Explicit control using Enable pin

My project uses two voltage regulators: S9V11F3S5 for the 3.3V processor and S9V11F5S6CMA for the 5V electronics.

My processor already has brownout detection, so I don’t need to use an enable pin to protect from low battery voltage. Instead, I just want to use the enable pin to shut down the 5V electronics for power savings. I’m finding it rather clunky to use it for this purpose because the enable pin is configured to, in essence, monitor the input voltage.

My specific problem is that there are really three states. When the processor is powered, the digital output pin controlling the enable pin is either logical high or low. That works fine. However, if the processor is switched off then the pin floats and the enable pin is driven by its own internal battery level detection. I actually don’t want this. I want the 5V regulator to be off when the processor is off.

I’m guessing there is a pullup resistor on the regulator that causes the pin to go high enough to trigger the enable. If so, can I just remove it and explicitly control the output with a logic level on the enable pin? There would also have to be a pulldown resistor that would cover the situation where the pin is floating. I suspect there is already a pulldown resistor, but I don’t know. Just looking for the best solution. Thanks.


You might be able to get the behavior you are looking for from that regulator if you adjust the potentiometer to its end where EN is lowest. Then it should be emulating a pull-down resistor.

If that does not work, then you could try removing the potentiometer and add a pull-down 0603 resistor to the pad the potentiometer currently covers. Here is a picture of a board with that pad populated:

- Patrick

If that is true then I already am using the S9V11F3S5 for the 3.3V regulator. If I use the SEL pin to set the output to 5V then I could potentially use the same enable trick, right? The description text says there is a 100K pullup from ENABLE to VIN. Would I pop that and then add a pulldown from ENABLE to GROUND?

It looks like the picture you provided is doing just that using the S9V11F3S5, except in the 3.3V configuration.

Yes, your plan for modifying your S9V11F3S5 regulator should work, and it would be simpler than modifying the S9V11F5S6CMA.

By the way, the picture I posted is actually our S9V11F3S5C3 regulator. I was just using it to show you where you could put a surface-mount pull-down resistor, but it might be easier just to add an external pull-down to the EN pin.

- Patrick