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TPS63805: TPS63805 output abnormal issue

William Wu
Prodigy 70 points
Part Number: TPS63805

Tool/software:

Hi Team,

Currently, I'm using the TPS63805 buck-boost to stabilize the input 3.3V to output 3.3V,

and the output is facing unstable issue during the power-on (as below pic), 

the output appears that it stay at 1.2V for about 200us before continuing to ramp up,

I tried to measure the L(input/output), and it doesn’t work during this period, the Tramp and Tdelay is 4.47ms and 327.8us

After I separated the EN and VIN pin, we can see the issue will be solved, 

but just want to confirm, the reference in the SPEC mentioned the Vin can connect with the EN pin,

In our measurement today, we know that delaying or reducing the enabled voltage will solve this issue, 

But can I understand what might be causing this issue, even though we have followed the reference circuit? Thank you.

EN pin input after the Vin

Reduce the EN pin to 1.8V

  • 0
    TI__Mastermind 24485 points

    Hi  William,

    Seems uvlo_off triggered in your screenshot below, once uvlo_off triggered, device will shunt down and switching again after uvlo_on.

    Regards

    Tao

  • 0 in reply to Tao Zhang
    Prodigy 70 points

    Thank you Tao, 

    but as you saw on the waveform, the input voltage (green line) keeps raising without abnormal, 

    how come the uvlo_off will enable?

  • 0 in reply to William Wu
    TI__Mastermind 24485 points

    Hi  William,

    Actually, here you can see a voltage drop at input side when device start switching. 

    Regards

    Tao

  • 0 in reply to Tao Zhang
    Prodigy 70 points

    Hi Tao,

    Thank you, the input voltage is dorp to 1.5V

    but according to my waveform, there are two times drop, why the second drop didn't enable the UVLO?

    and when I reduce the EN pin to 1.8V, the input still has voltage drop, but the output is normal,

    are there any other parts related to this issue?

  • 0 in reply to William Wu
    TI__Mastermind 24485 points

    Hi  William,

    It is a normal behavior but not issue.

    Compare with different voltage of EN, you can see that the input voltage when device start switching is higher one(2V, while it is about 1.8V with higher input voltage). Then, device can accept a deeper drop.

    For twice input voltage drop, i think their actual minimum input voltage is different. Anyway, the root cause is uvlo_off.

    Regards

    Tao

  • 0 in reply to Tao Zhang
    Prodigy 70 points

    Thank you Tao,

    But what confused me is that the same circuit combination on the EVM does not have this drop issue (Vin & EN connected with input 3.3V)

    that's why I want to consult with you are there any other parts related to this issue, thank you

  • 0 in reply to William Wu
    TI__Mastermind 24485 points

    Hi  William,

    This is very depend on your test system, such as a long cable used between your power source. Long cable means high DC resistance and parasitic inductor and will cause a deeper drop.

    Also, in the EVM, a 68uF cap is mounted and will make your input drop much smaller.

    Regards

    Tao