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Original question:

BQ25820: Need some help with the battery charger design.

BQ25820: Input and output capacitance and leakage questions for charger

Myles Scott
Intellectual 350 points
Part Number: BQ25820

Tool/software:

1) In some documentation I see the suggested minimum input and output capacitance is 160uF.  If I go considerably lower than that, say 40uF for example what are the possible issues?

My system has 4 charging circuits for 4 batteries (so 4 BQ25280 chips).  The batteries are hot swappable so I would like to to limit capacitance that is is attached to the battery.  I could add a fet to handle this but I'm trying to avoid this due to space since I would  need 4 copies.  In my design I am not using the input fets or the battery fet and the batteries are isolated from the system during charging. 

Vin is 18V
Battery 16.5V @ 9AH
Max charge current is 2.5A

2) Would the input capacitance need to increase by 4x since I have 4 chargers or would a 2x or something be adequate?

3) If I disconnect VAC using external opposing fets do I need to worry about any leakage current into the BQ25820 from other pins assuming the battery is connected?

  • +1
    TI__Mastermind 31571 points

    Hello Myles,

    Myles Scott said:
    1) In some documentation I see the suggested minimum input and output capacitance is 160uF.  If I go considerably lower than that, say 40uF for example what are the possible issues

    160uF is the recommended. 80uF is the minimum. Using a lower amount of capacitance, can cause an increase in ripple current.

    I recommend refering to this section in the datasheet for choosing the correct amount of bulk capacitance:

    Myles Scott said:
    2) Would the input capacitance need to increase by 4x since I have 4 chargers or would a 2x or something be adequate?

    I recommend using 4X, to reduce ripple current.

    Myles Scott said:
    3) If I disconnect VAC using external opposing fets do I need to worry about any leakage current into the BQ25820 from other pins assuming the battery is connected?

    I recommend refering to section 6.5 Electrical Characteristics to see the quiescent current:

    Best Regards,

    Christian.