We need to conserve as much power as possible for the capacitor bank to
keep the MCU and LCD powered during USB PD power cycles. Using a buck
driver saves ~30% power for the most power hungry component, the
LCD backlight.
When the external USB PD supply power cycles the board, we keep the
MCU and LCD powered for a few seconds with charge from the capacitor bank.
However, the 2 MOSFETs acting as level shifters for USB PD I2C communication
leaks voltage of ~1V to the I2C ports of the USB PD controller, which
leaks through to VBUS.
Some supplies refuse to complete the power cycle as long as there is any
residual voltage on VBUS.
Replace the MOSFETs with a PCA9306 I2C level shifter. It allows the USB
PD I2C and thus VBUS voltage to drop to ~0V.
With the ST pin of the TPS1H100 model A there is no way to distinguish
open load (which is an error) from current limiting (which is regular
operation).
