Table 11. System Design Input Parameters

8.2.2.1 Stepper Motor Speed

The first step in configuring the DRV8846 requires the desired motor speed and stepping level. The DRV8846 can support from full step to 1/32 step mode.

If the target motor speed is too high, the motor will not spin. Make sure that the motor can support the target speed.

For a desired motor speed (v), microstepping level (nm), and motor full step angle (θ_step),

Equation 2.

θ_step can be found in the stepper motor data sheet or often written on the motor itself.

For DRV8846, the microstepping levels are set by the M0/M1 pins and can be any of the settings in Table 2. Higher microstepping means a smoother motor motion and less audible noise, but increases the switching losses and requires a higher ƒ_step to achieve the same motor speed.

8.2.2.2 Current Regulation

The chopping current (I_CHOP) is the maximum current driven through either winding. This quantity will depend on the sense resistor value (R_XISEN).

Equation 3.

I_CHOP is set by a comparator which compares the voltage across R_XISEN to a reference voltage. Note that I_CHOP must follow Equation 4 to avoid saturating the motor.

Equation 4.

where

• VM is the motor supply voltage.
• R_L is the motor winding resistance.

8.2.2.3 Decay Modes

The DRV8846 supports four different decay modes: slow decay, fast decay, mixed decay, and AutoTune. The first selection to try is the AutoTune mode, which adjusts the decay mode automatically to improve current regulation. The current through the motor windings is regulated using a fixed-off-time PWM scheme. This means that after any drive phase, when a motor has reached the current chopping threshold (I_CHOP), the DRV8846 places the motor in one of the four decay modes until the PWM cycle has expired. Afterward, a new drive phase starts.

The blanking time, t_BLANK, defines the minimum drive time for the current chopping. I_CHOP is ignored during t_BLANK, so the winding current may overshoot the trip level during this blanking period.