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TPSM82864A: Output Capacitor vs Output load capacitance & loop stability

Brad Browning
Prodigy 10 points
Part Number: TPSM82864A
Other Parts Discussed in Thread: TPSM8287A06

I have a question about the TPSM82864 loop stability due to load capacitance.  Section 9.2.2.2 of the TPSM82864A/6A datasheet states the following about the output capacitors.

The capacitor value can range from 2 × 22 µF up to 150 µF. The recommended typical output capacitors are 2 × 22 µF or 1 × 47 µF with an X5R or better dielectric. Values over 150 µF can degrade the loop stability of the converter.

If the output capacitors are two 22µF but the rail powers a load with 330µF.  Assuming this is all within a 3" x 3" area with a power plane and ground plane.  Will that cause issue with the control loop stability or is this more for for the output caps right next to the TPSM82864. 

My understanding is the extra capacitance will dampen the loop response.  This will slow down the response but not cause instability issues.

Is there anything else in this size that would have a higher tolerance for large capacitance loads?

  • 0
    TI__Guru*** 146000 points

    Hi,

    Yes, you should check the stability on an EVM, but it should be ok in my experience.  There is a stable range outside of the 150uF limit.

    Is it a single 330uF ceramic or a tantalum or electrolytic cap (with ESR)?  The ESR helps stability a lot.

    You can also improve stability by adding a Cff, like in this app note: https://www.ti.com/lit/an/slva466a/slva466a.pdf 

    What requires so much capacitance at its input?  Can you reduce the 330uF?

    This is quite a small module, but we do have the TPSM8287A06 which supports much more output cap and has remote sense and I2C to make it a better fit for powering processor core rails, for example.

    Thanks,

    Chris