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Part Number: DRV8874
Other Parts Discussed in Thread: BQ25886, , DRV8873

Good morning,

I was discussing with one of your colleague on power management forum about the battery charger BQ25886, concerning the optimization of the design of the electronic board that my company is developing. We have already designed and produced a functional working prototype of the device. We now need to start the engineering phase.

The system is supplied from a 7.2V dual-cell Li-Ion battery, with nominal capacity of 2600mAh. The battery has a max discharge current of 5A (nominal operating current, limited by the safety board) and a peak discharge current of 10A (discharge cutoff current). We connect the battery to the battery charger thus to the system through BAT/SYS pins. SYS pin supplies digital power (through a 3V3 linear regulator) and directly provides power to the motor driver DRV8874 that drives a brushed DC motor. Such motor may draw up to 12A in stall condition (hence also in fast startup transients). However, BCD motor driver is limited to chop currents higher than 6A. Therefore, the intrinsic limit of the battery charger of providing 5A rms current and peak current (even if for a very short duration of 1us) of 9A does not constitute a problem for the functioning of our device. Morevover, the big bulk capacitor on the motor driver (470uF) helps the battery-battery charger system dealing with inrush currents peaks. The obtained results confirm such hypothesis, as the device operates correctly with accetable voltage ripples on VSYS during motor operations (note that we actually need to provide fast startups of the motor from 0V to max Voltage of 7.2V).

I discussed with your colleague about the possibility to connect the battery directly to the motor driver, if we want to increment the motor current.

1) First question is: since we want to increase the maximum current above 6A, could you suggest a similar part number with higher current limitations? (since DRV8874 has a peak output current of 6A and above such value overcurrent protection may trigger)

Moreover, since we are driving a DC motor, it may occur that during breaking some current is fed back into the battery charger. We performed some measurements of current consumptions of the battery. During device operation, there are positive current spikes (discharging) of ca.6A (RMS current is 2.5A), and reverse current spikes (current coming from SYS pin, exiting from BAT pin, entering into the battery) of ca.10ms, which repeat at 5Hz frequency. The unregulated negative current can cause safety issues with the battery. Therefore:

2) Second question is: could you please suggest an efficient way (considering the relatively high values of forward currents) of protecting the battery from reverse currents coming from the motor driver?

Thank you so much. Kind regards.

  • Hello Alessandro,

    Thank you for posting to the forum.

    Please give us a day to research your question and we will provide you with an reply by 3/12 or earlier.

  • Good Morning Pablo,

    thank you for working on my question. 

    May I add another one?

    3) Since we will always drive the motor in the same direction (thus one side of the motor can be shorted to GND), could we use DRV8874 in Independent Half-Bridge mode, drive both IN1 & IN2 inputs with the same control signal (PWM) and place the two driver's outputs OUT1 & OUT2 in parallel to give double the current? Is it guaranteed that the two internal half-bridges will actually work synchronously if driven with the same signal?

    Thank you! Regards.

  • Please look at the DRV8873 if you need up to 10A peak currents.  For the second question, if you brake with "slow decay", the current will recirculate around the low-side FETs and you will not have the issue or current back into the supply.  

  • Good Morning,

    Thank you for suggesting the DRV8873 part number.

    Concerning the second question, I actually brake the motor using slow decay. However, since the motor driving dynamic is quite fast and it is controlled by a PID, as the motor is braked and the inductance fully discharges, the motor back emf may kick in and when the high side of the half-bridge is switched on again, a portion of the current generated by the back emf + inductance may flow back into the battery, as the attached graph shows.

    Please see also the third question I asked to your colleague.

    Thank you so much! Regards.

  • Hello!

    For the third question, this is not recommended which is why I suggested the DRV8873.  The 1/2 bridges are not synchronized.  

    For controlling supply pumping, this can be controlled by adding larger bulk decoupling on the supply to absorb the current and/or adding TVS diodes to clamp the voltage. 

  • Dear Ryan,

    thank you so much for your answers.

    Best regards.