We currently use the DRV8837 in the following manner : it is a secondary low-voltage driver for a high-power torque motor. Its purpose is to allow a finer control of the motor current, when the torque needed is low. The DRV8837 is meant to supply loop control currents up to 1 A, after which the main driver takes over and drives up to 10 A in the motor.
Since the main driver works with 28VDC, the two drivers need to be separated. This is done with 2 MOSFET transistors : they are passing when the DRV8837 is in use, and open when the main driver is operating, to avoid busting through the low voltage rail.
We have however noticed a very high power loss in the DRV8837 when powered with its supply rail, P3V3A, when P3V3A = 3,3V. So we changed the board a bit to have P3V3A = 5.3 V, so we can compare with the Rdson data in the datasheet.
The motor is 2.35 ohms, 13.8 mH, no back-emf.
The command supplied to the IN1/IN2 pins is a 70% duty cycle forward/brake command : IN1 = 3,3V, and IN2=3,3V 30% of the time, and 0V 70% of the time. The PWM frequency is 20 kHz.
Below is the OUT1 (yellow) and OUT2(blue) waveforms. When in forward, V(OUT1)=4,4 V & V(OUT2) = 0.55V. When in brake, V(OUT1) = -0.55V & V(OUT2) = 0.5V.
The average voltage at the motor terminals is then approx. 2.35V, which is confirmed by our current probe which is showing approx. 1 A.
Why are we seeing such a high Rdson on both the high side and low side ? The calcultated total Rdson (TOP+BOT) is 1,5 ohms !
We then tried the same command type (forward/brake) with 16% PWM and the results are :
- V(out1) = 5.1V
- V(out2) = 100 mV
This corresponds to a motor current of 340 mA, and a total Rdson (TOP+BOT) of almost 1 ohm.
Even if our PCB was badly designed (which it isn't), there is no way to make a +600 mohms power or ground trace without having the full intention to do so.
There is approx. 120 µF (V-bias and worst-case tolerance applied) on the P3V3A rail, and the DRV8837 is the ONLY IC on that rail.
V(out1) and V(out2) are measured directly at the output of the DRV8837, and we also measured V(HB1) and V(HB2) (after the separation transistors), to make sure they weren't at fault. We have V(OUT1)=V(HB1) and V(OUT2)=V(HB2) within a few 10 mV.