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Original question:

BQ21040: BQ21040 charge current drop

BQ21040: Charge current dropping as Vin changes

Lauren Mayfield
Expert 6910 points
Part Number: BQ21040

Hi,

My customer seems to be having an issue similar to this one, but looks like they stopped responding so there wasn't a resolution.

Inquiry from the customer-

Please find our charging circuit as below. Per our design, Iset is set to ~415mA.

If the Vin is 5V or lower than 5V, when the charging current tapers to around 50mA (~10% of Iset), the CHG terminal will turns to high and the charging current will drops to below 10mA.

But, I found that if the Vin is higher than 5V, the termination current would become higher than 10% of Iset. And, the charging current would not stop and continue to charge the battery until the battery enters over-charge protection (4.25V). Please find my findings as below table.

 

Vin

termination current

Charging current after CHG terminal turn to high

5.0V

~50mA

<10mA

5.1V

~60mA

From ~40mA to ~20mA

5.3V

~240mA

From ~220mAto ~150mA

 

  1. Is it the expected result? The maximum Vin should be 6.5V. I don’t know why the termination current will be changed by the Vin and the charging current cannot terminate.
  2. Can I stop the charging current or only keep the charging current under 10mA, after CHG terminal turn to high (Condition: any Vin is under Max Vin)?
  3. Can I keep the termination current close to 10% of Iset (Condition: any Vin is under Max Vin)?

 

Our charging circuit:

 

  • 0
    TI__Mastermind 22240 points

    Hi Lauren,

    Are you measuring the voltage at VCHRG? What is the battery voltage when these data points are taken? Is it possible to look at the following voltage waveforms:

    • TS voltage
    • Battery voltage
    • IN voltage

    Looking at the schematic, we do expect a capacitor on the battery pin as well. 

  • 0 in reply to Anthony P Pham
    TI__Expert 6910 points

    Hey Anthony,

    From the customer-

    The voltage measured in previous email is VCHRG in the schematic( Pin 6 Vin of charging IC). The battery voltage is around 4.2V when I took the data. And, there are two capacitors (10uF) parallel to the battery, but they are not showed in the charging circuit.

    Today I take the data again by CRO. Please find the below data for your reference.

    (refer to TEK00002 & TEK00003)

    Voltage of Battery just before charging (Pin2) (CH1)

    Voltage of Battery

    (Pin2) (CH1)

    Voltage of Vin

    (Pin6) (CH2)

    Voltage of VCHRG

    (Pin3) (CH3)

    Voltage of TS

    (Pin 1) (CH4)

    Charging current after CHG terminal turn to high

    4.220V

    4.268V

    5.357V

    3.364V

    4.989V

    ~185mA

     

    (refer to TEK00004 & TEK00005)

    Voltage of Battery just before charging (Pin2) (CH1)

    Voltage of Battery

    (Pin2) (CH1)

    Voltage of Vin

    (Pin6) (CH2)

    Voltage of VCHRG

    (Pin3) (CH3)

    Voltage of TS

    (Pin 1) (CH4)

    Charging current after CHG terminal turn to high

    4.247V

    4.252V

    5.234V

    3.363V

    4.933V

    34mA

     

    (refer to TEK00007 & TEK00008)

    Voltage of Battery just before charging (Pin2) (CH1)

    Voltage of Battery

    (Pin2) (CH1)

    Voltage of Vin

    (Pin6) (CH2)

    Voltage of VCHRG

    (Pin3) (CH3)

    Voltage of TS

    (Pin 1) (CH4)

    Charging current after CHG terminal turn to high

    4.236V

    4.245V

    4.831V

    3.364V

    4.588V

    5mA

     

    Thank you!
    Lauren

  • 0 in reply to Lauren Mayfield
    TI__Mastermind 22240 points

    Hi Lauren,

    If the battery is close to 4.2V, the charge current will begin to taper to the termination current which is 10% at 5V which is expected behavior. I wouldn't expect the termination current to change with VIN and the fact that it is increasing with VIN is a concern.

    I also notice that the TS voltage is oddly high despite a 10k resistor. This may be putting it in TTDM mode and disabling termination but wouldn't explain the change in current with changing VIN. This voltage should be about 0.5V with a 10k resistor. Can we check that the IC is soldered properly? 

  • 0 in reply to Anthony P Pham
    TI__Expert 6910 points

    Hey Anthony,

    Their comments-

    Sorry! The voltage of TS measured before was wrong because I monitored it with the wrong place. The voltage of TS should be around 0.5V. Please find the attached wavefront for your reference.

    Apart from the termination current, the maximum charging current is also changed with the voltage difference between Vin of charging IC and battery voltage. Please find the below data for reference.

     

    1. Is it the expected result? Per the specification, the maximum Vin should be 6.5V. Why do the termination current and the maximum charging current be changed with the voltage difference between Vin of charging IC and battery voltage?
    2. Can I stop the charging current or only keep the charging current under 10mA, after CHG terminal turn to high (Condition: any Vin is under Max Vin)?
    3. Can I keep the termination current close to 10% of Iset (Condition: any Vin is under Max Vin)?
    4. Can I keep the maximum charging current  close to Iset (Condition: any Vin is under Max Vin)?

     

     

    Per our circuit design, Iset is set to ~415mA.

    Voltage of Battery

    Voltage of Vin

    Voltage Difference

    Charging current

    ~4V

    5.15V

    1.15V

    ~465mA

    ~4V

    4.86V

    0.86V

    ~430mA

    ~4V

    4.41V

    0.41V

    ~366mA

    ~4V

    4.40V

    0.40V

    ~341mA

     

     

     

     

    ~3.8V

    5.06V

    1.26V

    ~580mA

    ~3.8V

    4.84V

    1.04V

    ~437mA

    ~3.8V

    4.82V

    1.02V

    ~434mA

    ~3.8V

    4.39V

    0.59V

    ~369mA

     

     

     

     

    ~3.5V

    4.93V

    1.43V

    ~762mA

    ~3.5V

    4.81V

    1.31V

    ~525mA

    ~3.5V

    4.70V

    1.20V

    ~456mA

    ~3.5V

    4.47V

    0.97V

    ~425mA

    ~3.5V

    4.40V

    0.90V

    ~381mA
  • 0 in reply to Lauren Mayfield
    TI__Mastermind 22240 points

    Hi Lauren,

    I don't believe this is the intended result. I will attempt to explore any reasons why this might be happening in the lab. 

    Lauren Mayfield said:
    Is it the expected result? Per the specification, the maximum Vin should be 6.5V. Why do the termination current and the maximum charging current be changed with the voltage difference between Vin of charging IC and battery voltage?

    This is not the expected result for increasing VIN. For decreasing VIN below the VIN-DPM threshold, this would slowly decrease the charge current. For the lower VIN voltages, the corresponding lower charge currents is expected.

    Lauren Mayfield said:
    Can I stop the charging current or only keep the charging current under 10mA, after CHG terminal turn to high (Condition: any Vin is under Max Vin)?

    Once the /CHG terminal pulls high, there should not be any current flowing into the battery.

    Lauren Mayfield said:
    Can I keep the termination current close to 10% of Iset (Condition: any Vin is under Max Vin)?

    Termination current is set to 10% of the programmed fast charge and cannot be changed. 

    Lauren Mayfield said:
    Can I keep the maximum charging current  close to Iset (Condition: any Vin is under Max Vin)?

    It is expected that you should be able to keep the maximum charge current if there are no issues with the supply power when in constant current or fast charge mode. 

  • 0 in reply to Anthony P Pham
    TI__Expert 6910 points

    Anthony,

    Hopefully I caught you before you went into the lab and had to do some extra work!

    They found another path in the circuit from Vin to Vbatt. That's why the charging current cannot stop and the charging current is changed with changing Vin. They fixed it.

    Thanks for your help!

  • 0 in reply to Lauren Mayfield
    TI__Mastermind 22240 points

    Hi Lauren,

    Thanks for the update! That's good to know, I'll mark the thread as resolved. Feel free to reply should there be any new updates regarding this issue.