Debugging MAX77958/MAX77961 System - PD Negotiates to 9V, but Charge Current is 0A with POR Error

Hello,

I have designed a custom PCB for a 2S battery charging application using the MAX77958 (PD Controller), MAX77961 (Buck-Boost Charger), and MAX17320 (Protector). The MAX77958's I2C Master port is connected to the MAX77961's I2C slave port for control.

I am facing a persistent issue on our latest board revision and would appreciate some guidance.

Summary of the Problem:
When we connect a USB-C PD power adapter, the system behaves as follows:

1. Initially, the adapter provides 5V, and the board draws about 600mA for 4-5 seconds.
2. The input voltage then successfully steps up to 9V, indicating a successful PD negotiation by the MAX77958.
3. The moment the input reaches 9V, the input current drops to 0A, and the battery does not charge.

Key Findings from I2C Debugging:
I have successfully established I2C communication with the MAX77958 at address 0x25. After the 9V/0A state occurs, we are consistently reading the following register values:

Reg 0x04 (UIC_INT) = 0x04

Reg 0x09 (USBC_STATUS2) = 0x05

According to the datasheet, the 0x05 value in the SYSMsg field corresponds to SYSERROR_BOOT_POR, indicating the MAX77958 is continuously undergoing a Power-On Reset.

Hardware Configuration Details:

1. The VCIN and VIO1 / VIO2 pins on the MAX77958 are correctly powered (VIO2 to MAX77961's PVL, VIO1 to 3V3 from host mcu side).
2. The MAX77961 is configured with the following hardware strapping:

• CNFG: 86.6kΩ resistor to GND (Configured for 2S battery).
• ISET, ITO, VSET, INLIM: All tied directly to PVL (to use internal default charging parameters).
• OTGEN, DISQBAT, STBY: Each is pulled to GND with a 10kΩ resistor.

Questions:

1. Given the SYSERROR_BOOT_POR (0x05) error on the MAX77958, what are the most likely causes for its SYS power rail to be unstable after a successful 9V PD negotiation? Could the VBUS transition from 5V to 9V cause a temporary droop or glitch on the MAX77961's SYS output that is significant enough to reset the MAX77958?
2. I believe that because the MAX77958 is continuously resetting, it never reaches a stable state to send the necessary I2C commands to the MAX77961 to begin drawing current. Would this explain the 9V, 0A symptom?
3. Could you please validate our hardware configuration for the MAX77961? Is using the default autonomous settings (by tying ISET, INLIM, etc., to PVL) a valid approach when the charger is intended to be controlled by the MAX77958's I2C master?
4. What would be the most effective next diagnostic step? Should we be focused on probing the SYS rail of the MAX77958 with an oscilloscope, triggering on the VBUS transition, to look for voltage droops?

Thank you in advance for your help.