Other Parts Discussed in Thread: TMS320F28055,
Tool/software: Code Composer Studio
Hello, I found a recent forum thread with a similar topic but I want to be more precise, and possibly i would like to receive more precise response.
Recently we designed a control card with TMS320F28055 and DRV8841, and we made some simple tests driving a brushed geared DC 24V motor. The objective is power regulation with PWM to the motor, which can rotate in any direction (clockwise or counterclockwise). The motor drain , when rotates with no mechanical resistance appied to motor shaft, is about 500 mA.
DRV8841 has two kind of current limit: one is programmable and the other is fixed at 3 Ampere, to protect the driver itself. The motor is connected to AOUT1 , AOUT2 as in fig. 5 of your datasheet. No other reactive element (inductor or capacitor) between driver outputs and motor.
With no means of current limitation the driver goes immediately to fault state after the driver is programmed out of reset. using another device to switch motor on/off, a simple MOSFET switch, we measured at motor startup a short transient with current which actually goes above 3 Ampere for a very short period.
the internal current limitation is with a 0.22 sensing resistor with 3.3V reference, thus with a 3 Ampere full scale chopping current. We did not put any capacitor in parallel to sensing resistor. The chopping current (from datasheet) can be programmed at 0% , 38%, 71% and 100% of full scale. I have tried bringing DRV8841 out of reset with AI1, AI2 at various settings, but the result is always driver fault. Only putting a small 2.2 Ohm resistor in series to the motor solves the problem.
So the questions: what is wrong? I expected that the programmable current limitation could overcome the fixed current protection, why not? How can i solve the problem?
Thanks, Vasco
