9.2.1.2.1 Motor Current Rating
Motor specification selection is the most importance criteria for the design. Each half-bridge (OUTx) of the DRV89XX-Q1 device is designed to handle RMS current of 1-A and the peak current is limited by the minimum over-current (OCP) limit of 1.3-A. Therefore, a motor with peak starting current higher than 1.3-A is expected to hit OCP limit. For higher peak current motors (starting current higher than 1.3-A), following methods can be implemented:
- Current Chopping: During starting, if supply voltage is connected directly to the motor, then due to low back-emf (when speed is zero or low), a huge peak current is demanded by the motor. This peak current is only limited by the motor's winding resistance (RMOTOR). This peak current of motor can be limited by starting the motor with low-duty PWM switching operation and then gradually increasing (duty-ramping) the duty with speed to 100% PWM operation (equivalent to motor operating in continuous mode). This duty-ramping provides enough time to ramp motor speed and build sufficient back-emf which limits the peak current. The DRV89XX-Q1 device implements a 2-kHz PWM switching operation which is suitable for the HVAC damper motors.
- OCP Deglitch Time Adjustments: This method is applicable if the motor inertia is low and the motor can quickly pick up the speed. For this method, the motor starting current should settle to lower than minimum over-current limit (IOCP) before OCP deglitch time (tOCP) is over. The device provides multiple (8 settings) OCP deglitch time settings with a default deglitch time of 10-µs and can be increased to a maximum value of 60-µs.
NOTE
For multiple motor connection, it has to be ensured that the total device current should be lower than the maximum current-carrying capability of the power-supply (VM/GND) pins i.e. 6-A (maximum).