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LM51521-Q1: When Vin higher than Vout

Jeff YANG
Genius 5345 points
Part Number: LM51521-Q1

Hi,

For LM51521-Q1, If  I set Vout = 16V, if my input using >16V, is it will turn on low side MOSFET to keep Vout = 16V?

Thanks!

Jeff

  • 0
    TI__Guru 59825 points

    Hello Jeff,

    The device is not able to keep the output voltage below the input voltage as the backgate diode of the top FET directly connects the input to the output.

    So no, the low side FET is off and the high side FET is on to limit losses.

    Best regards,
    Brigitte

  • 0
    TI__Genius 10385 points

    Hi Jeff,

    Thank you for using E2E forums, In LM51521 the bypass feature is disabled, meaning when the VIn is one diode drop off voltage above the Vout the device will not go into bypass. In other words, when Vin>Vout+Vf(High side FET's body diode) the device will not go into bypass, which means the high side FET always ON, but instead the high side FET will be off and the Vout will be Vin - Vf(high side FET's body diode). In conclusion to answer your question, no the Low side FET will not be on when your Vin is >16V, but instead you low and high side FET will be OFF and the device will not be boosting and Vout will be equalt to Vin - Vf(body diode of high side FET) so Vout will be Vin - (~0.6)V.

    Hope this helps.

    BR,

    Haroon

  • 0 in reply to Haroon Wardak
    Genius 5345 points

    Hi Brigitte,

    What the difference for LM51521 and LM5152? LM5152 have bypass mode, so means LM5152 will short the body diode to reduce the Vf disapassion?

    Thanks!

    Jeff

  • 0 in reply to Jeff YANG
    TI__Guru 59825 points

    Hello Jeff,

    As long as you enable the charge pump for the high-side drive on pin 14, yes, LM5152 will turn on the high-side FET for lower power dissipation.

    Best regards,
    Brigitte