TPS40056

SLVS612 − APRIL 2006

13

www.ti.com

APPLICATION INFORMATION

LOOP COMPENSATION

Voltage-mode buck-type converters are typically compensated using Type III networks. Since the TPS40056

includes no voltage feedforward control, the gain of the PWM modulator must be included. The modulator gain

is described in Figure 5.

A

V

IN

V

S

or

A

log

V

IN

V

S

to output voltage modulator gain in terms of the input voltage and ramp voltage,

D

+

V

O

V

IN

+

V

C

V

S

or

V

O

V

C

+

V

IN

V

S

Calculate the Poles and Zeros

is located at the frequency calculated in equation (16).

f

1

2

p

L

C

O

(Hertz)

at the frequency calculated in equation (17).

f

1

2

p

ESR

C

O

(Hertz)

R

V

EA_REF

R1

V

W

The maximum crossover frequency (0 dB loop gain) is calculated in equation (19).

f

f

SW

4

(Hertz)

frequency, the control to output gain has a –2 slope (−40 dB/decade), while the Type III topology has a +1 slope

(20 dB/decade), resulting in an overall closed loop –1 slope (−20 dB/decade).

Figure 5 shows the modulator gain, L-C filter, output capacitor ESR zero, and the resulting response to be

compensated.

(14)

(15)

(16)

(17)

(18)

(19)