LVDSMC Operation Question

I recently received the LVDS motor controller, and I am concerned about having a voltage on VMot when there is no power supply hooked up to this pin. I am running this on a breadboard with a PIC18F452 sending the serial commands. I’m using a regulated power supply at 5V.

While running the controller with motors (dual motor mode) hooked up and 4.8V for the motor power supply, and ramping up the speed from 0 to 64, things start out good. As the motors freely spin up to speed, it gets to a point where the motors stop and the regulated power supply drops to 1.5V or so (I have the current limited and it seems as though the circuit is starting to draw too much current).

When I disconnect the motor power supply and motors, and then hook up a DVM to VMot, I notice there is a voltage reading on this pin. It will start out at about 5.7V and slowly drop as the motor speed is increased. Is there suppose to be this sort of voltage reading on this pin when nothing is connected to it?? Also, the LVDSMC and PIC are the only devices hooked up to the 5V powers supply.

I have the Micro LVDSMC and VMot is 0 when nothing is connected, if only it could handle the current draw from these motors, I’d just use this one.

Anyhow, I’ve been quite happy with the micro dual serial controllers so I’m hoping to get this LVDSMC going reliably.

Any help or input would be greatly appreciated.


It’s normal to see a voltage on the motor supply of the low-voltage DSMC (the p-channel MOSFETs at the tops of the H-bridges have pull-up resistors from the gates to the motor supply; when the logic-supply-powered gate control signal is applied, it will show up on the unpowered motor supply).

Almost all of the problems I’ve seen with the motor controller are noise-related. The reset pin can latch into a weird state that makes it get pulled low, which will cause excessive current if your microcontroller is trying to drive the pin high. One way to avoid the problem is to pull the reset line high with a resistor and then use an open-drain output to drive it low when you want to reset it (on a microcontroller, you can simulate an open-drain output by switching between driving low and being an input (high Z)).

You’ll also want to apply all the noise-reducing techniques you can: caps on the motors, twisted motor leads, caps on power supplies, routing power wires away from signal wires, and so on.

- Jan

Hi Jan,

Thanks for the reply and all the good information. The LVDSMC is working great for our project. Thanks again for the quick reply and providing hobby robotics enthusiasts like myself with a great product line-up.