Voltage drop of PWM input of Pololu 36v9 driver

When I connect the 5V 20kHz PWM signal from a 3.3-5V level converter (Sparkfun BOB-12009) to the 36v9 driver’s input, the signal gets reduced to 4.1 V. Here is a picture combining the measured signal before and after connecting the driver’s input.

The 5V supply of the level converter comes from an external regulated power supply. Its 3.3V supply comes from a Microstick II development board which runs the PIC microcontroller creating the 3.3V PWM signal. All three components’ grounds are connected together.

Is it normal to see that voltage drop? Does the system require any component between the level converter and the driver?



I am sorry you are having problems using your high-power motor driver. That drop in voltage does not seem normal, and I am not sure what might be causing it. Could you tell me more about your setup? What is the power supply you are using for 5V? Could you try measuring its output when the level converter is only showing 4.1V? Could you also post pictures of your setup that show all the connections?

There is a 47k pull-down resistor on the PWMH pin. It looks like the level shifter you are using uses a 10k pull-up resistor, which creates a voltage divider with the pull-down resistor on the motor driver. Is the driver working as expected? If not, you might try adding an external pull-up resistor between the level converter and motor driver.

- Jeremy

Thanks Jeremy. The driver seems to work fine. I was wondering about the behavior for the short duty cycles, but the main limitation in my setup seems to be the time constant of the level shifter. I will measure the speed curve of my motor to check that.

Is there a minimum duty cycle for the driver?


I measured one of those drivers and found that I could not get output pulse widths shorter than about 0.5us. Note that with most drivers like this the pulse width output is always going to be a bit smaller than the pulse width that is input because of a dead time added to prevent shoot-through. In the case of this driver the dead time is about 3us, so any input pulses shorter than 3.5us won’t result in an output pulse. Reducing the switching frequency would help you get smaller duty cycles without actually decreasing the pulse width of your input signal.