⚠️ 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

YO3HJV

Finally, I solved it...
No need for 5V output on 5V_BUS because it came from magnetic base charger.
Therefore an interlock code was added:
--When 5V from battery is needed for external acc., VBUS== OFF & EXTEN==1;
--When magnetic base charger (or external 5V is applied to Socket Power (J3) on Bottom2, VBUS == INPUT & EXTEN==0.

This way, the Bottom2 LED's and othe devices are powered with 5V in any situation.

Cheers,
Adrian

felmue

Hello @YO3HJV

glad to hear you figured it out.

I was just setting up a test to see if I get this 400 mA power loop too with my M5Core2; but I do not.

When I set input mode I see about 82 mA drawn from the battery. When I set output mode I see about 99 mA drawn from the battery. (Note: an LED is connected to the Grove port.)

In case your still interested in my UIFlow2 test program. It's in UIFlow2 Project Zone called: M5Core2_Input_Output_Test_UIFlow2.4.6

Thanks
Felix