8.2.2.3 Sense Resistor Selection

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 peak motor current is 1 A, RMS motor current is 0.7 A, and a 0.4-Ω sense resistor is used, the resistor will dissipate 0.7 A² x 0.4 Ω = 0.2 W. The power quickly increases with higher 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.