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Protection diode

Dennis Cascio
Prodigy 90 points

I am using an LM3480IM3-5 linear  regulator with 0.47uF of output capacitance.  Do I need to use a protection diode across the device to protect against damage when the input power goes away?

  • 0
    TI__Mastermind 22610 points

    Depends on definition of "... when the input power goes away ...".

    If the input power source simply goes open circuit there is little chance that there will be reverse current flow, so there would not be a need for the protection diode.

  • 0 in reply to Donald Jones
    Prodigy 90 points

    Well, assume it is a power supply output that would shut down.  It may bleed down quickly as there are other loads on it other than the 5V linear regulator, but the input circuit will not open up as with a switch.  There will always be the input capacitance connected. 

  • 0 in reply to Dennis Cascio
    TI__Mastermind 22610 points

    If there is input capacitance at the input, and the input voltage is being bled down by parallel load, it is unlikely that the input voltage will fall fast enough to cause a problem

    With the 0.47uF cap and a modest 10mA output load current (i.e. 500 ohms) the charge on Cout will largely dissipate in less than 1milli-sec.

    Based on this info, I don't see a reason to add a protection diode.