I'm trying to create a solution to power an ATtiny85v MCU for maybe two seconds after a power loss to perform some power down tasks, but I don't have much knowledge with electronics. I did some searching but couldn't find an answer to my specific scenario.
The MCU's power is coming from the output of a LDO (Microchip MCP1703T or similar). With my limited knowledge, my idea is to use a capacitor across VCC and GND of the MCU to keep it running for up to two seconds. Powering the MCU with a capacitor has been covered in other questions here, but in my case the capacitor to power the MCU has to be much larger than the specified largest output capacitor of the LDO. Without any components between the MCU's "power off" capacitor and the output capacitor of the LDO, these two capacitors will be parallel connected. Wouldn't that just be the same thing as a single larger capacitor as LDO output capacitor? It would be much larger than the allowed largest output capacitor for the LDO, possibly causing stability issues. How would I go about solving this?
A few notes:
I will be doing this on a 17mm board already populated with components for other things and have very limited space. Also, how to detect power loss and how to run the MCU in low power mode is not of interest for this question unless it is of importance to the solution.
Best Answer
Yes, regulator stability could be affected by the added capacitance.
Assuming your required current is relatively low, simply adding a series resistor to the much larger capacitor (or using an electrolytic type with high ESR) should ensure stability without causing too much voltage drop when the added cap is sourcing current.