Table 4. Design Parameters

8.2.2.1 Motor Voltage

The motor voltage to use will depend on the ratings of the motor selected and the desired RPM. A higher voltage spins a brushed DC motor faster with the same PWM duty cycle applied to the power FETs. A higher voltage also increases the rate of current change through the inductive motor windings.

For the DRV8832, TI recommends setting a motor voltage at the lowest system VCC. This will maintain a constant RPM across varying VCC conditions.

For example if the VCC voltage can vary from 4.5 V to 5.5 V, setting the VSET voltage to 1.125 V will compensate for power supply variation. The DRV8832 will set the motor voltage at 4.5 V, even if VCC is 5.5 V.

8.2.2.2 Motor Current Trip Point

When the voltage on pin ISENSE exceeds V_ILIM (0.2 V), overcurrent is detected. The R_SENSE resistor should be sized to set the desired I_LIMIT level.

To set I_LIMIT to 5 A, R_ISENSE = 0.2 V / 0.5 A = 0.4 Ω.

To prevent false trips, I_LIMIT must be higher than regular operating current. Motor current during start-up is typically much higher than steady-state spinning, because the initial load torque is higher, and the absence of back-EMF causes a higher voltage and extra current across the motor windings.

It can be beneficial to limit start-up current by using series inductors on the DRV8832 output, as that allows I_LIMIT to be lower, and it may decrease the system’s required bulk capacitance. Start-up current can also be limited by ramping the forward drive duty cycle.

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 I_RMS² × 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× 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 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.

8.2.2.4 Low Power Operation

Under normal operation, using sleep mode to minimize supply current should be sufficient.

If desired, power can be removed to the DRV8832 to further decrease supply current. TI recommends removing power to the FAULTn pullup resistor when removing power to the DRV8832. Removing power from the FAULTn pullup resistor will eliminate a current path from the FAULTn pin through an ESD protection diode to VCC. TI also recommends setting both IN1 and IN2 as a logic low when power is removed.