Wired OR for LDO with an enable

[robotico]

Hi All,

I'm using an LDO with an enable. I'd like the LDO to be active when either of two other voltages (+5VDC or +3.3VDC, both from other LDOs) are present. My proposed solution is a 100k pull-up to +3.3V in series with a diode and then a 100K pull-up and diode to +5V, both connected to the EN pin of the LDO. So, if either +3.3V or +5V are present the EN pin would be pulled high. Seems simple enough, just curious if anyone spots any flaws or if there would be a problem when both +3.3V and +5V are on at once.

Thanks for any thoughts.

 

[erikl]

Re: Wired OR for LDO enable

1. Your LDO's EN input will need a >100k pull-down to guarantee a safe disable in case both external voltages are missing (the diodes prevent a safe "logic 0").

2. Your LDO's EN input needs to stand the +5V-0.6V input voltage from the +5V power supply. But you can scale it down via the a.m. pull-down resistor.

3. No problem with both external power supplies on.

 

[robotico]

Re: Wired OR for LDO enable

Thanks a lot for your help. Hadn't thought about a pull-down for a safe disable so I greatly appreciate your feedback.