OT: SuperCap for SD memory 'loss of power' write completion

[Yashu]

I'm contemplating using a double layer capacitor with a power hi-side switch (like the TI TPS2081) which will detect inadvertent loss of the main supply voltage and subsequently switch to the supercap.

The super cap will then power the uC and SD card until the uC finishes performing a possible write to the SD card. At this point, the uC goes to sleep and any SD card flashing runs to completion.

I would appreciate any input from anyone has any experience -or- references to peruse concerning this topology?

[libor]

I use the normal smoothing electrolithyc capacitor for a similar problem. My device has a 220VAV power supply: it has a 220V to 9VAC transformer, a greatz, a 4700uF smoothing cap, and a 5V voltage reg.
A voltage divider direct after the transformer on the 9VAC (before the greatz) connected to the on-board comparator of the PIC serves as a zero-cross detector every 1/50 sec (i made a kind of a software counter watchdog that gets reset at every AC zerocrossing in an interrupt (as a by-effect this pulse also synchronizes my software real-time clock))
If a zerocross was missing, my program knows that the mains power has gone and I have only the power in relative large capacitor that has to decay from 9V to 6-7 VAC before the voltage regulator gives up yielding 5V. With all outside large consumers switched of at the beginning of this 'emergency' save, I have ca. 1 sec time of normal operation of PIC left. I use this time to save some operating parameters to eeprom before the power is completely lost.
Your SD card consumes much more power, and you need perhaps more than 1 sec, so if you have to use a much larger cap, it is less expensive to implement at the lower (regulated) voltage at 5V or 3V.