Z _ mod

ESR

O

1

ƒ

2π

R

C

=

´

´

P _ mod

L

O

1

ƒ

2π

R

C

=

´

´

dc

ps

L

A

gm

R

=

´

Z

O

dc

C

P

s

1

2π

ƒ

V

A

V

s

1

2π

ƒ

æ

ö

+

ç

÷

´

è

ø

=

´

æ

ö

+

ç

÷

´

è

ø

18

TPS65321A-Q1

SLVSE55 – NOVEMBER 2017

www.ti.com

Product Folder Links: TPS65321A-Q1

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Copyright © 2017, Texas Instruments Incorporated

Feature Description (continued)

Figure 12. Simple Small-Signal Model and Frequency Response for Peak-Current Mode

(6)

(7)

(8)

(9)

7.3.1.18 Small-Signal Model for Frequency Compensation

The buck regulator of the TPS65321A-Q1 device uses a transconductance amplifier as the error amplifier.

Figure 13 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 10 and Equation 11 show how to relate the frequency response of the amplifier to

Figure 13. See the Typical Application section for a design example with a Type 2A network that has a low-ESR

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

crossover frequency. Also, the crossover frequency must 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, add a lower-value capacitor (C2) in parallel with R3 to work as an optional pole. This capacitor can filter

noise at switching frequency, and is also required if the output capacitor has high ESR.