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USB charger for 3 x NiMH cells in series

Gary Poon
Intellectual 510 points
Other Parts Discussed in Thread: BQ2002, LM317

I am designing a charger which takes input power from computer USB port and fast charge for 3 NiMH cells(1.2V each, 300mAh) connected in series. I noted that somebody suggested BQ2002 in this forum earlier, so I studied the user guide: http://www.ti.com/lit/ug/sluu007b/sluu007b.pdf

However I do not understand the function of the LM317. How does it controls the constant charging current and voltage to the cells?

Also during the charging period, the USB needs to supply power to the device MCU as well. I planned to add a diode at the output pin of the charging IC to form an ORing diode supply, is it fine?

Can you suggest a solution?

Thank you.

  • 0
    TI__Mastermind 25296 points

    Please refer to figure 4 of LM317 datasheet, as constant current regulator / limiter...

    Basically LDO will regulate the equation on page 7 of the device datasheet changes (since the output has been moved) into

    Vref / R1 + Iadj = ILoad + Voutput / R2

    in which now

    Voutput = Vadj_pin

    R2 = open

    Iadj = 0 (almost, or ILOAD >> Iadj)

    so that

    ILoad = Vref / R1

    and since both Vref and R1 then we can assume the behaviour as a constant current regulator...

    I hopw this helped a bit...

    Vincenzo