⚠️ M5Stack Core2 v1.3 - Ouroboros-like power loop -

[YO3HJV]

Hi all, I’m investigating a power management issue on the M5Stack Core2 (AXP192 + external SY7088 boost) and I’m looking for confirmation or a proper solution.

🔧 System Architecture (as implemented)
From schematic analysis and testing:

AXP192 IPSOUT powers the system and the boost converter

// VBUS ALWAYS INPUT ENABLED  
 writeAxpReg(0x30, 0x80);

SY7088 generates 5V (when EXTEN = 1)
SY7088 output is directly connected to VBUS
VBUS is also connected back into the AXP192 as an input
ACIN is the real USB 5V input

So effectively:
BAT → IPSOUT → SY7088 → VBUS → AXP192 → IPSOUT

❗ Observed Problem
When running on battery:

-Enable 5V output (EXTEN = 1)
-Disconnect USB cable
-Immediately, battery current jumps to ~450 mA
-System enters a self-sustained power loop

🔍 Key Observations
EXTEN is correctly set (verified via register dumps, Reg 0x12 bit 6)
VBUS input is permanently enabled (Reg 0x30 = 0x80)
Boost converter behaves normally
Loop appears only when USB is disconnected

💡 Root Cause
SY7088 output is physically tied to VBUS
AXP192 does not fully isolate VBUS internally
The supposed N_VBUSEN mechanism does NOT break the electrical path

🔥 As a result:
BAT → boost → VBUS → AXP internal path → IPSOUT → boost...

This leads to:
continuous energy circulation
high battery drain (~400 mA)
no stable operating point

❓ Question to the community
Has anyone:

• Observed this behavior on Core2 or similar AXP192 designs?
• Found a way to safely use 5V boost to power external accssories from battery without loop?

Thank you,
Cheers,
Adrian YO3HJV