equal to or lower than 5 kΩ. Lower values of FB resistors achieve better noise immunity, and lower light load

efficiency, as explained in the Design Considerations for a Resistive Feedback Divider in a DC/DC Converter

technical brief.

OUT

OUT

FB

V

V

R1 = R2

1

= R2

1

V

0.8V

æ

ö

æ

ö

´

-

´

-

ç

÷

ç

÷

è

ø

è

ø

(6)

A feedforward capacitor (CFF) is not required for proper operation, but can further improve output noise.

However, care must be taken in choosing the CFF, because the power-good (PG) function can not be valid

with a large CFF during start-up, and can cause spurious triggering of the PG pin during a large load transient.

Refer to the Pros and Cons Using a Feedforward Capacitor with a Low Dropout Regulator application report for

a discussion of the pros and cons of using a feedforward capacitor.

8.2.2.2.6 NR/SS Capacitor Selection

As described in Section 7.3.8, the NR/SS cap affects both the total noise and the soft-start time. When no

NR/SS cap is connected, there is a default 2.35-ms soft-start time. The recommended value for a 5-ms soft-start

time and good noise performance is 220 nF, as there is an internal 220 nF capacitor already in the module. The

maximum NR/SS cap is 3.3 μF for a start-up time of 35 ms. Values greater than 1 μF have minimal improvement

in noise performance. Use Equation 7 and Equation 8 to calculate the soft-start time based on desired soft-start

time or the chosen capacitor value. Note that the CNRSS in the equation below is the combination of the internal

220 nF cap and any external cap from the CNRSS pin to GND.

tsss =  CNRSS×0.8

INRSS

(7)

CNRSSF = INRSS × tss

0.8

(8)

TPSM82913

SLVSGJ4A – OCTOBER 2022 – REVISED DECEMBER 2022

www.ti.com

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