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Original question:

TPS745-Q1: TPS745-Q1: VFB rised up to 0.75V, is it normal?

TPS745-Q1: Vfb out of spec. Get's worse after several power cycles

David D
Intellectual 580 points
Part Number: TPS745-Q1
Other Parts Discussed in Thread: TPS745

Hi,

I have the TPS745 used in the following manner.

After first power up, the Vfb was 0.567V (and the output 0.835V instead of 0.8V).

Which seems out of spec, though marginal.

Yet after several (10-20) power cycles the Vfb kept increasing till the output became equal Vin (1.78V).

Enable signal is driven by 1.8V open-drain buffer.

PG pull up voltage (V_3V3_MB) is up before Vin (V_1V8_MB).

Please advise what could be the reason for this failure.

Thanks,

David.

  • 0
    TI__Mastermind 38075 points

    Hi David,

    When you are doing the power cycling, are you cycling the input (V_1V8_MB) or toggling the EN pin (MS_AVDD_0V8_EN)? Is there any load on the device when you're doing these tests? 

    Did you use a DC meter or an oscilloscope to measure the output voltage?

    Thank,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 580 points

    Hi Nick,

    By power cycle I mean powering down the entire board, including 1.8V.

    There's a load of few tens of mA , <100mA.

    I measured with DVM and verified the voltage is stable with a scope.

    Thanks,

    David.

  • 0 in reply to David D
    TI__Mastermind 38075 points

    Hi David,

    I see. Did you continue to monitor the output voltage during both the on and off phase of the power cycle? I was wondering if the output was falling all the way to 0V between each power cycle. Either way this is weird behavior. 

    After cycling enough times that the device output equals the input, does it ever recover once powered for a while or if the board is unpowered for a while?

    Is this repeatable? I.e. have you put a fresh device on the board and ran the same series of tests? 

    Can you capture a scope shot of the startup?

    Thanks,

    Nick

  • 0 in reply to Nick Butts
    Intellectual 580 points

    Hi Nick,

    I'd like to update that we found out that the assembled part had push-pull PG stage, rather than intended OD.

    The PG was probably mistakenly driven by another device (which I omitted from the schematic for simplicity).

    We're investigating this further, but this issue most likely is not related to abnormal behavior of TPS 745.

    Thanks,

    David.

  • 0 in reply to David D
    TI__Mastermind 38075 points

    Hi David,

    Noted. I'll leave this post open in case there is still an issue that cannot be resolved on your end.

    Regards,

    Nick