Increase Speed Of DC Motor with DRV8711

Hi Mohammad,

Can you clarify?

To increase the speed of a DC motor, the duty cycle of PWM input is increased.

To increase the speed of a stepper motor, increase the step input rate (through the STEP pin or the SPI register)

This assumes that the current (Torque, Gain, sense resistor) settings will allow enough current through the motor.

For reference, please refer to the DRV8711EVM User Guide.

1 ) Thanks for your reply 2 ) when I set the OFF Register[7:0] to 0xFF , speed of DC motor reduced , and when I set the OFF Register[7:0] to 0x00 , Speed is the max value , Why ?? 3 ) What's the relation between the DECAY , OFF and BLANK register ? and relation between TBLANK , TOFF and TDECAY ?? What's the best chioce for initializing these register ?

Hi Mohammed,

2 and 3) The OFF register is designed to be used when chopping current. Refer to section 7.3.5 of the datasheet for details. The AINx/BINx pins are intended to control the speed of the motor. 

Additional reference material can be found in the DRV8711EVM user guide, the DRV8711 Decay Mode Setting Optimatization (SLvA637) and DRV8711 Quick Spin and Tuning Guide (SLVA632)

Initial settings of the registers depends on the motor, and the current chopping desired. In general the default settings of the DRV8711EVM is a good place to start. The settings can be seen in the DRV8711 User Guide, although the settings are for a stepper motor. If using a BDC motor, it is recommended to use slow decay.

Mohammed,

How fast are you trying to step?

When you set your OFF register to 0xFF, you are setting a minimum delay between PWM pulses of 128 us. 1/128 us = 7812 PWM cycles/sec. You can't step (microstep) faster than your PWM rate, so your maximum step rate is less than 7800 steps/sec. You really need to use all the timing parameters together to achieve your full current to consider your maximum step rate (< 7800 steps/sec). Figure 10 in the datasheet summarizes this.