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Original question:

TPS799-Q1: TPS799-Q1 unexpected output in EN low

TPS799: unexpected output while EN is low

Christian Lieber
Prodigy 30 points
Part Number: TPS799

Hallo experts,

was the issue "unexpected output in EN low" analyzed any further?

I experience the same issue in my current design, utilizing the TPS79925. 

For the picture below I left the output open, except for a 2.2uF ceramic cap.

If I attach the load, it looks very similar. Is this an expected behavior of this LDO?

Best regards,

Chritian

  • 0
    TI__Guru 50025 points

    Hi Christian,

    This issue has been documented in the past. From what I'm seeing, it seems that the issue only really exists in the fixed-output versions. 

    The explanation of this anomaly is that when the input voltage ramp rate is too slow, the internal circuitry is not properly biased until after the pass device has allowed some current through and charged the output. Speeding up the ramp rate seems to solve the issue. From your o'scope plot, I estimate that the rise time is about 2ms. In the documentation, speeding up the ramp rate to <50us solved the issue. Is it possible for you to speed up the VIN ramp rate? 

    Regards,

    Nick