MP2790 — battery-only cold start: DSG never turns on

Battery Management
1

Hi all,

I’m using the MP2790 as the protection AFE in a 5S Li-ion UPS, with a host MCU powered from the PACKP (post-DSG load) rail. I have a chicken-and-egg problem on a battery-only cold start (no charger / no mains):

Measured at cold start, battery only:

  • TOP and both FET drains ≈ 18 V (battery present) :check_mark:

  • CHG gate ≈ 18 V (driven) :check_mark:

  • DSG gate ≈ 120 mV → DSG stays OFF, so PACKP never comes up

  • Because the MCU is powered from PACKP, it never boots → it can never write ACT_CFG.ACTIVE_CTRL to command the FETs on → deadlock

This doesn’t look like a fault — DSGFET_ON_RUN_SC_DET_EN reads 0 (disabled). It seems the DSG turn-on is simply never commanded: power-up default is ACTIVE_CTRL = 0b00 (all FETs off), FET_SRC = 0 (register/MCU control).

My questions:

  1. For a system where the MCU is powered from PACKP, what is the intended way to start on battery alone? Should the IC bring DSG up autonomously, and if so how?

  2. Will committing ACT_CFG.ACTIVE_CTRL = on (+ FET_SRC = 0, FT_STATE_SEL) as the MTP/NVM default make the IC turn DSG on at its own power-up, before the MCU is alive? This is the core thing I need to confirm.

  3. I also read that an MTP store needs ~7.6 V on NSHDN (VNSHDN_PROG). Is that correct, and is in-system MTP programming therefore not possible without that voltage?

(For reference: FET_MODE reads 0x0513 and won’t accept RAM writes; MEM_STATUS = 0x000F.)

Thanks!