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Original question:

LM5121: Disconnection switch of LM5121

LM5121: overload condition during V_RES=2V~4V in figure

Ben Kuo
Expert 7550 points
Part Number: LM5121

Dear team,

According to datasheet Figure 28, H/L side FET will turn-of  first when device trigger to OC.

Circuit breaker function is enable, if load current still rise up.

During this OC condition(V_RES=2~4V in datasheet figure28), current will pass to load through HS FET bode diode. Is this a risk to damage MOSFET?

If yes, how to prevent this issue happened?

If no, why this issue will not happened?

Looking forward your feedback.

Regaards,

  • 0
    TI__Mastermind 21720 points

    Hi Ben,

    Thanks for reaching out with your question and for considering the LM5121 in your design.

    If the over current condition is triggered  (Vcs = 160mV)the breaker FET will be turned off. When the breaker FET is turned off no current will flow through the high-side switching MOSFET. This is due to the breaker FET disconnecting the input voltage from the inductor.

    Please let me know if you have any questions.

    Thanks,

    Garrett 

  • 0 in reply to Garrett Roecker
    TI__Expert 7550 points

    Hi Garrett,

    We have test with LM5121 EVM, the protection behavior is the same as datasheet.

    H/L side FET is turned off when trigger to OC, and FET of circuit breaker is still turned on.

    So all the current pass to output via body diode of HS FET and heat up HS FET.

    only if we increase current(OC+ delta-I), and then circuit breaker function is enable to disconnect VIN-Vout.

    Can we avoid HS FET heat up condition??

    Regards,

  • 0 in reply to Ben Kuo
    TI__Mastermind 21720 points

    Hi Ben,

    In the condition of operation where the breaker FET is still enabled and the switching FETs are off current can flow through the high-side switching FET. One way to work around this is finding a MOSFET that has a body diode with a low forward voltage drop. Another way to is add a schottky diode across the high-side MOSFET. This ensures that the high-side MOSFET will not over heat in the condition you mention. 

    Please let me know if you have any questions.

    Thanks,

    Garrett