Datasheet

Vref

5pF

COMP

VSENSE

Type 2A

Type 2B

p ´ ´

Iout max

p m od =

2 Vout Cout

p ´ ´

z m od =

2 Resr Cout

¦ ¦ ´ ¦

= p mod z mod

¦ ¦ ´

= p mod

TPS54478

www.ti.com

SLVSAS2 –JUNE 2011

Figure 33. Type-II of Frequency Compensation

The design guidelines for TPS54478 loop compensation will be addressed in the APPLICATION INFORMATION

section with more details. The approach is to run the Pspice model first to find the accurate response of the

power stage with slope compensation effect. The compensation network is then designed based on the desired

crossover frequency. The crossover frequency and phase margin are more closer to the measured results when

the slope compensation effect is included.

For type-II compensation, the modulator pole, fpmod, and the esr zero, fz1 can be calculated using Equation 13

and Equation 14. Derating the output capacitor (C

OUT

) is needed if the output voltage is a high percentage of the

capacitor rating. Use the capacitor manufacturer information to derate the capacitor value. Use Equation 15 and

Equation 16 to estimate a starting point for the crossover frequency, fc. Equation 15 is the geometric mean of the

modulator pole and the esr zero and Equation 16 is the mean of modulator pole and the switching frequency.

Use the lower value of Equation 15 or Equation 16 as the maximum crossover frequency.

(13)

vertical spacer

(14)

vertical spacer

(15)

vertical spacer

(16)

vertical spacer

The type-III compensation is recommended to achieve higher crossover frequency by introducing extra phase lift.

By adding a small capacitor C3 in parallel with R1, one-pair of zero and pole is generated as given by

Equation 17 and Equation 18. The following APPLICATION INFORMATION section provides step-by-step design

guideline for the type-III compensation wih the effect of slope compensation included.