BQ25638: Dual-Input Charger Design: SYS Power Path and Loop Control When Only Auxiliary Charger Has Input

Hi TI team,

I found the dual-input charger design in a TI application note. I noticed that with two chargers, only one SYS pin is active and both charger outputs are connected only at the BAT pin. I have the following questions:

When only the auxiliary charger has input (main charger has no input), how is the SYS pin powered? Does the main charger enter supplement mode to let BAT back-feed to SYS, or is there another power path?

When both main and auxiliary chargers have input, do they need to operate in the same charging phase (e.g., both in CC or both in CV)? Or can they operate in different phases (e.g., main in CC while auxiliary in CV)?

My understanding is that when both chargers are active, the auxiliary charger's control loop is actually governed by the main charger. Is this correct?

Please clarify the power path architecture and control loop interaction between the two chargers. Any relevant app note or technical reference would be appreciated.

Best Regards，

Luo Jiang