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Original question:

BQ25710: In-device charging with regen-capable BLDC device

BQ25731: Behavior and configuration at startup with battery disconnected

Vincent Politzer
Prodigy 60 points
Part Number: BQ25731
Other Parts Discussed in Thread: TPS25751

Hello,

I am working on a design where the BQ25731 can be used to charge our device's removeable battery (3S lithium ion). It will be supplied power via a TPS25751 USB-C PD controller. Our device has a host MCU, which is powered off of a 3.3V SMPS, which is supplied by the battery. There is also a 5V SMPS on-board for some other peripherals (plus logic level shifters).

The scenario I am trying to sort out is what happens when USB power is applied via the TPS25751, but the battery is not inserted. The ideal behavior is that the USB PD controller and charger ICs can be configured correctly after power up, and the system can remain idle until a battery is inserted, at which point the charger can begin charging.

However, if I want to configure the USB PD controller and charger ICs via the host, I would need the charger to output our system voltage (at least, enough for the 5V SMPS) for the system to properly start up. My understanding is, with CELL_BATPRESZ detecting no battery, the default output voltage would be 4.2V. Does this mean that our system will require an EEPROM for the TPS25751 to load the patch at boot?

Additionally, while the device is not intended to fully function without the battery installed, it would be useful if the BQ25731 can still supply enough power for the host processor to run for debugging purposes (approximately ~250mA and 12V would suffice). Can it support that?

We are considering the BQ25731 because we have a scenario where cannot allow the charger to block charge current into the battery. This is because our device has the ability to charge the battery via BLDC regenerative braking, where current would flow backwards into the battery. If the charger blocked this current, VSYS could rise very high, causing component failures.

Regards,
Vincent

  • 0
    TI__Mastermind 35435 points

    Hi, Vincent,

    You can set the CELL_BATPRESZ pin to 3S. There is no battery detection algorithm.  The pin is manually set the host MCU or user. Pulling the CELL_BATPREZ pin to low, sends a signal to the charger that no battery is presented. The charger will stop charging accordingly, otherwise, BQ25731 will charge the battery by a small current 128mA. 

    It will charge the output to 12.6V assuming the battery cap is a small battery.

    Regards,

    Tiger

  • 0 in reply to Tiger Zhou
    Prodigy 60 points

    Hi Tiger,

    Thank you for the clarification on the function of the CELL_BATPRESZ pin. Since our host MCU is downstream of the charger output, it seems the trickle charge would be desirable for our specific use-case. Is 128mA for a 10mΩ sense resistor?

    In the datasheet, it states, in section 9.6.2, the ChargeCurrent Register (I2C address = 03/02h) resets to 0A. Figure 9-15 says the register resets to 0000h, but the section header of 9.6.2 says it resets to 0080h. That seems to correspond to bit 7 of the second byte, or "charge current bit 1". With a step size of 128mA (5mΩ sense resistor), does that mean the POR output current may be 256mA? Either way, I am hopeful the POR output current would be enough for our host to configure the ICs.

    Regards,
    Vincent

  • 0 in reply to Vincent Politzer
    TI__Mastermind 37481 points

    Hello Vincent,

    Thank you for your patience. Tiger is out office today and will return on Monday to provide a follow up to your question.

    Best Regards,

    Christian.

  • +1 in reply to Vincent Politzer
    TI__Mastermind 35435 points

    Yes, BQ25731 default charge current is 0x0080, corresponding to 256mA with 5mohm current sense. 

    See attached default settings. 

    Fullscreen 20VIN-Default-Settings.txt Download 
    * Created: Tue Mar 16 11:22:10 CDT 2021
*
* Format: Register Name  tab Character,\t  Hexadecimal register value.
* Device: bq2573x
* BQZ Container: Charger_1_00-bq2573x.bqz
*
Charge Option 0	E70A
Charge Current	0080
Max Charge Voltage	20D0
OTG Voltage	09C4
OTG Current	3C00
Input Voltage	3BC0
Min System Voltage	4200
IIN_HOST	2000
Charger Status	8400
Prochot Status	B800
IIN_DPM	2000
ADC VBUS and PSYS	CE00
ADC IBAT	0000
ADC IIN and CMPIN	0000
ADC VSYS and VBAT	0000
Manufacture ID	D640
Device ID	00D6
Charge Option 1	3F00
Charge Option 2	00B7
Charge Option 3	0434
Prochot Option 0	4A81
Prochot Option 1	41A0
ADC Option	2000
Charge Option 4	0048
Vmin Active Protection	006C

  • 0 in reply to Tiger Zhou
    Prodigy 60 points

    Thank you very much Tiger! This gives us a great starting point. We really appreciate your help.