LM317L: Do I need a protection diode between input and output?

Hi Higa,

You may not need the diode in production if you have a well controlled source.  But even if your application will have a controlled source, will your test environment be controlled (especially if you plan on using bench power supplies to power the LM317)?  The protection diode is only necessary if Vin can drop to zero faster than Vout, such that Vout > Vin and this voltage can forward bias the parasitic diode within the internal pass device in the linear regulator.  Sometimes you need it during the testing phase but you can remove it during production. Sometimes you don't need it in either case, and sometimes you need it in both cases.

Thanks,

Stephen