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bq24040

Geoff Harvey
Prodigy 30 points
Other Parts Discussed in Thread: BQ24040

Hi,

I'm using a bq24040, were I want the current to be programmed by ISET.

So I have connected ISET2 to GND.

But the battery charge current never exceeds 100mA. (as if ISET2 is floating, which it isn't)

Is a transition required on ISET2 in order to put it into order for the device to sample the ISET2 pin?

Geoff

  • 0
    TI__Guru**** 165766 points

    What is the input voltage at the input pins?  What is the current limit of the power source?  What is the voltage of battery, i.e., is the IC stuck in pre-charge because battery voltage is low?

  • 0 in reply to Jeff F
    Prodigy 30 points

    Jeff,

    You are absolutely right.

    There was too much resistance in the various breakouts/cabling/metering we put inline with the USB outlet for measurement, such that although the input voltage was good at 100mA, it would have dropped below the limit at 420mA (the fast charge current we set).

    We were too focussed on the device perhaps being on the wrong charging mode and overlooked the obvious.

    When we removed the excess resistance the device was able to charge at 420mA.

    Many thanks for you help and insight.

    Geoff

  • 0 in reply to Geoff Harvey
    Intellectual 345 points

    Greetings,

    I'm having trouble in understanding the concepts of setting the value of Rpreterm for BQ24040.

    I want to charge my Li battery 3.7V 450mAh.

    My Fast charge current is 200mA, so the value of RISET becomes 2.7K. 

    Now what value of Rpreterm should I use with pin4.

    What will happen if I set the ISET2 = Low? (w.r.t above scenario what would be the input current and output current of this device?)

    Regards,

    Muhammad Rehan.

  • 0 in reply to Muhammad Rehan
    TI__Expert 7730 points

    Rpreterm is used to program the pre-charge and termination current. The formula Rpreterm / KPRE-CHG% is used to calculate the pre-charge is how much percent of the fast charge current. KPRE-CHG% is 100 ohm/% typical in the datasheet.

    For example, if Rpreterm is 2k ohm, then the pre-charge current is 2k/100=20% of the fast charge current. The termination current is always half of the pre-charge current.

    If ISET2 = Low, the input current limit will be determined by your RISET. If ISET2= High, the input and output current is limited to 500mA and if ISET2 is floated, the current is limited to 100mA.

    Thanks,

    Wenjia

  • 0 in reply to Wenjia Liu
    Intellectual 345 points

    Dear Wenjia,

    Thank you very much for your kind response.

    Regards,

    Muhammad Rehan.