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BQ24745 charge current limit

RAVI57144
Prodigy 50 points
Other Parts Discussed in Thread: BQ24745

What is the best way to limit the charge current to a maximum of 1A on BQ24745? I am not sure whether I can increase the value of sense resistor between CSOP and CSON from 0.01 Ohm to 0.1 Ohm. I am looking for a hard current limit (preferably cycly-by-cycle) regardless of what is being written with ChargeCurrent() command. I did not quite grasp what is meant by a upper limit of 5A when the chrage current drops below 3.3A.

My battery chemistry is NiMH @ 3.6V. Due to regulatory reasons for the mining industry, I need to limit it to below 1A.

Any help is appreciated. Thanks.

  • 0
    TI__Intellectual 2295 points

    first, the bq24745 do not have a cycle-by-cycle current limiting for OCP

    second, the sensing resistor better to be within 20mohm  for bq24745

    the over current protection is 145% of Ichg (which is setting command), Figure 27. give you a example the OCP protection

    there's a Maximum Charge current regulation at Low Voltage (<4V), (VFB_SHORT < VFB < 4 3), it's 3A typ.

    there's no hard limiting method to limit 1A maximum charge current

    what's the input/out voltage/current requirement? Do they must have SMBUS

    Let's see whether we can find another charger to do so.

     

  • 0 in reply to Bin Chen36084
    Prodigy 50 points

    Thanks Bin.

     

    Just to clarify a few things:

    If I use a 0.02 Ohm sense resistor, the maximum charge current will be limited to 80.64mV / 0.02 = 4.032A and the DAC steps will be reduced to 64 mA.

    The charger is using a 12V @ 2.5A adapter and the battery pack has built in OC protection with a fast acting 1A fuse. With a 0.01 sense resistor, I limited the current to 512+256= 768 mA. I have had a few fuse blowouts and noticed the current exceeding well over 1A during fast charge. At 145%, the current can reach 768 x 1.45 = 1.12A. Perhaps with component tolerances, I occasionally see 1.4A charge current.

     

    This board was designed by someone else and brought to me for failure analysis. Do you have any suggestions for a better charge controller with SMBus interface? 

     

     

  • 0 in reply to RAVI57144
    Prodigy 50 points

    Bin,

    Could you please comment on my previous post. The datasheet is vague in this regard and I would like to understand how this current limiting works.

    Thanks very much.

    -Ravi

  • 0 in reply to RAVI57144
    TI__Guru 56650 points

    Yes. If you use a 0.02 Ohm sense resistor, the maximum charge current will be limited to 80.64mV / 0.02 = 4.032A and the DAC steps will be reduced to 64 mA.

    bq24745/747 charging current accuracy is about 5%@20mVsns or 25%@10mVsns. If the charging current is 1A with 0.02 ohm sense resistor. The accuracy should be in 5% unless the sense resistor has a big error.