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LMG5200: Keeping Upper GaN FET on Indefinitely

Nicholas Carley
Genius 9915 points
Part Number: LMG5200

If I want to keep the output of the lmg5200 high for a long time, the bootstrap will decay. So, can I put a 5.2 volt zener diode across the bootstrap cap, and bleed a little current into the HB pin, and keep the upper GaN fet on indefinitely?

  • 0
    TI__Expert 4830 points

    Hi Nicholas,

    thanks for your interest in the LMG5200.

    The method you just described would only partially work, this is because to turn on the HS FET fully you would need to be able to maintain the gate to 5V above the Source. Once the FET is on, the Drain and Source voltages are equal (i.e. for the High side FET you would be getting the bus voltage), and your gate would be 5V below this.

    This will cause the FET to maintain a certain level of resistance in the channel to maintain a drop to allow ~6V drop.

    To ensure proper operation you would need to generate a 5V reference above HS. If you cannot use the bootstrap for a long time the solution can be:

    1. Size the HB capacitor to maintain change above UVLO to the maximum on time

    2. Use an isolated supply

    3. Use a charge pump

    4. use the method you described but use a voltage that is at least 5V above your Vin pin.

    Hope this helps but if you need further assistance, please feel free to respond to this post.

    Best regards,

    Alberto