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TPS7A92: Active discharge function

Yishan Chen
Intellectual 1670 points
Part Number: TPS7A92

Tool/software:

Hi,

Customer want to understand the active discharge function, but I don't see a lot of material in this function.

Could you please help me understand these questions?

When will the active discharge function start to work?

Enable low or Both the Input voltage and Enable low?

Will the active discharge function works when we don't have anything in input but we have 3.3V in output terminal?

what is the benefit of active discharge?

Thank you

Yishan Chen.

  • 0
    TI__Guru 50025 points

    Hi Yishan,

    The active discharge is active when the input is powered and VEN is low. If the input is not powered there is no voltage to turn on the pull-down transistor, so the input must be powered. 

    Yishan Chen said:

    Will the active discharge function works when we don't have anything in input but we have 3.3V in output terminal?

    This is not a recommended use-case. This device does not have a reverse current protection circuit, so current will conduct back to the input and there is a potential for damage if the current is not limited somehow. 

    Yishan Chen said:

    what is the benefit of active discharge?

    If the device is disabled with the EN pin when there is a very light load, the output voltage can take a long time to discharge (time constant = COUT * RLOAD), so the active discharge helps to discharge the output in cases like this. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Intellectual 1670 points

    Hi Nick,

    Thank you for your reply.

    Nick Butts said:
    Yishan Chen said:

    Will the active discharge function works when we don't have anything in input but we have 3.3V in output terminal?

    This is not a recommended use-case. This device does not have a reverse current protection circuit, so current will conduct back to the input and there is a potential for damage if the current is not limited somehow.

    So, this is not relevant to active discharge. At any moment in this application case, the current will conduct back from output to input terminal, right?

    Best Regards,

    Yishan Chen

  • 0 in reply to Yishan Chen
    TI__Guru 50025 points

    Hi Yishan,

    Yes that's correct. My point was that if they want the active discharge to *not* be on when the 3.3V is on the output, having no input does not solve the issue. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Intellectual 1670 points

    Hi Nick,

    I have received a couple of question from customer side and I think it is interesting.

    (1) No input but 3.3V output, Enable High.

    In this case, Will the output voltage conduct back to the input and cause the active discharge function working?

    (2) How much current can the body diode of MOSFET withstand? Will the TPS7A92 broke in case(1)? Will the current flowing through the body diode trigger the current limit?

    Thank you,

    Yishan Chen

  • 0 in reply to Yishan Chen
    TI__Guru 50025 points

    Hi Yishan,

    I believe they are correct - once enough current has conducted through the channel to the input, the internal circuitry may be powered. However, if EN is high the active discharge will not be on if the device does become powered. The guidance we usually give for reverse current is to limit the reverse current to 5% of the rated current, or 100mA for this device. In many cases the reverse current is not well-controlled, but if the 3.3V is only there from leakage current (from pull-up resistors or something similar where the OUT pin is not directly exposed to a power path) then the current can be minimal from that. 

    Yishan Chen said:
    Will the current flowing through the body diode trigger the current limit?

    I don't think so. I think the reverse current is unidirectional, and if the reverse current was on the order of the device's current limit then there will be worse issues than the current limit tripping anyways. 

    Regards,

    Nick

  • 0 in reply to Nick Butts
    TI__Intellectual 1670 points

    Hi Nick,

    (1) No input,3.3V at output, Enable Low

    Will this trigger the active discharge function?

    (2)They have a 3.3V in the output of TPS7A92 when they need to use JTAG to upload program

    I think this is directly exposed to the power path. Will this broke the body diode of TPS7A92 or any other forms of damage?

    Thank you,

    Yishan Chen

  • 0 in reply to Yishan Chen
    TI__Guru 50025 points

    Hi Yishan,

    Yishan Chen said:

    (1) No input,3.3V at output, Enable Low

    Will this trigger the active discharge function?

    I believe the active discharge will activate in this case once the input has charged enough.

    Yishan Chen said:

    (2)They have a 3.3V in the output of TPS7A92 when they need to use JTAG to upload program

    I think this is directly exposed to the power path. Will this broke the body diode of TPS7A92 or any other forms of damage?

    In that case they should add a diode from OUT (anode) to IN (cathode) to provide an alternative current path to protect the device, because yes the body diode can be damaged if the reverse current is not managed somehow. 

    Regards,

    Nick