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BQ24780S: How to do pre-charge?

chentsu chung
Mastermind 6180 points
Part Number: BQ24780S

Hi Team,

1. How BQ24780S do pre-charge? Need to write commend from MCU?

2. I can not find the "battery voltage" information for reading? Does BQ24780S has this register?

3. If BQ24780S has no battery voltage for reading, then how MCU knows the battery is low voltage and set charging current as pre-charge level?

Thank you very much,

C.T.

  • 0
    TI__Genius 11850 points

    Hi Chentsu,

    1) The BQ24780S will automatically apply pre-charge current when VBAT < BAT_LOWV. No MCU command is necessary (although the Charge Current and Charge Voltage registers must be non-zero). The rising threshold of BAT_LOWV is 2.7 V, and IPRECHG is set by a 5 mV drop across the Rsr sense resistor (please see the table below). This corresponds to IPRECHG = 0.5 A with the default Rsr value of 10 mΩ.

    To help illustrate this, the waveform below shows the behavior as VBAT rises above BAT_LOWV, at which point the BQ24780S applies a 1 A fast charge current. When VBAT falls below BAT_LOWV, the BQ24780S applies a 0.5 A pre-charge current instead.

    2) The BQ24780S does not have an integrated ADC to allow the user to easily read VBAT from the registers.

    3) The charger detects VBAT through the SRN pin and uses this voltage to determine whether to apply pre-charge current or fast charge current (see the comparator below).

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Mastermind 6180 points

    Hi Angelo,

    Thanks for your great help, it's very clear with the waveform.

    But this 2.4V is good for 1 cell application, may I know how BQ24780S do pre-charge for 2~4 cells application?

    Or BQ4780S can internal scale up the LOWV threshold to meet different cell application?

    Thank you very much,

    C.T.

  • 0 in reply to chentsu chung
    TI__Genius 11850 points

    Hi Chentsu,

    Unlike some of TI's other chargers which have a CELL pin to select the number of cells, pre-charge on the BQ24780S works the same way with different cell counts. When VBAT is below the fixed BAT_LOWV threshold, the charge current is clamped to the pre-charge current value (0.5 A with the default Rsr value of 10 mΩ).

    Best regards,

    Angelo

  • 0 in reply to Angelo Zhang87
    Mastermind 6180 points

    Hi Angelo,

    Got it, BQ24780S can only do pre-charge that using 2.5V LOWV threshold.

    So, what I want to know is, for 3 cells battery pack (pre-charge current will be around 7~8V), how do you recommend we use BQ24780S to do pre-charge?

    By external ADC to sensing battery voltage for MCU, and let MCU write commend to BQ24780S to config charging current?

    Best regards,

    C.T.

  • 0 in reply to chentsu chung
    TI__Genius 11850 points

    Hi Chentsu,

    Yes, the solution you suggested is fine. Your MCU can write your desired charge current to the registers based on the battery voltage.

    Best regards,

    Angelo