Table 12. Design Parameters

8.2.2.1 Stepper Motor Speed

The first step in configuring the DRV8880 requires the desired motor speed and microstepping level. If the target application requires a constant speed, then a square wave with frequency ƒ_step must be applied to the STEP pin.

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 (n_m), and motor full step angle (θ_step),

Equation 2.

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

For the DRV8880, the microstepping level is set by the Mx pins and can be any of the settings in the table below. Higher microstepping will mean a smother motor motion and less audible noise, but will increase switching losses and require a higher ƒ_step to achieve the same motor speed.

Example: Target 120 rpm at 1/8 microstep mode. The motor is 1.8°/step

Equation 3.

8.2.2.2 Current Regulation

In a stepper motor, the full-scale current (I_FS) is the maximum current driven through either winding. This quantity will depend on the TRQ pins, the VREF analog voltage, and the sense resistor value (R_SENSE). During stepping, I_FS defines the current chopping threshold (I_TRIP) for the maximum current step.

Equation 4.

TRQ is a DAC used to scale the output current. The current scalar value for different inputs is shown below.

Note that I_FS must also follow the equation below in order to avoid saturating the motor. VM is the motor supply voltage, and R_L is the motor winding resistance.

Equation 5.

8.2.2.3 Decay Modes

The DRV8880 supports several different decay modes: slow decay, fast decay, mixed decay, and AutoTune. The current through the motor windings is regulated using an adjustable fixed-time-off scheme. This means that after any drive phase, when a motor winding current has hit the current chopping threshold (I_TRIP), the DRV8880 will place the winding in one of the decay modes for t_OFF. After t_OFF, a new drive phase starts. For fixed decay modes (slow, fast, and mixed), the best setting can be determined by operating the motor and choosing the best setting.

8.2.2.4 Sense Resistor

For optimal performance, it is important for the sense resistor to be:

• Surface-mount
• Low inductance
• Rated for high enough power
• Placed closely to the motor driver

The power dissipated by the sense resistor equals I_rms ² × R. For example, if the rms motor current is 1.4A and a 250 mΩ sense resistor is used, the resistor will dissipate 1.4 A² × 0.25 Ω = 0.49 W. The power quickly increases with higher current levels.

Resistors typically have a rated power within some ambient temperature range, along with a derated power curve for high ambient temperatures. When a PCB is shared with other components generating heat, margin should be added. It is always best to measure the actual sense resistor temperature in a final system, along with the power MOSFETs, as those are often the hottest components.

Because power resistors are larger and more expensive than standard resistors, it is common practice to use multiple standard resistors in parallel, between the sense node and ground. This distributes the current and heat dissipation.