Hi,
We have a design which uses 2 x DRV8839 to drive a couple of small motors. The product is battery powered (4 x AA), so to maintain a (fairly) constant motor speed the DRV8839 IN line is PWM'd to keep the effective motor voltage at 4V as the battery voltage drops.
What I'm seeing is the current profile increase as the supply voltage drops. Measurements show that as the supply voltage decreases the voltage across the motor is being held at ~4V as desired.
The capture below shows the motor currents (yellow - 6.2V and blue - 4.8V) and supply voltages (pink - 6.2V and green - 4.8V) during operation. The difference between the yellow and blue profiles is ~100mA. The current through the motor, measured with a clamp, doesn't change so it looks like the extra current is being used in the chip.
What I'm struggling to understand is why extra current is being used if the voltage across the motor is being kept at a constant 4V.
Any ideas?
Thanks,
Joe
