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LTC4412 Power switch with MOSFET pair

Hi There,

I have a couple of questions regarding the use of the LTC 4412 with a pair of MOSFETS as shown in figure 4 of the datasheet here:

http://www.analog.com/media/en/technical-documentation/data-sheets/4412fb.pdf

What I am trying to achieve is a high-side switch with reverse current protection by using the LTC4412 with  a pair of MOSFETS. So similar to the primary path of Figure 4, without the auxiliary path. 

So my questions are as follows:

  1. Does the back-flow prevention functionality (sensing Vsense relative to Vin) still work as per Figure 1?
  2. The internal voltage clamping clamps the gate voltage to the higher of Vin or Vsense. Neither of which in this configuration are connected to the source of the MOSFET, leaving the MOSFET V(GS) unprotected. would operation be affected by either flipping the MOSFETS (so the path goes SDDS) or adding a TVS diode between the gates and sources of the MOSFETS?

Thanks,

David

Top Replies

  • Hi,

    Does the back-flow prevention functionality (sensing Vsense relative to Vin) still work as per Figure 1?

    Yes the back flow prevention functionality still works when you connect the device…

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  • Hi,

    Does the back-flow prevention functionality (sensing Vsense relative to Vin) still work as per Figure 1?

    Yes the back flow prevention functionality still works when you connect the device as shown in figure#4. As soon as voltage on SENSE pin goes higher than VIN + 20mV, both the MOSFETs are switched off. Body diodes of the MOSFETs are connected opposite to each other and prevents the back flow of current as well.

    The internal voltage clamping clamps the gate voltage to the higher of Vin or Vsense. Neither of which in this configuration are connected to the source of the MOSFET, leaving the MOSFET V(GS) unprotected. would operation be affected by either flipping the MOSFETS (so the path goes SDDS) or adding a TVS diode between the gates and sources of the MOSFETS?

    Maximum V(GD)  of a MOSFET is usually higher than Maximum V(GS) of a MOSFET. So, it is not recommended to connect Source terminal to the input and expose Source Gate junction to possible high input voltage transients. Adding TVS diode between gate to source of the external MOSFET will not affect the operation of the part.

  • Hi Manpreet,

    Thanks for your response. I just have a quick follow up question:

    Adding a TVS diode between the gates and sources in figure #4 has the potential to provide a reverse current path (albeit with a high voltage drop) that runs:

    Vload > through the body diode of the second FET > through the TVS diode > into the voltage clamping circuitry in the LTC4412 > Out to Vin.

    Given that in order for this to happen Vsense would need to be significantly higher than Vin would the clamping prevent that current path?

    Would this current path still be blocked if Vin is unpowered?

    Thanks,

    David

Reply
  • Hi Manpreet,

    Thanks for your response. I just have a quick follow up question:

    Adding a TVS diode between the gates and sources in figure #4 has the potential to provide a reverse current path (albeit with a high voltage drop) that runs:

    Vload > through the body diode of the second FET > through the TVS diode > into the voltage clamping circuitry in the LTC4412 > Out to Vin.

    Given that in order for this to happen Vsense would need to be significantly higher than Vin would the clamping prevent that current path?

    Would this current path still be blocked if Vin is unpowered?

    Thanks,

    David

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