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BQ25756: Startup without Battery

Karsten Bernhard
Prodigy 30 points
Part Number: BQ25756

Tool/software:

Hi,

in a design we use BQ25756 with a 1 cell battery with bms and a parallel load of approx 5W.

If the bms switches off the battery or battery is empty, the charger shall power the load.

The load is switched off during startup. Only mcu is connected to charger output.

Now we have the issue that the charger does not startup without battery and mcu is connected.

If i understand right, the charger should works in trickle charge mode and deliver I_Bat_Short.

Unfortunately I can't find any informations about I_BAT_Short in datasheet.

Furter I can't see any signal in SW1 or SW2.

How many mA is I_Bat-Short?

Can you explain how trickle charge works?

Thank you very much for your support!

  • 0
    TI__Mastermind 30815 points

    Hello Karsten,

    Karsten Bernhard said:
    How many mA is I_Bat-Short?

    Trickle charge is ≈150mA.

    Karsten Bernhard said:
    Can you explain how trickle charge works?

    Trickle charge mode works by making the charger send small pulses of current to the output. It works similar to PFM mode.

    I have a few questions to help troubleshoot this:

    • What's your normal VAC and battery voltage?
    • What's your system load?
    • Does the behavior change if you set EN_PRECHG=0? This will disable trickle charge and precharge modes.
    • What are the status and flag registers here?
    • Can you send me your schematic?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 30 points

    Hello Ethan,

    many thanks for your answer.

    We test the system with an input voltage of 6V, and a charger output voltage of 4.1V

    The load during startup in the test is a 1kR Resistor at charger output.

    If the charger is in the state and I set EN_PRECHG=0, the charger starts and I see the correct output voltage

    I attached the schematic and the register export.

    Thank you very much!

    Fullscreen 8304.Register.txt Download 
    * Created: Mon Dec 16 06:50:47 CET 2024
*
* Format: Register Name  tab Character,\t  Register Address  tab Character,\t  Hexadecimal register value.
* BQZ Container: Charger_1_00-bq25756.bqz
*
Charge Voltage Limit	00	10
Charge Current Limit	02	640
Output Voltage Limit	04	00
Input Current DPM Limit	06	640
Input Voltage DPM Limit	08	348
Reverse Mode Input Current Limit	0A	640
Reverse Mode System Voltage Limit	0C	3E8
Precharge Current Limit	10	140
Termination Current Limit	12	A0
Precharge and Termination Control	14	0F
Timer Control	15	1D
Three-Stage Charge Control	16	00
Charger Control	17	C9
Pin Control	18	C0
Power Path and Reverse Mode Control	19	20
MPPT Control	1A	20
TS Charging Threshold Control	1B	96
TS Charging Region Behavior Control	1C	57
TS Reverse Mode Threshold Control	1D	40
Reverse Undervoltage Control	1E	00
VAC Max Power Point Detected	1F	00
Charger Status 1	21	09
Charger Status 2	22	80
Charger Status 3	23	00
Fault Status	24	00
Charger Flag 1	25	00
Charger Flag 2	26	00
Fault Flag	27	00
Charger Mask 1	28	00
Charger Mask 2	29	00
Fault Mask	2A	00
ADC Control	2B	60
ADC Channel Control	2C	0A
IAC ADC - Use Field View	2D	00
IBAT ADC - Use Field View	2F	00
VAC ADC	31	00
VBAT ADC	33	00
TS ADC	37	00
VFB ADC	39	00
Gate Driver Strength Control	3B	00
Gate Driver Dead Time Control	3C	00
Part Information	3D	12
Reverse Mode Battery Discharge Current	62	02

    22600.Schematic.pdf

  • 0 in reply to Karsten Bernhard
    TI__Mastermind 30815 points

    Hello Karsten,

    Thanks for the new information.

    I have a few suggestions for the schematic:

    1. The minimum capacitance for the charger to be stable is 80µF on both VAC and VBAT. Can you increase VAC capacitance to around 80µF?
    2. I recommend installing an RC filter in parallel with the ILIM_HIZ and ICHG resistors when these resistors are greater than 5kΩ.
    3. Make sure R405 and R406 are DNP for the initial testing.
    4. Short R401, R402, R403, and R404 for initial testing also.

    If suggestions 2,3, or 4 doesn't help the circuit performance, the IC might be bad here. Can you replace the BQ25756 IC and see if the behavior changes?

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    Prodigy 30 points

    Hello Ethan,

    unfortunately the charger does not start with a load and default parameter.

    If i set EN_PRECHG=0 the charger starts.

    I can't see any signal in SW1 or SW2 in this state. (I see the 40ms compensation measurement but nothing after that)

    Capacity was increased to 80uF and 800uF for test.

    We tested 3 PCBAs and saw this with all 3 pcbas.

    Is there any precondition the charger needs for startup?

    Many thanks!

  • 0 in reply to Karsten Bernhard
    TI__Mastermind 30815 points

    Hello Karsten,

    Thanks for being patient with this this. We are pretty busy at the moment. I'll get back to you on this question later this week.

    Best Regards,
    Ethan Galloway

  • 0 in reply to Ethan Galloway
    TI__Mastermind 30815 points

    Hello Karsten,

    Just to clarify, when the IC is in trickle charge mode, you don't seen any switching on SW1 or SW2? Do you see any switching on any of the gate drive traces?

    If you remove the 1kΩ resistor on VBAT, do you see output voltage come up then?

    Karsten Bernhard said:
    Is there any precondition the charger needs for startup?

    There shouldn't be any other preconditions. The charger meets all the preconditions at the moment because EN_PRECHG=0 works.

    Best Regards,
    Ethan Galloway