Our system uses an i.MX7D as its core, powered by a PF3000. Most of the systems peripherals are powered by an external 3V3 buck-boost regulator. Two supercaps in a row provide a <5V backup power source.
I am looking for a solution for the following scenario:
- All I/O Banks on the MX7 are connected to V33 (default @3.15V)
- All peripheral devices are connected to BUCKBOOST_3V3
- The buck-boost regulator is enabled by V33
- MX7 and peripherals are connected via I/O
- The systems main power fails and backup power is used
The Supercaps will discharge, resulting in a dropping backup voltage. While the voltage at the peripherals remains at 3V3 (Boost regulator), the voltage of V33 will begin to drop once the backup voltage has gone below 3V15+RegulatorDropout. As the maximum difference allowed between the I/O voltage on a connected pin and the internal I/O supply is 0.3 Volts, we get into a critical area once the input voltage drops too low, before the PF3000 finally turns off.
I am looking for a solution to keep the internal I/O voltages of the MX7 at 3V3 while the backup voltage drops.
The easiest solution is probably to connect all I/O bank supplies on the MC7 to the external Buck-Boost regulator, that is turned off once V33 is turned off. Would that be an exceptable solution or is there a constraint I am overlooking in this setup?
EDIT: the boost converter creates a small peak to 3V3 on startup, then drops back to about 1V before rising to 3V3 in a ramp