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TPS7A4700 Reverse Current Protection - Sensing After Diode

Thomas Schmit9
Prodigy 70 points

I've been using the TPS7A4700 for awhile in various designs and have been pretty happy with it. I have a new application where the TPS7A4700 is disabled in a "low power" mode. During this low power mode, I need to provide reverse current protection to prevent a digital device from back-feeding through the TPS7A4700.

I tried the circuit in the image below but the voltage regulator doesn't seem to work in this configuration. If I jump the diode, it outputs the 3.3V I need but as soon as I remove the jumper shorting the diode, the output goes to 0 (about 0.2V at the output pins, actually). The hope was that, by connecting the sense pin to the load, it would adjust the output for the voltage drop across the diode.

Where did I go wrong? I can always connect the FB pin before the diode but then the output voltage can vary as the current through the diode (and thus Vf) changes.

Thanks!


 

  • 0
    TI__Guru 55940 points

    Hi Thomas,

    Yes while placing a diode in the feedback loop in order to account for the voltage drop across the diode looks appealing, this configuration does cause issues for the feedback loop.  You already identified the pros and cons of connecting a diode after the LDO so I will mention the other two options.

    The most common configurations for dealing with reverse current is shown below.  Here we provide a robust path for the reverse current through the external diode.  This current charges the input capacitor until Vout no longer exceeds Vin.

    The other option is to place a diode before the input capacitor.  This will block reverse current from upstream devices; however, it will do nothing to protect the LDO.

    Very Respectfully,

    Ryan