Table 3. Design Parameters

8.2.2.1 Current Regulation

The maximum current (I_TRIP) is set by the Ix pins, the VREF analog voltage, and the sense resistor value (R_SENSE). When starting a brushed DC motor, a large inrush current may occur because there is no back-EMF and high detent torque. Current regulation will act to limit this inrush current and prevent high current on start-up.

Example: If the desired chopping currents is 1.5 A:

• Set R_SENSE = 100 mΩ
• VREF would have to be 0.75 V
• Create a resistor divider network from V3P3OUT (3.3 V) to set VREF = 0.75 V
• Set R2 = 10 kΩ and set R1 = kΩ

8.2.2.2 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 × R. For example, if the RMS motor current is 1.5 A and a 200-mΩ sense resistor is used, the resistor will dissipate 1.5 A² × 0.2 Ω = 0.3 W. The power quickly increases with greater 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.