Part Number:DRV8704
We have been having some problems with implementing the drv8704 in our dual brush DC motor design. We are part of the Rice Electric Vehicle team here at Rice University, and are preparing to go to competition in two weeks.
The problem is, that when we accelerate up to a speed, and then choose to decelerate, the entire circuit goes haywire, and begins to oscillate at a very high frequency of 50 MHz.
After a handful of seconds of operation, the motor driver circuit starts oscillating at about 50 MHz and then the DRV chip goes into a fault state.
We know because we have scoped out the signals from multiple nodes, gates on the high and low side transistors and on the loads. The most recent time we observed this, a couple of FETs and the DRV chip were damaged.
We have a couple of questions and theories about how to fix this.
1. the ceramic bypass capacitor on the VM pin, not the bulk capacitance:
Under (5) Pin Functions, VM is stated as requiring a 0.1uF bypass capacitor to GND, however, under (8.2) Typical Application, VM is shown as having a 0.01uF bypass capacitor to GND. VM is also shown as having a 0.01uF bypass cap in (10.2) Layout Example.
I am wondering if this is an error in the datasheet? If so, which is the correct bypass capacitance to use? Does it make a difference?
2. The filtering on the current sense resistor. We are using a current sense resistor of 10 mOhms, and expect to draw up to 30 amps from each motor, but so far have only tested with a power supply at about 2 Amps. What should the cutoff frequency of the filtering circuit be? Is our current sense resistor large enough to get an appreciable value out of it?
We are shipping the car out for competition this Tuesday, so getting some feedback soon would be greatly appreciated from our team!
Thank you,
Manuel
I have attached eagle files of our design, and some screenshots of our problem.
(Please visit the site to view this file)(Please visit the site to view this file)
Thanks!