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.
