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LM74610-Q1: Reverse polarity behavior

Paul Beijer
Intellectual 460 points
Part Number: LM74610-Q1
Other Parts Discussed in Thread: LM5050-2, LM74700-Q1, SM74611

Hi all,

I'm considering using a LM74610 in my application in order to emulate an ideal diode. In my case the connected MOSFET should be turned off as soon as the current reverses (flows from drain to source), or even slightly before that since the body diode can handle small currents.

The datasheet shows however that this only happens once the Anode voltage is 20mV below the Cathode voltage. With a typical Rdson of the used MOSFET of say 2 mohms, that would imply that the device would have to conduct at least 10A in the reverse direction before the emulated diode stops conducting.

I realize that in case the reverse current is too small to hit the -20mV threshold, the chargepump would eventually not be able to sustain the required gate voltage and therefor the device will be switched off anyway.

But this seems to be a bit of an uncontrolled mechanism because the time it takes depends on how fast the gate charge leaks away which is typically not very well controlled.

Is my analysis correct or am I overlooking something? If correct, is there a workaround to make the MOSFET turn off before or at the point where the current reverses?

Best regards,

Paul Beijer

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  • 0
    TI__Genius 16305 points

    Hello Paul Beijer,

    Your analysis is correct and the device behaves this way.

    For fast reverse current blocking response, we recommend to use LM5050-2 or LM74700-Q1.

    Reverse blocking response of 0.5us of LM74700-Q1 is shown in its datasheet first page.

    Unlike LM74610, the LM74700-Q1 regulates the forward voltage of ANODE-CATHODE to 20mV, this will ensure the MOSFET will turn off even when there is slow decay of ANODE voltage (compared to cathode).

    Let us know if this device works for you application requirements.

    Regards,

    Kari.

  • 0 in reply to Karikalan Selvaraj
    Intellectual 460 points

    Hi Kari,

    Thanks for the reply.

    Unfortunately we can't use the ICs you suggest because they need an auxiliary power supply of at least 4V. In our case the diode must conduct when the rest of the circuit is completely de-energized, so there's no 4V available. The LM74610 takes it's power from the current itself which eliminates that problem.

    I also considered the SM74611 which is specifically targeted at PV applications and has the MOSFET internal. But apart from the fact that the current and voltage ratings are not quite what I need, the datasheet of that device isn't very specific about the reverse behavior.

    Best regards,

    Paul

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  • 0 in reply to Paul Beijer
    TI__Genius 16305 points

    Hi Paul,

    One way to tackle your problem with LM74610 or SM74611 is increase the capacitance after the diode. This means to achieve a 20mV drop for reverse turn off lot of current needs to be sourced and the capacitance helps to provide that current to trigger reverese turn off.

    Let me know if you have more questions on why the 20mV threshold is not a big problem in you design.

    Regards,

    Kari.