=

£

p ´

´

SW

0

OUT

f

1

f

2

ESR C

4

R1

R2

Voltage

Divider

+

VFB

+

VREF

PWM

Control

Logic

and

Divider

L

ESR

UDG-12010

Switching Modulator

Output

Capacitor

TPS51225

TPS51225B

TPS51225C

DRVH

DRVL

TPS51225, TPS51225B, TPS51225C

www.ti.com

SLUSAV0A – JANUARY 2012 – REVISED JUNE 2012

Light-Load Condition in Out-of-Audio™ Operation (TPS51225B)

Out-of-Audio™ (OOA) light-load mode is a unique control feature that keeps the switching frequency above

acoustic audible frequencies toward a virtual no-load condition. During Out-of-Audio™ operation, the OOA

control circuit monitors the states of both MOSFETs and forces them to transition into the ON state if both of

MOSFETs are off for more than 40

μs. When both high-side and low-side MOSFETs are off for 40 µs during a

light-load condition, the operation mode is changed to FCCM. This mode change initiates the low-side MOSFET

on and pulls down the output voltage. Then, the high-side MOSFET is turned on and stops switching again.

Table 1. SKIP Mode Operation (TPS51225/B/C)

SKIP MODE OPERATION

TPS51225

Auto-skip

TPS51225B

OOA

TPS51225C

Auto-skip

D-CAP™ Mode

From small-signal loop analysis, a buck converter using D-CAP™ mode can be simplified as shown in Figure 1.

Figure 1. Simplifying the Modulator

The output voltage is compared with internal reference voltage after divider resistors, R1 and R2. The PWM

comparator determines the timing to turn onthe high-side MOSFET. The gain and speed of the comparator is

high enough to keep the voltage at the beginning of each ON cycle substantially constant. For the loop stability,

(2)

determined by the capacitor chemistry. For example, specialty polymer capacitors have output capacitance in the

not suitable for this operational mode.

Copyright © 2012, Texas Instruments Incorporated

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