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LM51521-Q1: High side MOSFET acting as diode

Part Number: LM51521-Q1


I'm having an issue with my LM51521-Q1 boost circuit. Here's the schematic of the circuit:

Not pictured is a 380uF capacitor on the input side, and an additional 1000uF on the output side.

The circuit boosts to the desired voltage, however the high side MOSFET seems to behave as a diode instead of a FET, getting very hot as a result. I'm measuring a V_sd of around .8V and a V_gd of around .8V. V_th of the MOSTFET is 2V. How can I increase the voltage of the gate to increase Vgd past V_th?


  • Hi John,

    Thanks for using the e2e forum.
    To further look into this, could you share the following design details?
    - the input voltage range
    - output voltage
    - output load
    - inductance

    Thanks and best regards,

  • The input voltage range I believe is set from around 4V to 10.5V, Output voltage is 13.8V, Output load during the test was 2A, and the inductance was 3.3 uH. The maximum output current was configured to be around 8A.

    When testing at an input voltage of 9V, and output load of 2A, the high side NMOS was reaching case temperatures of 140F, and at 3A was reaching around 190F.

  • Hi John,

    Thanks for the update and the data.
    The spec of this application look okay to me and I did not see any problem within the schematic either.
    I noticed you are operating the device in Skip mode. (MODE pin is floating)

    Could you change the mode to FPWM operation? To do this, please connect the MODE pin to VCC or put an external source of >2V to the MODE pin.
    This way the high side switch should be forced to keep operation.

    Best regards,