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TPS562210A: Soft-start time formula discrepancy. Power-good output

Damian Popielarski28
Prodigy 70 points
Part Number: TPS562210A

Hi

My design rest on the soft-start time and the delay of power-good. I see a strange discrepancy in the soft-start timing.

  • I set 10nF as soft-start timing capacitor.
    • Tss calculated from formula in datasheet is 1ms
    • Measured Tss is about 2ms.
  • Eval board use 8.2nF as soft-start timing capacitor.
    • Charts from evaluation of example design in datasheet shows, that for SS capacitor Tss is about 2ms.

Please state correct formula for soft-start design.

State the accuracy of soft-start circuit over various conditions: 

  • Css charging current tolerance.
  • PG delay accuracy.
  • PG trigger thershold accuracy is stated already.

There is a second issue with the TPS562210A datasheet. The description of Power good delay timing is not as clear as should be.

  • Please draw chart that explain timings.One picture will be more easy to interpret than 10 paragraphs.

Third issue: Charts of PG outputs in datasheet and EVM evaluation are wrong. The real behaviour is different. PG signal is held low, until some time pass after 100% of Vout. Please correct.

BR

  • 0
    TI__Genius 9550 points
    hi Damian,
    I am looking into these questions and will give you feedback on Monday.
  • 0
    TI__Expert 4875 points

    Hi Damian,

    1. For soft start time, according to the system engineer who define the SS time equation, the SS time should be measured from 10% to 90% Vout rise time.

    2. Css charging current tolerance.   Please refer to datasheet EC table

        PG delay accuracy. 1.7 times soft start time. Please refer to below figure for demo

    3. The PG waveform is wrong in datasheet. Thanks for pointing out. The validation team connect PG pull up resistor to Vout. We are discussing on how to correct the datasheet for the TPS56x21xA family. Thanks!