SC2542 datasheet(14/27 Pages) SEMTECH

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SC2542

POWER MANAGEMENT

The compensator in Figure 10 includes an error ampli-

fier and compensation networks Zf and Zs. It is imple-

mented by the circuit in Figure 12. The compensator pro-

vides an integrator, double poles, and double zeros. As

shown in Figure 11, the integrator is used to boost the

gain at low frequency. Two zeros are introduced to com-

pensate excessive phase lag at the loop gain crossover

due to the integrator (-90deg) and the complex pole pair

(-180deg). Two high frequency poles are designed to com-

pensate the ESR zero and to attenuate high frequency

noise.

Figure 11. Bode plots for control loop design.

Rbot

E/A

Rtop

0.75V

Out

Figure 12. Compensation network.

Applications Information (Cont.)

A resistive divider is used to program the output voltage.

The top resistor, Rtop, of the divider in Figure 12 can be

chosen from 20k

Ω to 30kΩ. Then the bottom resistor,

Rbot, is found from:

TOP

BOT

∗

−

where 0.75V is the internal reference voltage of the

SC2542.

The other components of the compensator can be cal-

culated using following design procedure:

(1). Plot the converter gain, including LC filter and PWM

modulator.

(2). Select the open loop crossover frequency Fc located

at 10% to 20% of the switching frequency.

(3). Use the first compensator pole Fp1 to cancel the

ESR zero.

(4). Have the second compensator pole Fp2 at half the

switching frequency to attenuate the switching ripple and

high frequency noise.

(5). Place the first compensator zero Fz1 at or below

50% of the power stage resonant frequency Fo.

(6). Place the second compensator zero Fz2 at or below

the power stage resonant frequency Fo.

A MathCAD program is available upon request for the

calculation of the compensation parameters.

Design Procedure for a Step-down Power Converter

Selection criteria and design procedures for the follow-

ing parameters are described:

1) Output inductor (L) type and value

2) Output capacitor (Co) type and value

3) Input capacitor (Cin) type and value

4) Power MOSFETs

5) Current sensing and limiting circuit

6) Voltage sensing circuit

7) Loop compensation network

The following step-down converter specifications are

needed:

ESR

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