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BOOST-DRV8305EVM

Robert Rice1
Prodigy 150 points
Other Parts Discussed in Thread: EK-TM4C1294XL, BOOST-DRV8711

Hi,

I just discovered this Motor Driver Forum. Previously I was posting to the Tiva micro controller forum because I am using the EK-TM4C1294XL processor board.

I was doing well developing firmware for the 8305 until my motor developed a short circuit on one winding and it took down the BOOST-DRV8305EVM with it. That was probably my fault for having protections disabled. So now I have another motor and driver board on order.

I am using the 3-PWM mode and generating a sinusoidal commutation which gives me a smooth low speed motor rotation - great for positioning. I think only trapazoidal commutation is possible with the 1-PWM mode.

I see the specification recommends a switching frequency of 200 kHz but I found that the complementary outputs could not respond that fast. The duty cycle becomes very distorted at that frequency, so I slowed the PWM period down to 15 kHz and that gave me a smooth and quiet response. Is the 200 kHz recommendation correct?

The problem I have with both the 3-PWM mode and the 6-PWM mode is that the LaunchPad boards have a very limited number of PWM capable BoosterPack output pins. Even the new Delfino board brings out only 14 PWM pins. Since my business is motion control, I want to be able to control several motors from one micro-controller. So I really want to be able to control the driver entirely with the SPI interface. I can do this with the BOOST-DRV8711 board using the "force step" and dynamically changing the step mode - kind of like shifting gears. But that solution is Kludgy.

So what I would want for a 3-phase driver is an SPI to set the signed duty-cycle for each phase independently. I think the SPI interface is fast enough to control several motors in this mode of operation.

Similarly, if I want to bypass the indexer on the 8711, I would need to use four PWM outputs for 1 stepper motor. I also need to use four PWM outputs to use the driver in split mode to drive two self-commutated motors. So what I want is to have a mode that bypasses the indexer, a trivial µP function, but lets me set the signed duty-cycle for each H-bridge independently.

Thanks,

Bob Rice

  • TI__Guru** 114905 points
    Hi Bob,

    Welcome to the forum.

    You are correct; only trapezoidal commutation is possible with one PWM.

    The 200kHz is a maximum recommended switching frequency for the device. The actual frequency can be much lower, but it typically above 20kHz to remain above the audible frequencies.

    The actual switching frequency varies based on the gate charge of the FET. An approximation can be calculated using equation 2 in the datasheet.
  • in reply to Rick Duncan
    Prodigy 150 points
    Hi Rick,

    When using the BOOST-DRV8305EVM in 3-PWM Mode I observe a delay between the input going high and output going high. Is this the dead time delay?

    I can easily compensate for the active delay, but I also observe a minimum output pulse width. Is it possible to reduce this minimum output pulse width? Would 6-PWM mode enable a smaller minimum output pulse width?

    The minimum output pulse width is more difficult to deal with when trying to run the motor smoothly at low speed and low power. One way I could deal with it is to go to a pulse number modulation rather than a pulse width modulation. Hear I set a timer interrupt to the minimum output pulse width and periodically pulse to maintain the desired average duty-cycle. This works but it produces audible beat frequencies.

    Thanks,
    Bob Rice