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BQ25703A: How does the charging current change when DPM is working due to system load increased

Part Number: BQ25703A
Other Parts Discussed in Thread: BQ25703

Hi,

   As title, we test the mechanism by reduce the input voltage continuously and slowly.

  When the input voltage goes to be lower than VDMP setting, the charging current( input to a battery simulator) is changed sudenly. ( 1.3A   --> 0.7 ~0.8A)

  Can you clarify the DPM( dynamic power management) mechanism in detail?   The charging current can be reduced but in which method or algorithm?

  What is the reduce current step?  500mA, looks too much in a step.Leo

 Leo

  • Hello Leo,

    The assumption of the VDPM setting is that you are using an adapter that has a nominal voltage above VDPM but is sagging below VDPM because its power capability has been exceeded. The VDPM is an analog loop (no fixed step size) that will attempt to restore input voltage above VDPM by reducing current load. This is done first by reducing the charging current, then by entering supplement mode (where battery is discharged to supplement adapter input) and finally by current limiting the switching regulator which, once the discharge level of the battery is reached will cause V_SYS to begin to drop.

    From your description, you are in the first stage where battery charge current is reduced. However, I think that your test does not accurately simulate the real-world condition that VDPM is meant to address. With a real adapter (as opposed to bench power supply), the adapter nominal voltage would be above the VDPM level and would only sag below it as the I_SYS/I_BAT load is increased. Then, when BQ25703 reduces the I_BAT, the input voltage would rise back over VDPM as a result.

    In the test you are running where you are dropping the nominal voltage of the bench power supply, its voltage does not "relax" back up to nominal input voltage when the I_BATT is reduced. I think that this is why you are seeing such a large jump in I_BATT for a small reduction in input voltage.

    Instead, please set the voltage of your power supply above VDPM, but set a current limit on the supply. Then, instead of dropping PS voltage, you can steadily increase a load on I_SYS until the input current limit of the PS is reached. At this point, the input voltage of your PS should drop as I_SYS continues to increase, but when the input voltage reaches VDPM, the I_BATT will smoothly reduce in order to hold up the input voltage at VDPM level.

    Regards,
    Steve