R1

Vref = 0.8V

COMP

R2

R3

C1

C2

FB1

Error

Amp

x

x

Inside

TPS65320-Q1

Type 2A

Type 2B

Type 1

+

±

C2

R3

C1

Z _ mod

ESR

out

1

f

2

R

C

=

p ´

´

P _ mod

L

out

1

f

2

R

C

=

p ´

´

dc

ps

L

A

gm

R

=

´

Z

out

dc

C

P

s

1

2

f

V

A

V

s

1

2

f

æ

ö

+

ç

÷

p ´

è

ø

=

´

æ

ö

+

ç

÷

p ´

è

ø

TPS65320-Q1

SLVSAY9A – DECEMBER 2012 – REVISED APRIL 2013

www.ti.com

(5)

(6)

(7)

(8)

Small-Signal Model for Frequency Compensation

The buck converter of the TPS65320-Q1 device uses a transconductance amplifier for the error amplifier.

Figure 19 shows compensation circuits. Implementation of Type 2 circuits is most likely in high-bandwidth power-

supply designs. The purpose of loop compensation is to ensure stable operation while maximizing dynamic

performance. Use of the Type 1 circuit is with power-supply designs that have high-ESR aluminum electrolytic or

tantalum capacitors. Equation 9 and Equation 10 show how to relate the frequency response of the amplifier to

Figure 19. See the Application Information section for a design example with a Type 2A network that has a low-

ESR output capacitor. For stability purposes, the target is to have a loop-gain slope that is –20 dB/decade at the

crossover frequency. Also, the crossover frequency should not exceed one-fifth of the switching frequency (120

kHz in the case of a 600-kHz switching frequency).

For dynamic purposes, the higher the bandwidth, the faster the load-transient response. A large dc gain means

high dc regulation accuracy (dc voltage changes little with load or line variations). To achieve this loop gain, set

the compensation components according to the shape of the control-output bode plot.

form the dominant pole (P1). A resistor (R3) and a capacitor (C1) in series to ground work as zero (Z1). In

addition, one can add a lower-value capacitor (C2) in parallel with R3 to work as an optional pole. This capacitor

can be used to filter noise at switching frequency, and it is also needed if the output capacitor has high ESR.

Figure 19. Types of Frequency Compensation

16

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