SC4525CEVB datasheet(14/22 Pages) SEMTECH

14 / 22 page

SC4525C

Applications Information (Cont.)

Loop Compensation

The goal of compensation is to shape the frequency

response of the converter so as to achieve high DC

accuracy and fast transient response while maintaining

loop stability (see Figure 7).

Resr

COMP

REF

CONTROLLER AND SCHOTTKY DIODE

PWM

MODULATOR

Vramp

Figure 7. Block diagram of control loops

The block diagram in Figure 7 shows the control loops of a

buck converter with the SC4525C. The inner loop (current

loop) consists of a current sensing resistor (R

s=3.53mW)

and a current amplifier (CA) with gain (G

CA=8.5). The

outer loop (voltage loop) consists of an error amplifier

(EA), a PWM modulator, and a LC filter.

Since the current loop is internally closed, the remaining

task for the loop compensation is to design the voltage

compensator (C

5, R7, and C8).

For a converter with switching frequency F

SW, output

inductance L

, output capacitance CO and loading R, the

control (V

C) to output (VO) transfer function in Figure 7 is

given by:

)

()

(

)

(

ESR

PWM

This transfer function has a finite DC gain

PWM

≈

It has an ESR zero F

Z at

ESR

Z =

It has a dominant low-frequency pole F

P at

p ≈

and double poles at half the switching frequency.

Including the voltage divider (R

4 and R6), the control to

feedback transfer function is found and plotted in Figure

8 as the converter gain.

Since the converter gain has only one dominant pole at

low frequency, a simple Type-2 compensation network

is sufficient for voltage loop compensation. As shown in

Figure 8, the voltage compensator has a low frequency

integrator pole, a zero at F

Z, and a high frequency pole

at F

P. The integrator is used to boost the gain at low

frequency. The zero is introduced to compensate the

excessive phase lag at the loop gain crossover due to the

integrator pole (-90deg) and the dominant pole (-90deg).

The high frequency pole nulls the ESR zero and attenuates

high frequency noise.

Fsw/2

Fz1

Fp1

10K

100K

10M

-60

-30

FREQUENCY (Hz)

CONVE

RTER

GAIN

LOOP

GAIN

COMP

ENSAT

OR GA

Figure 8 — Bode plots for voltage loop design

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