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BQ24250: IC charging battery at too high current

Jean Smith1
Prodigy 40 points
Part Number: BQ24250

Good day

I am having issues with the BQ24250 IC. I am building a battery charger with adjustable charge current (which I set through I2C). The problem is that whenever I select a charge current higher than 1A, the current being delivered to the battery seems to be too high.

For instance, if I set the charge current to 2A, I am measuring 2.1A and sometimes higher with my current clamp (on the positive wire of the LiPo battery). I have also tried lower currents such as 1.5A. With this setting, I am measuring 1.7A.

I have also found that the charge current jumps around to different values sometimes before it settles on a charge current.

When the charge current is higher than it should be, I also find the IC to go into current regulation loop mode (which I can see in the registers).

When the charge current is below 1A, the charge current is accurate.

What could be causing this unstable charge current behaviour?

Here is my circuit:

Here is my PCB layout:

Thanks in advance

John

  • 0
    TI__Guru*** 146015 points

    Hi Jean,

    Is your current clamp a hall effect based clamp?  Does its accuracy change above 1A?  

    What is physically connected to the ISET pin?  Are you disabling the watchdog timer?  

    Can you send the I2C registers settings at charge current > 1A and < 1A?

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    Hi Jeff

    I don't think it is hall effect based. I have measured currents larger than 1A on other circuits successfully accurately before, so I think it is accurate above 1A.

    I have a few resistors connected at ISET to set the charge current to 2A as follows:

    I have disabled the watchdog timer as you suggested, but it doesn't seem to make a difference.

    At 2A charge current, the registers read as follows:

    Reg 0: 10
    Reg 1: DC
    Reg 2: 8C
    Reg 3: F0
    Reg 4: 82
    Reg 5: A8
    Reg 6: E0

    At 1A charge current, the registers read as follows:


    Reg 0: 10
    Reg 1: DC
    Reg 2: 8C
    Reg 3: 40
    Reg 4: 02
    Reg 5: A8
    Reg 6: E0

    At 1.2A charge current, the registers read as follows:

    Reg 0: 10
    Reg 1: DC
    Reg 2: 8C
    Reg 3: 70
    Reg 4: 02
    Reg 5: A8
    Reg 6: E0

    My input current limit is set to 2000mA.

    Thank you for the help

    Kind regards

    Jean

  • 0 in reply to Jean Smith1
    TI__Guru*** 146015 points

    HI Jean,

    When in default (standalone) mode after the WD timer expires, the charge current is set by the external resistor.  At that time, the charge current variation is a function of both the external resistor tolerance (for example +/- 1%) and the K(ISET) variation (297 - 267.5) = +/-7%.  When in host mode (WD timer not expired or disabled), the ICHG variation is also +/-7%.  So if you set for 2A, the actual current value could be 1.86A-2.14A.  And, the charger will likely not provide the same value in standalone or host mode.

    Based on the charger's I2C registers that you provided, when you set the charge current to 2A, the charger is in input current limit, which means the input current is being limited to 2A.  Is that actually happening? If so, I would expect the charge current to be below the 2A setting.

    Regards,

    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    Hi Jeff

    Interesting about the 7% variation. That would explain the variation in charge current.

    I am measuring that the current draw from the power supply is in fact about 1.7A when the charger is drawing about 1.7A. In this state it shows that the charger is in input current limit, even though I set the input current limit to 2A. The voltage that I am measuring on the battery while charging is 3.98V. The input voltage on the charger is 5.19V.

    What could be causing the charger to go into input current limit mode even when I am drawing less than 2A?

    Thanks in advance

    Jean

  • 0 in reply to Jean Smith1
    TI__Guru*** 146015 points

    Hi Jean,

    The charger limits the buck converter duty cycle so that it can only pull the input current limit setting from the input source. The input current limit regulation accuracy is about +/-11% if set by the external resistor on ILIM pin and KILIM or 1850mA-2050mA if set by the internal register.  Not sure why you are only measuring 1.7A.  At VBAT=3.98V and ICHG=2A -> Pout=8W, you will need Pin=VIN*IIN= 8W/efficiency=8W/0.87 = 9.2W ->9.2W/5.19V=1.77A =IBUS.  That's assuming that the charger's IN pin is actually seeing 5.19V and not lower due to a drop due to line resistance.  Also, if there is a drop from the charger BAT pin to the battery itself due to line resistance, the BAT could be at a higher, which would require even more input current. 

    Regards,
    Jeff

  • 0 in reply to Jeff F
    Prodigy 40 points

    Hi Jeff

    I think the measured value could have been closer to 1.77A when I measured.

    With the 87% efficiency and the input limit regulation accuracy that you explained, it makes sense what I am observing.

    Thanks for the help

    Kind regards

    Jean