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BQ24040: Li-Ion battery charging from 3.8V supply

Yuriy Romanovsky
Prodigy 80 points
Part Number: BQ24040
Other Parts Discussed in Thread: CC1310, TPS61099

Hello!

I have a task to charge standard Li-Ion Battery (3.7V / 1000mAh). My device has one step-down converter from 24 to 3.8V (to power GSM-module), and LDO (from 3.8 to 3.3, to power cc1310 RF-part), so I am limited to a source of 3.8 Volts. BQ24040 datasheet says, that input voltage range must be above 3.5, to begin charging process. Here is my schematic:

First I connected discharged battery, which voltage was 3.1V. When I'm turning charge on, voltage on "IN" pin raises to 3.85V, "PG" goes LOW (50 Ohm, so I assume that applyed voltage is enough), but voltage on "OUT" pin remains 3.1V.

I tryed to disconnect battery. In this case voltage on "OUT" pin is 0.145V.

What could be the reason that the charge does not start?

  • 0
    TI__Guru* 75806 points

    Hello

    Operating the BQ24040 from a 3.8V supply has several problems and would not be the best solution. 

    Have you considered adding a boost converter to increase voltage from 3.8V to 5V?  The TPS61099 is a possible boost converter.

  • 0 in reply to Bill Johns
    Prodigy 80 points

    Hello!

    Thank you for your reply!

    I understand, that charging Li-Ion Battery with 3.8V instead of 4.2V is not be the best solution. I planned to use a boost converter, but it was necessary to reduce the cost of the device.

    All charger IC's that support 3.8V input voltage I found using part selector (BQ257... family: Vin - 3.5V) are I2C/SMBus controlled and require a large number of external elements.

    Is there realy no option but use a boost converter?

    In this case I think, that BQ24040 datasheet is a little misleading ("Good" voltage source does not start to charge battery).

  • 0 in reply to Yuriy Romanovsky
    TI__Guru* 75806 points

    Hello

    The BQ24040 will require a voltage drop of about 350mV to 700mV,  Vin to Vout to regulate.  This is common for linear regulators.

    Options to consider:

    1.) Is a higher input voltage available, power supply for the 3.8V? We have some older linear regulator that have higher input voltage.

    2.) Is a lower voltage battery type an option, 3 X NiMH cells that would be about 3.6V.