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TPS7A57: noise source, seems like some resonance?

Harrison Le
Prodigy 20 points
Part Number: TPS7A57

Tool/software:

Hi - using the below schematic, we supply our IC with 0.75V as its core voltage. Further downstream on the IC, there is an additional 290uF outside of the package, and 100uF on the package (feedthrough caps in a shunt configuration). With this, we consistently see a 25kHz noise on the supply, roughly 5mVpp - as current increases, the peak to peak voltage increases (at about 1A draw, it's 40mV).

Is there anything in the LDO that might help explain this phenomenon? Thank you!

  • 0
    TI__Expert 5080 points

    Hi Harrison,

    Thank you for reaching out to us!

    This sounds like a stability problem. Devices will oscillate if they don't have a positive phase margin.

    With that many output capacitors, the stability problem is usually because capacitance is too large or the ESR has dropped below the minimum requirement. Given that I'm only adding up to 1680uF on a part that should be able to take 3000uF, ESR may be the culprit depending on the capacitors you used. This device also has a total impedance ESL requirement for stability that could be contributing.

    • What type of output capacitors are you using?
    • Are you seeing a clean sinusoid?
    • At 1A/40mV, is it still 25kHz or has the frequency shifted a little bit compared to 5mVpp?
    • Just to confirm, when you say you supply your IC with 0.75V, this means a downstream IC is being powered by the 0.75Vout of the LDO and the LDO itself is being powered by a 1.4V rail? 

    Best,

    Gregory Thompson

  • 0 in reply to Gregory Thompson
    Prodigy 20 points

    Hi Gregory -

    - Mixture of Al poly caps and ceramic caps (X5R/X6S/X7R). 

    - No, it's more sawtooth.

    - Frequency is still 25kHz

    - Correct

    Thank you for that particular chart as well. ESL did not come to mind.

  • 0 in reply to Harrison Le
    TI__Expert 5080 points

    Hi Harrison,

    Thank you for the answers, it seems like it's best we investigate this as a stability problem.

    Try removing all the output capacitors except for a 100uF one and see if the issue persists.

    Best,

    Gregory Thompson