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BQ27427: Fuel Gauge and BQ24040 Charge IC Schematic Review and Queries.

Arnav Jain
Prodigy 10 points
Part Number: BQ27427
Other Parts Discussed in Thread: BQ24040, , GPCCHEM

Tool/software:

Hello Team,
We have created an schematic using BQ27427Fuel Gauge and BQ24040 Charging IC for a compact circuit that will be powered by Li-Ion or LiPo battery around 1000-2000mAh Capacity. We wanted you to review the schematic before we proceed to the layout. 

Here is some explanation of the Schematic:

  1. BQ24040 Charge IC:
    1. VBUS is the 5V input from USB connector, we have 10uF and 100nF Caps at USB input (and another 100nF near the Charge IC).
    2. ISET is set to 1A as of now, so 540ohm.
    3. PG-Good Pin is fed to the host MCU.
    4. ISET2 is tied to Ground effectively setting the USB Limit to 1A.
    5. OUT is fed to BAT pin of Fuel Gauge.
  2. BQ27427 Fuel Gauge IC:
    1. SRX pin is used as System Output, hence the main System Voltage (VSYS) is sourced from here.
    2. BIN Pin is connected to Battery Input Pin.
    3. VDD_FG is output of Fuel Gauge's Internal LDO.
    4. GPOUT pulled up to VDD_FG and connected to a 3V3 MCU.
    5. SDA & SCL also connected to a 3V3 MCU (10K pull ups implemented).
  3. Queries:
    1. Pre-Term in BQ24040, we did not fully understand the significance of this Pre-Term (any good explanation will be appreciated).
    2. Where is Battery supposed to be connected to for Input, I am unable to understand.
    3. GPOUT is pulled up using 1V8 but MCU is 3V3, can there be issues.
    4. What value of NTC in BQ24040 should we use.
    5. Will our Battery be supported by this Fuel Gauge (I was reading about GPCCHEM so have some queries on what we would need to do, we can discuss those after finalizing schematics).

Sorry if these questions seem noob, as I am a first time user of Fuel Gauge with Separate Charge ICs.

Regards

Anmol

  • 0
    TI__Mastermind 24885 points

    Hello Anmol,

    This question has been assigned and is being reviewed.

    Thank you,
    Alan

  • 0 in reply to Alan Frias
    TI__Mastermind 37741 points

    Hello Anmol,

    A. This question can be answered by the charger team.

    B. The battery is connected to the BAT pin.

    C. This is fine, it is within the spec for the pin.

    D. This question can be answered by the charger team.

    E. The GPCCHEM tool will have to be run to see if one of the pre-programmed chemistries is a good match with your battery.

    Regards,

    Adrian

  • 0 in reply to Adrian
    Prodigy 10 points

    Thanks Adrian for the reply.

    Adrian said:
    B. The battery is connected to the BAT pin.

    If I connect the Battery to the BAT pin as you suggested then I would need to connect the Output of the Charger to VSYS, is that the correct way charger input to Fuel Gauge? Also this would mean that VSYS will be shared by Battery Output (SRX pin of Fuel Gauge) and Charger Output (OUT of Charging IC), Is this correct?

    Adrian said:
    E. The GPCCHEM tool will have to be run to see if one of the pre-programmed chemistries is a good match with your battery.

    Are normal LiPo and Li-Ion Cells supported by this Fuel Gauge?

    Meanwhile I am waiting for the Charger team to reply to my other queries.

    Regards

  • +1 in reply to Arnav Jain
    TI__Mastermind 40305 points

    Hi Anmol,

    A. As shown in the datasheet, Pre-Term "Programs the Current Termination Threshold (5 to 50% of Iout which is set by ISET) and Sets the Pre-Charge Current to twice the Termination Current Level." The current transitions from pre-charge to fast charge when the battery reaches VLOWV. The termination current, provided the right conditions, is the charge current out of the charger that will cause the charger to stop charging.

    D. As shown in the datasheet, "The TS feature is implemented using an internal 50μA current source to bias the thermistor (designed for use with a 10k NTC β = 3370 (SEMITEC 103AT-2 or Mitsubishi TH05-3H103F) connected from the TS terminal to VSS."

    Regards,

    Mike Emanuel

  • 0 in reply to Michael Emanuel35
    Prodigy 10 points

    Thanks Michael for the reply,

    Michael Emanuel35 said:
    D. As shown in the datasheet, "The TS feature is implemented using an internal 50μA current source to bias the thermistor (designed for use with a 10k NTC β = 3370 (SEMITEC 103AT-2 or Mitsubishi TH05-3H103F) connected from the TS terminal to VSS."

    Thanks for pointing this out, I missed this part.

    This did solve my Charger queries, however still waiting for fuel gauge follow-up queries to be answered..

  • 0 in reply to Arnav Jain
    TI__Mastermind 40305 points

    I will hand it off to the gauge team to finish the questions.

  • 0 in reply to Arnav Jain
    TI__Mastermind 37741 points

    Hello Arnva,

    Yes, normal LiPo and Li-ion cells are supported by this gauge.

  • 0 in reply to Adrian
    Prodigy 10 points

    Adrian thanks for letting me know the above, please answer one more question that I asked.

    Arnav Jain said:
    If I connect the Battery to the BAT pin as you suggested then I would need to connect the Output of the Charger to VSYS, is that the correct way charger input to Fuel Gauge? Also this would mean that VSYS will be shared by Battery Output (SRX pin of Fuel Gauge) and Charger Output (OUT of Charging IC), Is this correct?
  • +1 in reply to Arnav Jain
    TI__Mastermind 37741 points

    Hello Arnav,

    You understanding is correct.

    Regards,

    Adrian