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MX7 with PF3000, Boost regulator and Supercaps

Question asked by Lars Heinrichs on Sep 9, 2019
Latest reply on Sep 11, 2019 by igorpadykov

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

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