SS

RIPPLE

OUT(max)

LOAD

OUT

t

I

C

ILIM

I

V

2

æ

ö

æ

ö

=

´

-

-

ç

÷

ç

÷

è

ø

è

ø

FB

11.5 kW

+

Error

Amplifier

0.8 V

REF

BP

GND

+

Offset

f(I

PWM to

Switch

I

SLOPE

I

COMP

UDG-09128

I

x 2

C

COMP

R

COMP

COMP

TPS54290, TPS54291, TPS54292

www.ti.com

SLUS973 – OCTOBER 2009

MAXIMUM OUTPUT CAPACITANCE

With internal pulse-by-pulse current limiting and a fixed soft-start time, there is a maximum output capacitance

which may be used before startup problems begin to occur. If the output capacitance is large enough so that the

device enters a current-limit protection mode during startup, then there is a possibility that the output never

reaches regulation. Instead, the TPS5429x simply shuts down and attempts a restart as if the output were

short-circuited to ground. The maximum output capacitance (including bypass capacitance distributed at the

load) is given by

(4)

where

•

•

ILIM is the current limit level

FEEDBACK LOOP COMPENSATION

(COMPx pin) to ground serves as the compensation network for the converter. The signal from the error amplifier

output is then buffered and combined with a slope compensation signal before it is mirrored to be referenced to

the SW node. Here, it is compared with the current feedback signal to create a pulse-width-modulated (PWM)

signal-fed to drive the upper MOSFET switch. A simplified equivalent circuit of the signal control path is depicted

in Figure 15.

NOTE

Noise coupling from the SWx node to internal circuitry of BOOTx may impact narrow

pulse width operation, especially at load currents less than 1 A.

Figure 15. Feedback Loop Equivalent Circuit

A more conventional small-signal equivalent block diagram is shown in Figure 16. Here, the full closed-loop

signal path is shown. Because the TPS5429x contains internal slope compensation, the external L-C filter must

be selected appropriately so that the resulting control loop meets criteria for stability.

Copyright © 2009, Texas Instruments Incorporated

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