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TPS62812-Q1: negative voltage on output

Shinji Tsuji
Genius 12338 points
Part Number: TPS62812-Q1

Hi team,

I would like to know if there is any risk if Vin disconnected while there is a bulk cap (ex 1000uF) fully charged on output. 

I concern if output cap discharged through Low side FET and boosting VIN through the power inductor. 

Is there Low side sink current limit (negative valley current limit?) to limit this phenomenon?

regards,

  • 0
    TI__Genius 13440 points

    Hi Tsuji-san,

    There is the negative valley current is define in the datasheet which is 1.8A. However, device also have output discharge resistor that will help to discharge output caps faster.

    You can also place diode or load switch to avoid excessive reverser current through LS FET. Can you share application circuit ? it will help to understand in detail.

    Best regards,

    Tanvee

  • 0 in reply to Tanvee
    TI__Genius 12338 points

    Hi Tanvee, 

    This is a lesson learned type request for the devices used in general purpose, so there is no specific circuit in my mind now. 

    For the output discharge resistor, I think it is enabled only when disabling the device (EN=Low) and it will not work if Vin<UVLO correct?

    The case here is the sudden loss of supply and EN and VIN should drop at the same timing. 

    In this case, I cannot rely on the output discharge. Is my understanding correct?

    Could you explain how the LS negative valley current limit work? 

    Does it work cycle by cycle every time the low side sink current hit the limit?

    For example,

    1. remove VIN supply (VIN and EN drops at the same rate)

    2. LS FET sink current from bulk cap on output.

    3. Hit the LS negative valley current limit and LS FET close

    4. VIN is boosted through HS body diode

    5. LS turns ON at the next switching cycle

    6. repeat 2 to 5 

    regards,

  • 0 in reply to Shinji Tsuji
    TI__Genius 13440 points

    Hi Tsuji-san,

    Output discharge function will active till input supply will not drop below 2V. If device is in UVLO then discharge functional still work.

    LS current  will work cycle to cycle as you explain to discharge the remaining voltage on output and need more time to discharge. However, device can support 450uF capacitors on output and I don't expect it will have discharge problem during disabling devices.

    Best regards,

    Tanvee