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TPS65133: Soft Start Circuit for negative 5V

Michael Sauter73401
Prodigy 40 points
Part Number: TPS65133

Hello,

I'm testing the TPS65133 in my application and I have the same problems as discussed in the forum. The negative 5V is not working correctly because of the heavy load from my opamps. My opamps need about 70mA at -5V.

I cannot switch the opamps because the don't have any enable pin or power down pin.

Is there a possibility to add a soft start circuit for the negative 5V at the end of the TPS65133? I've seen that TI has several load switch IC but only for positive supply voltages.

What can I do?

Thanks, Michael

  • 0
    TI__Expert 6330 points
    Hi,

    I have notified our expert regarding this topic. Please expect a response by 2/12/19.

    Thanks,
    Aaron
  • 0
    TI__Mastermind 18550 points

    Hello Michael,

    TPS65133's VPOS and VNEG outputs are both current limited at startup (soft-start) and output voltage increases in a controlled manner so I don't think any additional soft-start circuit is needed. I powered up a TPS65133EVM with 75mA load present on VNEG at start-up and did not see any issues starting up and reaching regulation. Please see scope plot below. Do you have the recommendd component value capacitosr and inductors in your design?

    Kind Regards,

    Liaqat

  • 0 in reply to Liaqat Khan
    Prodigy 40 points

    Hello Liaqat,

    yes I have the recommended components. I tested the TPS65133 with the eval board. My input voltage is 3.3V and I don't get 75mA @-5V, it's about -4,4V. When I take normal load resistors the TPS65133 works better than with my opamps. I think the problem are my opamps, they don't act as a real load resistors at start up. This could be because the don't get the +5V and -5V at the same time. I have attached a scope shoot form the TPS65133 eval board with my opamps as load. The positive voltage reaches 5V and the negative voltage goes to +0,7V and the current of vneg is in the current limitation, I think. That's the reason why I'm asking about a soft start circuit or a load switch for the negative voltage.

    Is there a possibility to add a soft start circuit for the negative 5V at the end of the TPS65133 or a load switch?

    Thanks,

    Michael

     

  • 0 in reply to Michael Sauter73401
    TI__Mastermind 18550 points
    Michael,

    I am also not seeing load switches for negative supplies in our products portfolio.
    What if you disconnect positive supply from your op-amp load using a load switch and enable it only after the negative supply has reached regulation? I hope that this approach works for your application.

    Kind Regards,
    Liaqat