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TPS746: Version with Shorter Power-Good Rise Delay Not Turning On

Allan Englehardt
Prodigy 10 points
Part Number: TPS746

In production we have been using the TPS74601PBDRVR for about a couple years with no issues. Due to supply constraints, we just swapped in TPS74601PDRVR (no B) in place of TPS74601PBDRVR.

Our recent production run using TPS74601PDRVR has a very low yield (< 10%). On the failing units we do not see any output from the TPS74601PDRVR regulator; not even an aborted start. Removing TPS74601PDRVR and replacing it with TPS74601PBDRVR makes the circuit function as intended. This swap has been performed 10 times, and always fixed the problem.

The only advertised difference between the "B" and "non-B" versions of the part is the "Power-Good Rising Delay". I don't see how that difference alone can cause this issue. The PG pin is pulled up through a 10kohm resistor to the input voltage (+3.8VDC). The PG signal isn't used anywhere else in the design.

For what it's worth, the output voltage of the regulator is +3.3VDC. The EN line is pulled up to the input voltage (+3.8VDC) through a 4.7kohm resistor.

Are there any ideas on why these two parts could be behaving differently?

  • 0
    TI__Genius 12040 points

    Hello Allan,

                       Has this issue been observed across different units, in other words was a different unit selected when the swap was performed? What is the load current and the operating ambient temperature? Could you share a schematic showing the feedback resistors, input and output capacitors? Thanks!

    Regards,

    Srikanth 

  • 0 in reply to Srikanth Lolla Venkata
    Prodigy 10 points

    Hello Srikanth,

    Thank you for your reply.

    The swap was performed on several different circuit card assemblies. To clarify the circuit cards where we did the swap all had the same circuits implemented, but they were physically distinct cards. On all 10 of those distinct cards swapping to TPS74601PBDRVR fixed the issue.

    The load current is ~20mA. The issue was seen at room temperature (around 20C). The part will need to be operational in the full industrial temperature range.

    Below is a re-created schematic that is annotated with the information requested.

  • 0 in reply to Allan Englehardt
    TI__Genius 12040 points

    Hello Allan,

                      I don't see anything wrong with the schematic. Barring soldering mistakes (unintended shorts, unclean boards, leakage paths etc.) and the device entering thermal shutdown, there is no reason why the device (TPS74601PDRVR) shouldn't work. Its rather unlikely that there would be soldering mistakes on 10 different units. Could you provide us scope shots showing VIN, VOUT, VEN and IOUT at start-up as well as steady state for both TPS74601PBDRVR and TPS74601PDRVR? Thanks!

    Regards,

    Srikanth