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TPS76633 - LDO output too high

JamieB
Prodigy 170 points
Other Parts Discussed in Thread: TPS766

Hi

I have 3 instances of a new test pcb which use TPS76633 to produce my 3V3 rail.

1 board gives me 3V3, the other 2 produce an output higher than 3V3 that seems to roughly scale with the input.

An input of 3V9 gives an output of 3V5 (Obviously should be 3V3)

An input of 6V gives an output of 4V8 

 

I have tried varying the ESR series resistor - 5R,10R,20R doesnt seem to make any difference

PG pin is high on both working device and non-working device 

Device is cool to the touch so I assume thermal shutdown is not the issue

Im not sure if the device could be switching off due to internal current limiting. Measuring resistance from 3V3 to GND with power off shows the same on working test PCB and non-working PCB so it isn’t a short on the board drawing massive current and causing the Reg to shutdown.

Scope seems to show switching on and off of the output (See pic below)

I would massively appreciate any suggestions on this one.

Thanks

Jamie

  • 0
    TI__Guru 55940 points
    Hi Jamie,

    Is there anything downstream from the LDO during your measurements?

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 170 points

    Hi Ryan

    Thanks for getting in touch

    Yes the 3V3 regulator powers:

    - EEPROM

    - Tactile switch

    - Resistor divider

    - Motor driver (Motor is disconnected)

    - Bluetooth module


    And the 3V3 regulator provides reference for:

    - Clamping 2 off board inputs (Inputs currently disconnected)

    - reset signal pullup

    - I2C pullups

    I swapped the actual IC between the board and works and the one that doesn't and there is no effect.

    i.e. the working board still works and the failed board is still failed.

    This obviously points to something in the rest of the system being different however the boards are brand new, I cant visibly see any differences

    The resistance from 3V3 output to GND with power turned off is the same on both boards, so it doesnt seem to be a power short.

    The question is what could you connect to the output of the 3V3 reg to make it exhibit the behavior shown in my screenshot where the output keeps rising higher then 3V3 then resetting to 3V3 in a repetative manner..

     Any help would be appreciated

    Thanks

    Jamie

  • 0 in reply to JamieB
    TI__Guru 55940 points
    Hi Jamie,

    It sounds like something downstream is biasing the output downstream. From your waveform, it looks like the low point is ~3.3V. This implies that something downstream is charging and discharging.

    Very Respectfully,
    Ryan
  • 0 in reply to Ryan Eslinger
    Prodigy 170 points

    Hi Ryan

    Thanks for your message I really appreciate the help.

    As the regulator is rated for 250mA should it not be able to supply something charging and discharging with 250mA without its output changing from the fixed 3V3?

    Do you think there is something drawing a big surge current larger than 250mA?

    Thanks a lot

    Jamie

  • 0 in reply to JamieB
    TI__Guru 55940 points

    Hi Jamie,

    TPS766 can supply 250mA; however, the LDO does not sink current.

    My intention was to point out that the voltage appears to be biased higher than the regulated 3.3V for most of your scope shot.  The low point of 3.3V is the only point on the curve where the LDO is regulating.  When the output is biased higher than 3.3V by something downstream, the LDO is not able to regulate the voltage as it does not have sinking capabilities.

    Very Respectfully,

    Ryan

  • 0 in reply to Ryan Eslinger
    Prodigy 170 points

    Hi Ryan

    Ok that makes a bit more sense to me.

    I have fount if I hold my bluetooth module (which is powered from 3V3) in reset, the 3V3 regulates correctly.

    I should be ok now Ive isolated the issue.

    Thanks for all your help, it is very appreciated.

    Best regards

    Jamie