Hello TI Team,
We are facing three shipmode related issues on the BQ25120 in our design and we could use some help.
When the battery voltage (VBAT) goes under 3.1V (measured with a fuel gauge - STC3115), our MCU sets the BQ to shipmode to enter the lowest possible quescient current.
As a fail-safe feature, we want to keep the BUVLO at 3.0V to make sure the BQ will at least shut the VSYS output down.
What happens is the following:
- Issue n°1: BQ does not always enter shipmode unless we set the shipmode threshold higher (3.5V instead of 3.1V). As our application is low power, this is not an acceptable workaround as we lose significant autonomy (tested on 19 boards with 30% failure rate)
- Issue n°2: when in shipmode (with 3.5V threshold), pressing MR button does not exit shipmode unless VBAT<3.2V
- Issue n°3: setting BUVLO at 2.8V with 3.5V shipmode threshold seems to prevent BQ from entering shipmode (but BUVLO = 3.0V in the same conditoins is OK)
Some details on how we enter shipmode:
- Charger is not attached (VIN = 0V)
- CD pin is pulled low for 50ms (CD is previously low but sometimes toggled to resume I2C while in Hi-Z mode)
- CD pin is pulled high
- 50ms sleep
- 00h register is read for debug purposes
- 01h register is read to clear any fault flag
- 00h register is set to 0x20 (retry if not successful)
Issue n°1: Shipmode threshold = 3.1V, BUVLO = 3.0V, VIN = 0V, unable to enter shipmode on 6 out of 19 boards.
Here is a register dump prior trying to enter shipmode:
Register: 00 : 01 : 02 : 03 : 04 : 05 : 06 : 07 : 08 : 09 : 0A : 0B
Value: 03 : 40 : 00 : 9c : ba : 78 : ba : 00 : 68 : 12 : 3c : c2
Here is a register dump after failing to enter shipmode:
Register: 00 : 01 : 02 : 03 : 04 : 05 : 06 : 07 : 08 : 09 : 0A : 0B
Value: 23 : 40 : 00 : 9d : ba : 78 : ba : 00 : 68 : 12 : 28 : c2
00h register seems to indicate both CD state and EN_SHIPMODE bits are corrects, however shipmode is not entered.
Issue n°2: Shipmode threshold = 3.5V, BUVLO = 3.0V, VIN = 0V, unable to exit shipmode with MR button when VBAT<3.2V
Section 9.3.1 'Ship Mode' of the datasheet states the following: "The battery voltage must be above the maximum programmable BUVLO threshold in order to exit Ship mode with a MR button press when VIN is not present."
Referring to 09h register configuration, the maximum programmable BUVLO is 3.0V.
Section 9.3.3 states there is a 100mV hysteresis on BATUVLO, which means the MR button should not be able to get the BQ out of shipmode if VBAT<3.1V, correct?
However the MR button is uneffective if VBAT<3.2, and we do not understand why.
Issue n°3: Shipmode threshold = 3.5V, BUVLO = 2.8V, VIN = 0V, unable to enter shipmode
With 3.5V shipmode threshold, shipmode can be entered if BUVLO = 3.0V but not if BUVLO = 2.8V.
We don't get why setting BUVLO lower while keeping the exact same conditions otherwise prevents the BQ from entering shipmode.
Thanks in advance for your help,
Best regards,
Damien
