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Original question:

TPS610981: TPS610981

TPS610981: Startup Latch-Up with Vin Current Limit

Stefan Wyss
Prodigy 80 points
Part Number: TPS610981

I have the following (simple) setup:

- TPS610981EVM-674 Rev.A Evaluation Module
- 100 Ohms resistor at the Output between J4/Vmain and J8/GND
- JP1 Low
- Bench Power Supply (SUP) with V_sup = 3V between J1/Vin and J3/GND

If I setup the current limit of my Bench Power Supply to e.g. I_sup_max = 30mA and then connect the EVM, the TPS610981 seems to permanently short V_sup to ~0.73V. If I then increase the current limit to e.g. I_sup_max = 100mA, the short circuit stays(!!!) Until I further increase the current limit to approximately I_sup_max = 140mA, the permanent short disappears and V_sup return to 3V normal.

It seems that when the supply has high output resistance (at first), the TPS610981 falls into a short circuit mode from which it is not possible to recover easily (at least not with a battery with relatively high output resistance).

So my questions:

1) What happens here?
2) Why do I need ~140mA to recover from this startup condition?
3) How can I prevent this from happening? (Assuming that my supply has a relatively high output resistance at first and I can not change this).

Thanks for your support,
Stefan

  • 0
    TI__Mastermind 21565 points
    Hi Stefan:
    Thanks for your details description. I thought the key question from you is: "What's the maximum input current that the TPS610981EVM need?"

    The device of power supply are different, and it may reach the limit of the device. Have you test the current of the inductor and input? Please share the waveforms with me for better analysis.
    It doesn't matter if you haven't the current waveform. I will do the bench test after I get the EVM, and I will give the comments back within a week.
  • 0
    TI__Mastermind 46905 points
    Hi Stefan,
    please try to remove the 100ohm before the device startup. as the device switches from low input voltage, it is normal to see this behavior if the loading is large before the device finishes startup.
  • 0 in reply to Minqiu Xie
    Prodigy 80 points

    Hi Minqui Xie

    Thanks for your reply and suggestions.

    I think my key question is: "What's the minimum input current that the TPS610981EVM needs?"

    I have recorded the waveforms of Vin and Vmain when the TPS610981EVM is in this misterious "short-circuit mode".

    It is very strange. If I alter the current limit of the supply below 60mA, it shows the following waveform:

    But if I turn the current limit of my supply above 60mA, it shows the following (I_sup_max = 100mA):

    Thanks for helping me clarify this.

  • 0 in reply to Jasper Li
    Prodigy 80 points
    Dear Jasper Li

    In my final product, I will need a current of ~30mA at the output Vmain. So it is not an option to remove the 100ohm resistor. My final product will also have large output capacitance that needs to be charged at startup.

    Are you saying that the TPS610981 is not capable of starting up with (relatively) large loads? (The switch peak current of the TPS610981 is at least 350mA according to the datasheet.)
  • 0 in reply to Stefan Wyss
    TI__Mastermind 46905 points
    Hi Stefan,
    although the final loading is 30mA or higher, the behavior of the real loading is not the same as a resistor. many system load can be disable before the output voltage is ready. the large capacitance is also not the same as a resistor.

    the device limits its peak current at 350mA. its average current is approximately 300mA. the input power depends on the input voltage.
    at only 0.7V input voltage, the maximum input power is 0.7*0.3=0.21W, which means 63mA@3.3V output current if not considering efficiency.
    if the input voltage increase to 1.4V, the output current can be doubled.

    not sure about the input voltage range in your application. but we can get more energy if waiting for the input voltage ready before applying large loading.
  • 0 in reply to Jasper Li
    Prodigy 80 points
    Hi Jasper, and thanks for your reply and explanation.
    I think I will have to follow your recommendation of disabling the system load before the output is ready. This should resolve my issue. Thanks again!