BQ24640: BQ24640

Part Number: BQ24640

On the BQ24640 part, what is the lowest operational charge current I can set using iset with the sense resistor?

Can I program it for 100uA charge current?


3 Replies

  • Hi Douglas,

    Can you give me more information about your application? 100 uA of charge current seems very low.

    Using the formula above with the default Rsr value of 10 milliohms, that would require Viset = 20 uV. This would also mean that your voltage divider from Vref to Iset to GND would need a resistor ratio of 165000, which doesn't seem very practical and could easily be incorrect due to part tolerances.

    Best Regards,

    Angelo Zhang

    Applications Engineer | Battery Charging Products

  • In reply to Angelo Zhang:

    Hi Angelo,

    I have an energy harvesting application such that I'm using a CT and Solar to harvest energy, but in some case, the solar is not installed since it's an option. The solar harvesting uses an LTC3129 energy harvesting part and the CT harvesting uses an LTC3630A since the input has a wide Vin up to 60V. I have a supercap charger LTC3225 to charge and actively balance the two 60F caps in series. The output of the LTC3630A and LTC3129 are 5V and feed the LTC3225, however, when there is no solar option installed, the CT can only harvest 1mA. In the low end condition for CT to harvest energy, all it can do is 1mA, which the system (processor, etc) needs this to run (perhaps 800uA). There is not much energy remaining to charge the supercaps. It's ok for them to take a lot of time to charge, but with these supercap chargers, they don't allow for setting the charge current to a very low value of ~100 to 200uA. Therefore, the only thing I can think of is to use a discrete implementation such as an LDO with a series resistor at output and another resistor controlled by a FET for firmware to change the charge current, then have a discrete active supercap balancer on that as well. 

    Basically, right now, if the LT3225 or probably any other supercap charger part that tries to draw more than 200uA, the LTC3630A Vout of 5V will collapse. It's doesn't have MPPC function like the LTC3129, which behaves a lot better since its output gets pulled down but at least the Vin of the LTC3225 stay above threshold of ~2.55V to charge. But again, at some point, there is too much power being demanded which isn't available to charge the supercaps.

    So, long story short.... I need a way to limit the current when the CT is the only thing implemented. If solar is present with the CT, they can handle the charging at the low end.

    Hope that makes sense.

    Thank you,


  • In reply to Douglas Macauley:

    Hi Douglas,

    Thanks for the description of your application!

    100 uA charge current is lower than almost all of our chargers can provide. I'd recommend that you take a look at the bq25570, which is an energy harvesting boost charger with a nanopower buck converter that can provide an output current from <10 µA up to 110 mA.

    The bq25570 also works with supercaps, can sustain up to 5.1 V input, and has an output voltage range of 2-5.5 V (not sure what your capacitors' voltage ratings are though).

    Best Regards,
    Angelo Zhang
    Applications Engineer | Battery Charging Products