Typically when a depleted sink device is plugged into a USB PD charger, it has dead battery resistor (Rd-DB) to trigger the charger to provide power until its internal battery is charged enough, then it replaces Rd-DB with "regular" Rd to negotiate a PD contract.
For sink devices with no internal battery, I am concern that the delay that occurred during the switch from Rd-DB to Rd will cause the PD charger to stop providing power and cause a power cycle issue:
- Charger provides power to sink controller due to present of dead battery
- Sink controller sees power, removes/replaces dead battery resistor
- Charger doesn't see dead battery resistor, stop providing power
- Since there is no internal battery, sink controller loses power temporarily when switching from dead battery mode to normal mode
- Repeat steps 1-4
We've seen some DRP controllers that have this power cycling/oscillating effect.
Does TPS25750 have this effect? Can the TPS25750 be the sink controller for a device with no battery? Is there a delay during the switch from Rd-DB to Rd for TPS25750?