Automatic switching over from voltage source for RTC backup

Thread Starter

Frits Jan van Kempen

Joined Jan 18, 2016
Hi all,

I am trying to make a RTC backup voltage switcher (since my RTC ic does not have a separate pin for this).
Under normal conditions, the RTC is powered by 3.3V coming from an LDO (powered from battery or usb)
Under backup conditions, the power source is the battery (lipo 3,5 - 4,2V).

I need a circuit to SWITCH OFF the battery power from the RTC when the LDO gives 3.3V output.

Using two diodes won't work here because these will pass the HIGHEST voltage to the RTC, and I need it to pass the 3.3V when it is available (the lowest voltage of the two in this case).

I thought I was smart and use a P-MOSFET to switch off the batt power when a voltage was applied to the gate from the 3.3LDO.
BUT unfortunately, the mosfet will only switch off when the same or higher voltage is applied to the gate as the drain-source voltage (which is the battery at 3.5-4,2V).
So this circuit below always provides the batt power to the RTC.


Can anyone help me out on how to make this switch over work?

Note: I could just power the RTC on +batt, and it would work, but the problem is that the battery includes a charging circuit and then this charging power is powering the RTC also (4,2V) which might mess up the measurements for the charger.