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Implementing charge control with the BQ24401

tolu collins
Prodigy 110 points
Other Parts Discussed in Thread: BQ24401

i'm trying to understand in more details how to design around the bq24401.

Can i please have an explanation of the current control circuit in figure 7, page 10 of the BQ24401 datasheet?

thanks.

  • 0
    TI__Genius 13700 points

    Hello Tolu,

    The current sense works in the following way. I have highlighted the current path used for charging in the circuit seen in Fig. 7.

    The BQ24401 then determines the voltage that is present across R3 between the the chip ground, which is represented by the triangle ground symbol, and the sense point of SNS.

  • 0 in reply to Ryan May
    Prodigy 110 points

    Hello Ryan,

    thanks.

    I understand that the chip functions as a buck converter controller, switching Q1 ON through Q3 to control the output current to the battery,


    I want to design for 4NiMh batteries in series, for a charge current of up to 2A, so i'm trying to understand in details how to select the components. I've understood how to configure the IC, my concern is the network of Q1, Q2, D5, C9, R12, D2 and L1.

    I would like to understand how they work, and how to select their values.

  • 0 in reply to tolu collins
    TI__Genius 13700 points

    Hello Tolu,

    A majority of these components simply need to be selected to match the characteristics of the chip and the charging profile you are wanting to implement.

    Q1: This needs to be a PNP transistor that has a stopping voltage above the anticipated input voltage and capable of conducting the desired charging current in continuous operation.

    Q2:This needs to a signal level transistor. I would recommend using the characteristics of the MMBT3904LT1 to guide your selection. This is simpler to select since its only purpose is to pull the base of the Q2 back up to the input voltage quickly

    D5: Similar explanation to Q2 when it comes to selection since its purpose is to charge C9 to reduce the voltage strain on Q2

    C9: Follow Fig 7 from the datasheet. See D5.

    R12: Follow Fig 7 from the datasheet.

    D2: The selection of this power diode is going to be similar to Q1 and I would recommend looking at switching speed characteristics of ZHCS1000. This needs to be able to spot the input voltage and handle the full charging current in continuous operation while being able to switch fast enough to keep up with the buck converter switching. Also the lower you can get the forward diode voltage drop the more efficient it will be

    L1: This is the power inductor and must be sized to handle the peak charging current in continuous operation with out saturating. The size of this and the C5 are important for managing the switching ripple which if left unchecked could damage the battery.