I have a low power IC (an RTC module) which draws about 2.5uA average draw at 2.5V. I want to be able to power this from a coin cell (2V to 3.1V) directly, or from a larger battery pack (3.5 - 20V).
It is also possible for both batteries to be present at the same time.
Here's a way to do that:
Problem is, the load is very low power so if we use Schottky diodes and power the device from a coin cell, leaving the battery pack disconnected then the high leakage current of the schottky diode will likely dominate the overall power draw, flowing backwards through D2, into the regulator and to ground.
If a regular diode were used instead of a Schottky diode for D2, then a larger voltage drop would be introduced. If we were powering the devices from the Bat Pack then the 2.5V regulator will no longer work because the minimum voltage that the load will run at is 2.4V. If a 3.3V regulator was used instead, a low-dropout regulator (< 200mV) would be required. These are quite hard to find with quiescent currents of < 2.5uA. If the regulator has a high quiescent current then obviously the efficiency is reduced significantly. Not to mention the significant energy is dissipated in D2 due to the voltage drop across it.
Is there a better way to efficiently OR low current, low voltage power supplies together?