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TPS784: How to choose reverse current protection diode

Nikos Agianniotis
Prodigy 40 points
Part Number: TPS784

Hi,

in our application we are using the TPS78450 as a second stage regulator, powered from a DC-DC converter as shown in Figure 8-8 of the datasheet.

I would like to use a reverse current protection diode across TPS784, from the output to the input, like it is mentioned in section 8.1.7 and Figure 8-5 of the datasheet.

There is however the question about what diode should be selected and what criteria it should fulfill.

  1. Why is it mentioned that a Schottky diode with low forward drop voltage should be selected? Why low forward drop is important?
  2. Then, how much forward current should the diode withstand?
  3. Are there any other important parameter to consider?
  • +1
    TI__Expert 4805 points

    Hello,

    You need a Schottky diode with a low forward drop because you want to make sure the reverse current is flowing through the external diode rather than the parasitic body diode of the pass FET. If the Schottky has a lower forward drop then the body diode, it will activate first.

    Sizing the forward current of the Schottky will depend on the nature of the reverse current situation. Are you expecting an indefinite short from VOUT to VIN (which is being tied to GND)? Or is VOUT shorted to VIN which is going high impedance? It can be helpful to try to measure the current in this scenario to determine how big of Schottky you need. If it's a momentary short, you may be able to find an appropriate Schottky diode that can handle the associated surge current. 

    In general, you can assume parasitic body diodes to turn on around ~0.6V - 0.7V. Try to find a Schottky with a lower forward voltage across temperature, current and any other relevant conditions.

    ~ Aaron

  • 0 in reply to Aaron Paxton
    Prodigy 40 points

    That makes sense Aaron, thanks for your comment.

    The scenario that I am mostly worried about is applying an external, higher-voltage potential to the LDO output. In that case I suppose there will be current flowing towards the LDO, which is what I want to avoid. I guess I need to figure out what exactly happens in this case, so as to select the diode correctly.