LM1577 datasheet(16/27 Pages) NSC

16 / 27 page

Application Hints (Continued)

2. Compensation Network (R

C,CC) and Output Capacitor

OUT) Selection

C and CC form a pole-zero compensation network that sta-

bilizes the regulator. The values of R

C and CC are mainly de-

pendant on the regulator voltage gain, I

LOAD(max), L and

OUT. The following procedure calculates values for RC,CC,

and C

OUT that ensure regulator stability. Be aware that this

procedure doesn’t necessarily result in R

C and CC 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 RC.

Select a resistor less than or equal to this value, and it

should also be no greater than 3 k

Ω.

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 CC .

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

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

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

C and CC needed to stabilize the regulator. As a result,

the preceding calculations for C

C and RC 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

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