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Always On LDO with TPS65320

Walter Brumm
Prodigy 160 points
Other Parts Discussed in Thread: TPS65321-Q1, TPS65320-Q1

I am powering a low power MCU (3.3v) and ADC (5v ref) from a switched 12v supply, with a dirty 5v standby supply.  My current implementation is to use the TPS65320 running a 5v buck, and a 3.3v LDO.  Running off 12v, this gives a clean 5v (after additional filtering) and 3.3v rail.  

I'd like to use one of the TPS211xA power muxes to switch between the standby rail and buck output.  We tried applying an input voltage to only VIN_LDO using a diode-OR configuration, and appeared to fry the PMIC.  I'm not sure if this was a bench error, or if the auto-sourcing circuitry was not made to be operated in this fashion.   Are there any foreseeable issues with this approach?  

(I didn't see anything in the datasheet about operating them separately)

Thanks.

  • 0
    TI__Genius 16410 points
    Hi Walter,

    Let me move this to the automotive forum. I've assigned your question to the appropriate applications engineer, and he will respond.

    Regards,
    Karl
  • 0 in reply to Karl Wallinger
    TI__Genius 9655 points

    Hi Walter,

    I'm afraid, but this operation is not supported. The VIN pin needs to be supplied at all times as it also powers the internal biasing (see pin description).

    Furthermore, the LDO will always use VIN as a supply when Buck is disabled, even if a voltage is present at VIN_LDO.

    I think for your approach, the TPS65321-Q1 is better suited, as it does not have the auto-source-function but has LDO and Buck independent (and high input voltage range for VIN_LDO), however, even here VIN needs to be supplied for LDO-operation as well.

    Best regards,

    Frank

  • 0 in reply to Frank Dehmelt
    Prodigy 160 points
    Thanks. Just out of curiosity, is a reliability difference between the TPS65320 and the TPS65321? I would think the 65321 would still have the same single point of failure as it shares the same die.
  • 0 in reply to Walter Brumm
    TI__Genius 9655 points
    Walter,
    the parts do not share the same die, even though there are a lot of similarities.
    For reliability data, please see
    www.ti.com/.../estimator.tsp
    (within product folder select "Quality & packaging).
    Best regards,
    Frank
  • 0 in reply to Frank Dehmelt
    Prodigy 160 points

    Hi Frank,

    Sorry for the confusion, I was referring to the individual supplies on the IC.  Ideally I would like the LDO and buck to fail separately, though I understand if the die overheats, both supplies will fail regardless.  That said, In the event that the buck is out of regulation, would the TPS65320 auto-source from Vin?  That would seem to be as robust as the 65321 but still offer higher efficiency during normal operation.

    Thanks.

  • 0 in reply to Walter Brumm
    TI__Genius 9655 points

    Hello, Walter,

    in that respect yes, they share the same over-temperature protection, see datasheet section 7.3.1.15 for TPS65320-Q1 and section 7.3.3 for TPS65321-Q1.

    Best regards,

    Frank