Hello All,

I am developing a very small I2C controlled Motor Driver PCB Board for a robot for my master's thesis.
The idea is simple: Use ATTINY816 to send two PWM signals to the DRV8872 Motor driver to actuate an electromagnetic coil back and forth between two magnets. The PWM signals switch polarity at a rate of about 10 Hz.

Specs:

• ATTINY816 MCU
• DRV8872 Motor Driver
• Supply voltage for Driver: 7.0V
• Coil: 30 ohm (42AWG) air-core

This is one small board that is daisy-chained with identical circuits to make a chain of I2C controlled motor drivers.
I have the I2C working perfectly, but it seems the motor driver is limiting the current being supplied to the coil.
With 7V going through a 30ohm coil, the current shouldn't exceed 233mA, and I have experimented with this to confirm.

Troubleshooting:
I tried substituting a resistor for the coil, and the driver would not supply current to a 30ohm resistor, but would work with a 47ohm resistor.
I then tried putting a 20ohm resistor in series with the coil to get ~50ohms resistance, but it would still not work.

Could there be something generating as spike that triggers the fault detection in the DRV8872?

This is the first PCB I've ever designed, so any advice is welcomed in that design department, or even ways that you would troubleshoot this problem as I am a mechanical engineering grad student and still learning.

Attached are the schematics and PCB layer images.
(The schematic with just the motor driver is to help illustrate how the coil is connected to the circuit, there is only 1 coil connected to each PCB between the points Coil_1 and Coil_2)

 

Where are your bypass and bulk energy capacitors for the driver chip and controller?

 

Where are your bypass and bulk energy capacitors for the driver chip and controller?

I realized I forgot them on the design, but I was able to add them to a breadboard to supply the power with capacitance to the motor driver that way.

 

I realized I forgot them on the design, but I was able to add them to a breadboard to supply the power with capacitance to the motor driver that way.

They need to be on the board near the actual chips (controller and driver). This is especially true for the bypass capacitance.

 

Yep, you need those caps on your PCB and connected as close to the parts as you can get them. Your board looks pretty good I'll say, I've been doing PCB design for 30+ years and you're looking pretty good for a starter but you need the caps as was mentioned. Good Luck...

 

The R1/R3 0.22 Ohm is in the wrong place, it should be from ISEN to ground to provide current monitoring.
FAULTn is open-drain, pull it up with about 10K to provide an over-current signal to your micro

 

When you say "is limiting the current being supplied to the coil" or "would not supply current" etc. what are the values of current you are measuring?