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LM5109A: Is there a pull-down to hold the upper FET off ?

George Pontis46
Prodigy 100 points
Part Number: LM5109A

I am concerned about the possibility of an undefined state when both inputs are low for a long time. Under this condition, I wonder if FET or circuit leakage could pull the top gate positive slightly. If so, then the gate could eventually charge up enough to enhance the MOSFET slightly, which would then start pulling up the midpoint of the totem pole and HS. Once HS goes above VDD, there is no bias for the top driver and thus no way for it to hold the MOSFET off.

Some gate drivers come with a recommendation to include a high value gate-source resistor to address conditions like this. I don't see any reference to this in the data sheet.

Do I need to include a gate-source pull-down to protect against this condition ?

  • 0
    TI__Mastermind 23025 points

    Hello George,

    There are a couple of details to consider regarding the driver keeping the output in a low state with 0 or low voltage bias applied (below VDD or VHB).

    The internal driver stage is a pull up and pull down MOSFET and each has a body diode as the power MOSFETs. With 0V bias, there is a body diode path from the driver output to the bias capacitor which will clamp the driver to a diode Vf to VDD or VHB. There are internal initialization circuits that force the driver output stage into the off state which is effective when VDD or VHB is ~1.5V, which is the typical threshold voltage for the internal driver devices, When VDD or VHB is ~1.5V the driver is forced into the off state until the UVLO is achieved. At this time the driver output will follow the driver inputs.

    I think it is good practice to have a  gate to source resistance mainly to address leakage current that may exist from the MOSFET drain to gate.

    Please confirm if this addresses your questions or you can post additional questions on this thread.

    Regards,

  • 0 in reply to Richard Herring
    Prodigy 100 points

    Thank you for this quick response. I will address this in the next spin of my design. It might be worth a paragraph in the data sheet to mention the concern and possible solution.