leads to a high frequency L-C double pole which causes the overall loop gain to stay high until the L-C double

frequency. Given the zero from the internal ripple generation network is a relatively high frequency as well, the

loop with very small output capacitance can have too high of a crossover frequency which can cause instability.

The internal zero is selected by the resistor at the MSEL pin, as described earlier.

In general, where reasonable (or smaller) output capacitance is desired, output ripple requirement and load

transient requirement can be used to determine the necessary output capacitance for stable operation.

For the maximum output capacitance recommendation, select the inductor and capacitor values so that the L-C

double pole frequency is no less than 1/100th of the operating frequency. With this starting point, verify the small

signal response on the board using the following criteria: The phase margin at the loop crossover is greater than

50 degrees. The actual maximum output capacitance can go higher as long as phase margin is greater than 50

degrees. However, small signal measurement (Bode plot) should be done to confirm the design.

If MLCCs are used, consider the derating characteristics to determine the final output capacitance for the design.

For example, when using an MLCC with specifications of 10 µF, X5R and 6.3 V, the derating by DC bias and AC

bias are 80% and 50%, respectively. The effective derating is the product of these two factors, which in this case

is 40% and 4 µF. Consult with capacitor manufacturers for specific characteristics of the capacitors to be used in

the application.

For large output filters with an L-C double pole near 1/100th of the operating frequency, additional phase

boost might be required. A feedforward capacitor placed in parallel with RFB_HS can boost the phase. Refer to

the Optimizing Transient Response of Internally Compensated dc-dc Converters With Feedforward Capacitor

application report for details.

Besides boosting the phase, a feedforward capacitor feeds more VOUT node information into the FB node

through AC coupling. This feedforward during load transient event enables faster response of the control loop

to a VOUT deviation. However, this feedforward during steady state operation also feeds more VOUT ripple and

noise into FB. High ripple and noise on FB usually leads to more jitter, or even double-pulse behavior. To

determine the final feedforward capacitor value impacts to phase margin, load transient performance, ripple,

and noise on FB must all be considered. TI recommends using frequency analysis equipment to measure the

crossover frequency and the phase margin.

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TPS54KB20

SLVSGP3 – MAY 2023

Copyright © 2023 Texas Instruments Incorporated

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