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bq24620: Additional bulk input capacitance needed if multiple chargers share single PSU?

John M
Expert 2135 points
Other Parts Discussed in Thread: BQ24620, TPS5450, TPS2543

If I have two bq24620-based chargers on a PCB (each 5A, 14.4V output) sharing a single 24V switching PSU via a single PCB power connector. Would you recommend additional bulk input capacitance either at the PCB connector or near the bq24620 inputs?

Also sharing the 24V PSU are a TPS5450 providing power to two TPS2543 USB chargers (1.5A max each) and another circuit drawing 1.5A (occasionally). All are mounted on a single, 4-layer, 2oz. PCB with careful attention to power routing and effective ground planes and ground plane current flow. The PSU's output connector has about 4" of wire to the PCB power connector and the power plane/traces to the bq24620's will probably be 2" in length (or less).

Each charger will be used randomly, perhaps one on at a time or all.

If additional cap. is recommended, I could add a small power plane at the PCB connector (with room to mount any additional cap. needed) or just mount the additional cap. near each bq24620's other input caps. Whichever is better.

Thanks!

  • 0
    TI__Mastermind 31120 points

    It is best to have the appropreate amount of input capacitance next to each design since this is where the high frequency pulsed current comes from (having it across the board adds inductance and resistance, which is not good for a negative impedance switcher.

    Hopefully your comment about a single layer is for the component side and you have a second layer ground plane.  Expect much higher impedances with a one layer design, higher noise floor and more issues.

  • 0 in reply to Charles Mauney
    Expert 2135 points

    Thank you for your response Charles! I'm using a 4-layer board with a layout very similar to the bq24620's EVM board. Ground planes everywhere. :-)

    We're currently using the same input cap. setup as the EVM, a pair of 10uF MLCC X5R caps near the chip. Is there any need to increase that because of the multiple chargers drawing input current from the same PSU? Or is the 20uF enough?