I am trying to decide where to put the main power switch on a circuit with a Li cell, charger PMIC, and switching regulator. The switching regulator I'm considering (ON semi NCP1522B) has an ENable input. The charger PMIC (MCP73831) is basic, and will only be parallel to Vbat as the Vin on the regulator.

I see in the datasheet that the standby current when EN is low is max 1.5uA. If I use a simple SPST switch to pull EN high as a power switch, will that be sufficient for the system to not drain the battery much when "off"? Would it be better to put the power switch in front of Vin, or is there a better way to do this altogether?

  • 3
    \$\begingroup\$ It's not really clear what you are asking here: if you have a way to achieve 1.5 uA that is low for most battery types. But if your overall current and desire to use a mechanical switch means that you can just break the circuit entirely, why wouldn't you do that? \$\endgroup\$ – Chris Stratton Jul 14 '19 at 17:42
  • 2
    \$\begingroup\$ I would not. It's more for power sequencing and [selective] powering down, not actually turning off. You are using a mechanical switch anyways. Put it in the location of greatest effect. \$\endgroup\$ – DKNguyen Jul 14 '19 at 18:06
  • \$\begingroup\$ I guess my question is more of trying to understand the purpose of the EN pin, and if using a switch to it makes sense or not. Both of your comments are spot on - and I think I'll just put the switch in line with the main power. I have been toying with the idea of trying to do "soft-switch" power control instead, but that definitely won't happen until a different revision in the future, so that's probably why I considered even using the EN line in the first place. \$\endgroup\$ – Paul Jul 15 '19 at 1:52
  • 1
    \$\begingroup\$ Note you can still do a soft power "off" even with a hard power switch. What you do is tie the enable to the input with a capacitor, and also drive the enable from the MCU with a resistor. Turning it on starts up the regulator, then the MCU takes over holding it as long as it wants. If the MCU shuts off the enable, the user then has to turn the switch off and back on. Seen in lots of toy drones that shut off in a low battery condition. \$\endgroup\$ – Chris Stratton Jul 15 '19 at 15:25

It would seem to me that in this case, using the EN pin doesn't make sense.

If putting the mechanical switch between the battery and your circuit is an option, you should - as this will save you an unnecessary 1.5uA stand-by current drawn from the battery, and your question suggests no reason to believe this would have adverse effects.

In other designs the EN pin can be used for a variety of reasons and I'll mention a few that I've come across below:

  • Power sequencing. Some systems require power to be turned on/off in a specific sequence such as first 5V, wait 10mS, then 3.3V.

  • Partial power-down. When you need to power down part of a system and there is reason to run that part off a separate regulator - one with an enable input saves components.

  • UVLO and battery protection functions.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.