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LM74704-Q1: En pin

Ohad Fundoianu
Intellectual 320 points
Part Number: LM74704-Q1
Other Parts Discussed in Thread: LM74800-Q1,

Tool/software:

Hello, 

Regard using the En pin for disabling the Ideal Diode.

In case that En='0' which means disabling the Ideal Diode Gate drive and put the Ideal diode in Shut down mode. The N-channel MOSFET Vds is still keeping the same value as before. Then what we get is that FET tunnel is closed while all the Vds voltage is now falling on the N-channel FET bulk diode like a forward voltage while gate is closed. Is that a legal state where the FET diode is in high forward voltage Vds=12v of a battery for example while its gate is lock and FET tunnel is closed ? Won't there be a leakage current through the bulk diode? It is actually a diode under high forward voltage of 12v. It sounds suspicious. 

BR, 

Ohad

  • 0
    TI__Mastermind 26035 points

    Hi Ohad,

    When EN= '0', body diode will still conduct and the VDS will be <1V. Power dissipation will be high = 0.7 x ILOAD. When EN is low, high load through the body diode is not favorable.

    Regards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 320 points

    Hi Shiven, 

    So it doesn't really deactivate the FET. From your answer I understand that the Load (on the Drain) will still be connected to the Source and the I-load current will keep flow from Anode to Cathode this time through the bulk diode instead of the Rds-ON tunnel which will even make more dissipation. If so than what is the purpose of the En pin? Why is it use-full?    It doesn't really cut-off the FET by disabling the Gate. 

    Thanks, 

    Ohad

  • 0 in reply to Ohad Fundoianu
    TI__Mastermind 26035 points

    Hi Ohad,

    It is true that the path is not cut-off.  LM74704-Q1 is developed to replace a diode in power path to minimize power dissipation. If you also need to cut-off the power path, you will need a controller that can drive back-to-back FETs like LM74800-Q1. EN pin exists when the system is in shutdown and low Iq is expected. When EN is low, all the internal circuits of LM74704-Q1 are shutdown and very low Iq is achieved.

    Rehgards,

    Shiven Dhir

  • 0 in reply to Shiven Dhir
    Intellectual 320 points

    Hi Shiven, 

    Thanks for the information. I looked over this device, it still doesn't have an option of just "Cut-off" while VBAT still exists. As the LM74704-Q1 it has an EN pin which shut down the device. But If I want only to cut the second N-FET and that way cut off the conduction while VABT still exists? Will that En signal enable it ? 

    Ohad

  • 0 in reply to Ohad Fundoianu
    TI__Mastermind 26035 points

    Hi Ohad,

    I am assuming we are talking about LM74800-Q1 now. In LM74800-Q1, we can cuttoff the power path with EN pin. As there are two FETs, DGATE and HGATE.

    DGATE is an ideal diode FET and HGATE is a blocking FET.

    When EN is low, both the FETs are pulled low, and the blocking FET cuts off the power path.

    Regards,

    Shiven Dhir