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BQ25121A: Using BQ25121A with minimal parts/layout without via-in-pads

Vasim Valeev
Prodigy 20 points
Part Number: BQ25121A
Other Parts Discussed in Thread: BQ51013B, BQ25120

Tool/software:

Since my attempt to build BLE beacon with wireless charging (nRF52833 + BQ51013B + BQ25101Y) have ended with kind of fiasco (on nRF52833 side as i couldn't get it to work reliably in "high-voltage mode" when powered directly from Li-Ion  battery), i decided to switch to BQ25121A as battery charger and to power nRF52833 in normal mode from switching regulator. To save cost i have to use pcb process without via-in-pads and with minimal parts on board as the beacon is quite small (15x20mm).

My design looks this (TS is floating as i turn off TS faults via I2C, ILIM/ISET/PRETERM are grounded as i set these currents by I2C too; using internal nRF52833 13K pull-ups resistors to pull I2C lines):

 

MCU periodically polls BQ25121A by pulling CD high, reading TS control register and if the register has TS function enabled then sends 0x08 (TS_EN=0) to disabe TS function and sets currents and voltages (SYS_VOUT to 2.5V, BUVLO to 2.6V), then reads fault and status registers to clear possible faults.

Also during this routine it reads battery monitoring register and sends 0x80 (VBMON_READ=1) into it to initiate new battery measure cycle. As battery voltage reaches around 2.9V - MCU sends 0x20 (EN_SHIPMODE=1) to status/shipmode reigster to turn off the system completely and protect its battery.

My tests with BQ25120_EVM board looks good but to be sure and avoid re-designing again, i'd want to be sure it'll work good with BQ25121A too. Just few questions:

1. As i understand, pulling CD high disables charging and restarts charging process when i release it or pull down. There is a way to avoid this? Currently i try to read I2C register without pulling CD at first and pull it high if get error at the read, but that makes excess read cycle with timeout that is not really good for power consumption.

2. What does "HZ_MODE" bit in fast charge control register?

3. BQ25121A exits shipping mode always after i simply apply VIN voltage (5V from BQ51013B as i put the beacon on charger), with MR pin just floating?

4. Floating TS won't cause any trouble if TS_EN=0, right?

  • 0
    TI__Genius 14805 points

    Hi,

    Vasim Valeev said:
    1. As i understand, pulling CD high disables charging and restarts charging process when i release it or pull down. There is a way to avoid this? Currently i try to read I2C register without pulling CD at first and pull it high if get error at the read, but that makes excess read cycle with timeout that is not really good for power consumption.

    To clarify, you want to avoid restarting the charging process after /CD is pulled high, then pulled low again? If this is the case then you can avoid it by disabling charge via I2C, though this bit will have to be flipped again to re-enable charge.

    Vasim Valeev said:
    2. What does "HZ_MODE" bit in fast charge control register?

    This won't have an impact if VIN is applied. If you are in active battery mode then it will result in the device entering HZ mode without /CD being pulled low. To exit this HZ mode, you will need to pull /CD low then high again.

    Vasim Valeev said:
    3. BQ25121A exits shipping mode always after i simply apply VIN voltage (5V from BQ51013B as i put the beacon on charger), with MR pin just floating?

    This is correct. Ship mode exits take place after an /MR pull down or VIN being applied. If /MR is floating it will just remain at VBAT, and only a VIN being applied will result in shipmode exit.

    Vasim Valeev said:
    4. Floating TS won't cause any trouble if TS_EN=0, right?

    This is correct. Charging won't be affected by a floating TS pin so long as TS_EN = 0.

    Best Regards,

    Juan Ospina