Table 3. Design Parameters

8.2.2.1 Current Regulation

In a stepper motor, the set full-scale current (I_FS) is the maximum current driven through either winding. This quantity depends on the xVREF 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. The gain of DRV8812 is set for 5 V/V.

Equation 2.

To achieve I_FS = 1.25 A with R_SENSE of 0.2 Ω, xVREF should be 1.25 V.

8.2.2.2 Decay Modes

The DRV8812 supports three different decay modes: slow decay, fast decay, and mixed decay. The current through the motor windings is regulated using a fixed-frequency PWM scheme. This means that after any drive phase, when a motor winding current has hit the current chopping threshold (I_TRIP), the DRV8812 will place the winding in one of the three 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_TRIP is ignored during t_BLANK, so the winding current may overshoot the trip level.

8.2.2.3 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 Irms² x R. For example, if the rms motor current is 2-A and a 100-m Ω sense resistor is used, the resistor will dissipate 2 A ² × 0.1 Ω = 0.4 W. The power quickly increases with greater current levels.

Resistors typically have a rated power within some ambient temperature range, along with a de-rated 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.