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Original question:

LM74700-Q1: LM74700-Q1 - Can it drive two "back-to-back" FETs?

LM74700-Q1: LM74700-Q1

Gary Li
Prodigy 40 points
Part Number: LM74700-Q1
Other Parts Discussed in Thread: LM5060

Hi - I am using LM74700 as an ideal diode controller for reverse battery/current protection.   I am concerned about the inrush current since the body diode will conduct first. 

The other concern I have is the since the body diode of the N-MOSFET will always conduct, if the LM74700 fails for whatever reason, that will leave the N-MOSFET unprotected and potentially damaged.

Can LM74700 work with the below setup?

 

Thanks!

  • 0
    TI__Genius 16305 points

    Hello Gary,

    LM747000-Q1 on its own cannot drive the second MOSFET due its gate drive.

    MOSFET is turned off by LM74700-Q1 by shorting Gate to Source (Anode pin of LM74700-Q1).

    This means that second MOSFETs gate to drain voltage is 0V, so it is not off fully, instead acts a voltage follower (output voltage will be = Gate Voltage - Vth of MOSFET).

    We recommend to use LM5060 after LM74700 to drive the second MOSFET.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 40 points

    Hi Kari, 

    Thanks a lot for your response!

    I agree that the second MOSFET will not be fully off when the first MOSFET is turned off due to a reverse current. 

    Do you see any issues with my circuit for both of the MOSFET's to turn on when battery voltage is applied at C10 and EN pin of LM74700?

    Best Regards, 

    Gary 

  • 0 in reply to Gary Li
    TI__Genius 16305 points

    Hi Gary,

    During Turn ON also there is a concern due to the linear ORing mechanism, where source to drain voltage is regulated to 20mV by controlling the gate - source voltage.

    Gate voltage is controlled to regulate source to drain voltage to 20mV. Which means at low load current, gate voltage will be just above Vth and at higher load current, gate voltage is close to maximum. Please refer to the application section of datasheet for further explanation.

    When gate voltage is regulated, we will see impact of this for the second FET as well.

    We recommend to use LM74700 along with LM5060 for this application.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Prodigy 40 points

    Hi Kari, 

    Thanks for your clarification on LM74700, and recommendation of LM5060!

    According to the data sheet, LM5060 would be able to drive back-to-back N-MOSFET configuration for reverse polarity protection and in-rush control.   And if the LM5060 controller is at fault for any reason, the back-to-back MOSFET would not conduct, so there would be no current flow through the FETs.

    If LM5060 could fulfill the reverse protection functionality, do you see the need of having LM74700 together with LM5060 just for reverse battery protection?  In other words, what is the value of adding LM74700?   

    Best Regards, 

    Gary 

  • 0 in reply to Gary Li
    TI__Genius 16305 points

    Hello Gary,

    Yes LM5060 can drive back to back MOSFET for reverse polarity protection but it does not have reverse current blocking capability.

    As you need reverse current blocking, LM74700-Q1 will provide this.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    TI__Genius 16305 points

    Hello Gary,

    Let us know if you have more questions on this.

    If you think your question is answered, click "this resolved my issue" at the end.

    Regards,

    Kari.