As explained in the Step-Up Regulator Design Procedure,
C

, R

and C

OUT

must be selected as a group. The following

procedure is for a dual output flyback regulator with equal
turns ratios for each secondary (i.e., both output voltages
have the same magnitude). The equations can be used for a
single output regulator by changing

LOAD(max)

to I

LOAD(max)

in the following equations.

A. First, calculate the maximum value for R

Where

LOAD(max)

is the sum of the load current (magni-

tude) required from both outputs. Select a resistor less than
or equal to this value, and no greater than 3 k

B. Calculate the minimum value for

OUT

(sum of C

OUT

at both outputs) using the following two equations.

The larger of these two values must be used to ensure regu-
lator stability.

DS011468-17

FIGURE 17. Flyback Regulator Waveforms

DS011468-18

T1 = Pulse Engineering, PE-65300
D1, D2 = 1N5821

FIGURE 18. LM1577-ADJ/LM2577-ADJ Flyback Regulator with

Outputs

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Application Hints

(Continued)

Calculate the minimum value of C

Calculate the maximum ESR of the +V

OUT

and -V

OUT

output capacitors in parallel.

This formula can also be used to calculate the maximum
ESR of a single output regulator.

At this point, refer to this same section in the Step-Up Regu-
lator Design Procedurefor more information regarding the
selection of C

OUT

3. Output Voltage Selection

This section is for applications using the LM1577-ADJ/
LM2577-ADJ. Skip this section if the LM1577-12/LM2577-12
or LM1577-15/LM2577-15 is being used.

With the LM1577-ADJ/LM2577-ADJ, the output voltage is
given by

OUT

= 1.23V (1 + R1/R2)

Resistors R1 and R2 divide the output voltage down so it can
be compared with the LM1577-ADJ/LM2577-ADJ internal
1.23V reference. For a desired output voltage V

OUT

, select

R1 and R2 so that

4. Diode Selection

The switching diode in a flyback converter must withstand
the reverse voltage specified by the following equation.

A suitable diode must have a reverse voltage rating greater
than this. In addition it must be rated for more than the aver-
age and peak diode currents listed in

Figure 19.

5. Input Capacitor Selection

The primary of a flyback transformer draws discontinuous
pulses of current from the input supply. As a result, a flyback
regulator generates more noise at the input supply than a
step-up regulator, and this requires a larger bypass capacitor
to decouple the LM1577/LM2577 V

pin from this noise. For

most applications, a low ESR, 1.0 µF cap will be sufficient, if
it is connected very close to the V

and Ground pins.

Duty Cycle

Primary Current Variation

Peak Primary Current

P(PK)

Switch Voltage when Off

SW(OFF)

Diode Reverse Voltage

OUT

N (V

SAT

)

Average Diode Current

D(AVE)

LOAD

Peak Diode Current

D(PK)

Short Circuit Diode Current

Power Dissipation of
LM1577/LM2577

DS011468-78

FIGURE 19. Flyback Regulator Formulas

www.national.com

Application Hints

(Continued)

Transformer

Input

Dual

Maxi-

mum

Type

Voltage

Output

Voltage

Current

= 100 µH

10V

325 mA

N = 1

12V

275 mA

15V

225 mA

10V

700 mA

10V

12V

575 mA

= 200 µH

10V

15V

500 mA

N = 0.5

12V

10V

800 mA

12V

700 mA

12V

15V

575 mA

= 250 µH

15V

10V

900 mA

N = 0.5

15V

12V

825 mA

15V

700 mA

In addition to this bypass cap, a larger capacitor (

47 µF)

should be used where the flyback transformer connects to
the input supply. This will attenuate noise which may inter-
fere with other circuits connected to the same input supply
voltage.

6. Snubber Circuit

A "snubber" circuit is required when operating from input
voltages greater than 10V, or when using a transformer with
L

200 µH. This circuit clamps a voltage spike from the

transformer primary that occurs immediately after the output
switch turns off. Without it, the switch voltage may exceed
the 65V maximum rating. As shown in

Figure 21, the snub-

ber consists of a fast recovery diode, and a parallel RC. The

RC values are selected for switch clamp voltage (V

CLAMP

)

that is 5V to 10V greater than V

SW(OFF)

. Use the following

equations to calculate R and C;

Power dissipation (and power rating) of the resistor is;

The fast recovery diode must have a reverse voltage rating
greater than V

CLAMP

FLYBACK REGULATOR CIRCUIT EXAMPLE

The circuit of

Figure 22 produces

15V (at 225 mA each)

from a single 5V input. The output regulation of this circuit is
shown in

Figure 23 and Figure 25, while the load transient

response is shown in

Figure 24 and Figure 26. Switching

waveforms seen in this circuit are shown in

Figure 27.

Transformer

Manufacturers' Part Numbers

Type

AIE

Pulse

Renco

326-0637

PE-65300

RL-2580

330-0202

PE-65301

RL-2581

330-0203

PE-65302

RL-2582

FIGURE 20. Flyback Transformer Selection Guide

DS011468-19

FIGURE 21. Snubber Circuit

www.national.com

Physical Dimensions

inches (millimeters) unless otherwise noted (Continued)

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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.

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www.national.com

5-Lead TO-263 (S)

Order Number LM2577S-12, LM2577S-15 or LM2577S-ADJ

NS Package Number TS5B

LM1577/LM2577

Series

SIMPLE

SWITCHER

Step-Up

oltage

Regulator

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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