Table 9. Design Parameters

8.2.1.2.1 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 xVREF analog voltage, and the sense resistor value (R_SENSE). AVREF and BVREF can be configured to drive different currents, but in this example the same full-scale current is used in both coils.

Equation 2.

TRQ is a DAC used to scale the output current. The current scalar value for different inputs is shown in Table 10.

Note that I_FS must also follow Equation 3 in order to avoid saturating the motor. VM is the motor supply voltage, and R_L is the motor winding resistance.

Equation 3.

8.2.1.2.2 Stepper Motor Speed

Next, the driving waveform needs to be planned. In order to command the correct speed, determine the frequency of the input waveform.

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 4.

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

The frequency ƒ_step gives the frequency of input change on the DRV8881P. 1/ ƒ_step = t_STEP on the diagram below.

Equation 5.

Figure 25. Example 1/2 Stepping Operation

8.2.1.2.3 Decay Modes

The DRV8881 supports several different decay modes: slow decay, fast decay, mixed decay, and AutoTune (DRV8881E only). 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 DRV8881 will place the winding in one of the decay modes for TOFF. After TOFF, a new drive phase starts.

8.2.1.2.4 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_rms² × R. For example, if the rms motor current is 1.4 A 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.

Table 11. Design Parameters

8.2.2.2.1 Current Regulation

The maximum current (I_TRIP) is set by the TRQ pins, the xVREF analog voltage, and the sense resistor value (R_SENSE). In parallel mode the winding current is set by AVREF only and BVREF is ignored. When starting a brushed-DC motor, a large inrush current may occur because there is no back-EMF. Current regulation will act to limit this inrush current and prevent high current on startup.