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TPS82085: TPS82085 output capacitance for FPGA application

JasonT
Intellectual 260 points
Part Number: TPS82085
Other Parts Discussed in Thread: TPSM82813

I am using an FPGA which shows the following decoupling requirement for a power rail that I would like to source from a TPS82085:

two 330uF ceramic

one 100uF ceramic

five 47uF ceramic

three 10uF ceramic

three 470uF aluminum polymer

The polymer caps are supposed to be placed near the regulator. The maximum current draw on this rail is 3A but will be closer to 2A in my application. I have submitted a question to the FPGA supplier about the polymer caps since they take up a ton of board space and don't seem to provide any benefit. My other concern is that the roughly 1000uF of ceramic capacitance might be an issue for the TPS82085 at startup. There should not be any load on the supply as it ramps since the FPGA logic will not be operational until well after the supply is stable. I noticed this comment in the TPS8205 spec:

"Values over 150 μF may be possible with a reduced load during startup in order to avoid triggering the Hiccup short circuit protection."

Is 1000uF going to work here, assuming the load is <100mA during powerup ramp?

Thanks - Jason

  • 0
    TI__Guru*** 142750 points

    Hi Jason,

    Yes, it would be good to ask the FPGA vendor why so much capacitance is required.  This is too much for most regulators and unnecessary.  Modern regulators have much higher bandwidths and much better transient response and do not need so much bulk capacitance.

    Yes, my app note explains a possible issue with startup with large output caps:  In addition, we need to make sure the regulator is stable with such output caps.  This would need to be tested on your system or an EVM.

    Finally, you might look at the newer TPSM82813 power module.  This supports larger amounts of output caps and has a SS pin to avoid startup concerns.

  • 0 in reply to Chris Glaser
    Intellectual 260 points

    Hi Chris,

    Thank you for replying so quickly.

    It looks like the TPSM8281x availability is limited but I will contact sales about that.

    Jason

  • 0 in reply to JasonT
    TI__Guru*** 142750 points

    Yes, it is sampling and should release this quarter.

    It also has a 4A version, which is pin to pin, in case you need more current.