Question: How is charging supported in a system with a dead battery?
A system with a dead battery that supports being charged by USB shall apply Rd to both CC1 and CC2 and follow all UFP rules. When it is connected to a DFP or Charging UFP, the system will receive the default VBUS.
The Cypress implementation of dead battery charging is illustrated in the following figure:
At the dead battery condition, the CCG1 device will not be able to turn on the FETs to present Rd termination resistors on its CC lines. When a DFP presents an Rp resistor, the CC line will be at 5 V. This will turn on either Q2 or Q4 FETs by self-biasing through R1 or R2 resistors respectively, and Rd is presented resulting in establishing the connection.
Using this method,you can get only 5 V to VBUS. The external circuitry must be able to receive this 5 V and power CCG1 to negotiate higher power. The design should include a low drop out (LDO) voltage regulator or a regulator that can receive a higher VBUS voltage and provide 5-V output. After the battery in the system is charged by higher VBUS, it can start providing the 5-V output as required by the CCG1 device. Then, you can cut off the VBUS powering the CCG1 and start receiving the 5 V coming from the battery.