LM1577 datasheet(17/27 Pages) NSC

17 / 27 page

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

IN)

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

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