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BQ24780S: Charge current accuracy?

Christian 1
Prodigy 20 points
Part Number: BQ24780S
Other Parts Discussed in Thread: BQSTUDIO

Hi!

We use BQ24780s in our design and when we verfy charge current we get very good accuracy (-2%) with the lowest charge current setting but poor accuracy with high charge current.

This result is opposite to what we expect because in BQ24780s data sheet page 6, the accuracy is the other way around; it gets worse with lower charge current.

We have 20mOhm sense resistor, so if we write 128mA in register we expect to get 64mA (and we measure 63mA).

The question is why this phenomena; as suggested in data sheet it is not expected so is there some setting or other we have done wrong?

Written register value (mA) Expected current (mA) Measured charge current (mA) Diff (%)
128 64 63 -2%
192 96 94.5 -2%
256 128 126 -2%
320 160 157.5 -2%
384 192 188.5 -2%
448 224 219 -2%
512 256 249.5 -3%
576 288 279 -3%
640 320 308 -4%
704 352 335.5 -5%
768 384 362 -6%
832 416 386.5 -7%
896 448 408.5 -9%
960 480 428 -11%
1024 512 446 -13%
1088 544 462 -15%
1152 576 474 -18%
1216 608 480 -21%

 

Thanks

  • 0
    TI__Genius 11850 points

    Hi Christian,

    Here are my test results using the BQ24780S EVM with a 20 V adapter, 10 V battery, 12.592 V charge voltage register setting, and no system connected. As expected, the charge current accuracy tends to be higher as the charge current increases.

    Register Setting (mA) Measured Charge Current (mA) % Difference
    128 124 -3.1%
    192 187 -2.6%
    256 249 -2.7%
    320 314 -1.9%
    384 377 -1.8%
    448 438 -2.2%
    512 502 -2.0%
    576 569 -1.2%
    640 633 -1.1%
    704 699 -0.7%
    768 763 -0.7%
    832 825 -0.8%
    896 884 -1.3%
    960 947 -1.4%
    1024 1010 -1.4%
    1088 1074 -1.3%
    1152 1137 -1.3%
    1216 1202 -1.2%
    2048 2034 -0.7%

    It looks like you're running into something else that's dialing back your charge current. There could be several causes for this, so can you give me more information on the following questions?

    1) Are you testing this with a system connected, or is it just the adapter, the charger, and the battery?

    2) What is your adapter voltage, and what is its current limit set to?

    3) What is your battery voltage when you try to test this, and what is your charge voltage register setting?

    4) What is your input current register setting in BQStudio?

    5) What is the value in register "Charge Option 2"?

    I'd also be curious to see what would happen if you tested significantly higher charge currents (2 A or higher, like in the last row of my table). How big is the error then?

    Best Regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Prodigy 20 points

    Hi Angelo,

    Thanks for your support! Here are inputs

    1) with system connected

    2) adapter voltage is 5V, capable of delivering 1.8A

    3) battery voltage is about 2.6V, charge voltage register setting is 3088mV

    4) Haven't used BQStudio but value in register 0x3FH "Input Current Setting" is 0x1000H

    5) the value in register 0x38H "Charge Options Control 2" is 0x0384H

    I will test with significantly higher charge currents as you suggest but first need to modify my prototype.

    Thank you,

    Best regards

    Christian

  • 0 in reply to Christian 1
    TI__Genius 11850 points

    Hi Christian,

    Thanks for the info! I think I’ve found your issue.

    Your battery voltage is only 2.6 V. However, for safety reasons, the BQ24780S does not apply the full fast charge current when the battery is at a low voltage. Instead, it applies a lower pre-charge current to safely charge the battery up to a voltage high enough for fast charge. This voltage threshold is called BAT_LOWV, as shown in the picture below.

    The waveform below illustrates this. My Charge Current register is set to 1024 mA, and you can see that the transition from 500 mA pre-charge to 1024 mA fast charge only occurs at around Vbat = 2.79 V when Vbat is rising and Vbat = 2.6 V when Vbat is falling. This is close to the datasheet's typical values of 2.7 V and 2.5 V, respectively.

    Since your battery voltage of 2.6 V is below the BAT_LOWV threshold, the BQ24780S is only applying pre-charge current, which is why your Ichg stays low for higher Charge Current register settings. No matter how high you set your Charge Current register, it won’t matter if your battery voltage is below BAT_LOWV because the Charge Current register only sets the fast charge current, not the pre-charge current.

    To fix this issue, you would just have to increase your battery voltage above BAT_LOWV, and then you'll be able to apply the full fast charge current.

    Best regards,

    Angelo