• State
  • Locked Locked
  • Replies 6 replies
  • Subscribers 64 subscribers
  • Views 334 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TPS79133 issue when disabled

MikeW9894
Intellectual 540 points

Hi,

I'm using the TPS79133 in a fairly straight forward application.  The chip is connected to a 3.7V LiPo battery with the enable line pulled up to the battery voltage through a 499K resistor (also tried 49K9).  An N-channel FET connected to a power button provided the low signal on the enable line to turn the regulator on.

The issue I am noticing is that most of the time when the regulator is off (enable pin pulled high), the output sits at about 0.7V.  It doesn't seem to affect the power and the off current is about 60uA.  I've tried shorting the enable pin right to the battery while the FET was not enabled but there was no change to the output.  I've tried shorting the output of the regulator while it was disabled and it pops right back up as soon as I open it back up.

Is there anything I can do about this?  Is it even a concern?  I feel like that 0.7V sitting on the power pins of the controller ICs in the circuit might be making some operations act a little funny between power cycles but I haven't narrowed anything down.  It just doesn't feel like a clean design based on this part.

 

Thanks,

Mike

  • 0
    TI__Genius 9620 points

    Hi Mike,

    To answer your question regarding the effect of the .7v on the TPS79133, this should have none on the device.

    We would ask if you can provide more information related to the application such as a partial schematic?    Are you using our EVM?

     

  • 0 in reply to Jack Bourke
    Intellectual 540 points

    Hi Jack,

     

    I'm not sure how I would post a schematic to you online but it is such a simple circuit that I could describe it in a second.  I'm not using your EVM but I can tell you my values based on that schematic.  Since it is not adjustable, R3 and R4 do not exist.  R2 doesn't either.  C1 and C3 are 0.1uF.  C2 is 4.7uF.

    VIN is from 3.3V (dead battery) to 4.2V and of course this is a fixed 3.3V output part.  An N-channel FET drain pin is connected to the EN pin.  Source is grounded and the battery voltage applied to gate creates the low level on the enable pin since it is one of your few active low enabled regulators.  A 499K resistor pulls the EN pin to the battery voltage and keeps it off when not needed.  Most of my testing is done with a battery voltage of about 3.8V since it is a pretty low power circuit and the battery holds up for weeks.

    I feel like there have been times that I have measured the output voltage when off and it was at zero but most of the time it's pretty steady at 0.7V.  I don't know why though.  It looks like a standard diode drop voltage and maybe just what gets through the pass transistor in the LDO somehow.

     

    Thanks again!

     

     

    Mike

  • 0 in reply to MikeW9894
    TI__Expert 8890 points

    Hi Mike,

    And what is the output load?  Or better, could the 0.7V be leakage from the load?

    Bill

  • 0 in reply to Bill Stokes
    Intellectual 540 points

    Hi Bill,

    The load ranges from 6mA to 60mA and is under 30mA most of the time.  It's just some RF circuitry and a microcontroller.  This LDO provides the power supply for them so if this supply is off, nothing could be active.  I have another board without the LDO installed that gets its power from an external supply and there is no enable pin on that one so I can't test similar conditions.

    I just did some more testing and found some interesting stuff.  After turning the power off, the LDO output was at 0.77V, board current at about 54uA.  Wait 30 seconds or so and the output starts dropping off slowly.  After 10 seconds more the output had dropped to about 34mV.  This is why I was seeing near zero sometimes but usually I am checking right after turning the power off.  The output was also near zero upon first connecting the battery to the circuit.  Something on the supply output must be causing this and I will have to check my design more.  I have an idea about where this is happening and I do have jumper resistors I can remove to isolate circuits.

     

    Thanks for the help,

     

    Mike

  • 0 in reply to MikeW9894
    TI__Genius 9620 points

    Hi Mike,

    We tested the device in the lab with  510k ohm resistor and other sizes and had no voltage or current on the output with the regulator off.

    You might want to check over the application to see if maybe the rf circuitry or micro are causing the situation.

  • 0 in reply to Jack Bourke
    Intellectual 540 points

    Hi guys,

    I think the problem is with this FET.  I will continue testing when I order some new parts but I thank you again for testing things.

     

    Mike