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LM317A: Bypass cap on the Adj pin

Brian Wang0928
Genius 14090 points
Part Number: LM317A

I'm wondering to know what the C2 and D2 function for? In the D/S, it is not very clear to explain the reason. 

Why C2 needs high value capacitance? For noise immunity? 

What's the function of the D2?

Why ADJ pin needs discharging during IN and OUT shorted?

Where is the 50-Ohm resistor located in device to limit the peak discharge current?

Regards

Brian W

  • 0
    TI__Guru 50170 points

    Hi Brian,

    C2 is used to improve PSRR (input supply ripple) you can see the difference here:

    D1 and D2 are present to protect the LM317A in the event VIN suddenly falls. This help to discharge the voltages on the adjust and output pin quickly to avoid violating the absolute max ratings of the device:

    I hope this helps to answer your question.