power failure backup circuit

[daryldee]

Hi all!

I'm designing a board with an atmel MCU (atxmega32e5) that drives an oled, 4 high power LEDs with its relative buck drivers and all the frills that a little device needs.
What I would accomplish is to have a power failure advice/trigger when the main power is lost to actuate trough an ISR call an NVM write and save all the configs of this device.
I did not calculate already the NVM data write timing, but I guess we're talking about few mS.

So, the circuit I'm missing is a method to keep the power to the MCU alive for enough time.

The circuit runs with 3 different power rails:
- 3.3v (MCU and OLED)
- 5v (rs232 driver)
- Main 12/18v (led driving stage)

As of my knowledge, the best way would be to go analog and keeping energy storage up enough to allow the NVM write cycle.
What I miss, is a method to separate power domains and avoid that simple capacitors will discharge with the rest of the circuit.

what would you do?

Thanks!
D.