Overcurrent Protection

If the circuit draws more than the maximum allowable current caused by commutation failures, excessive load or internal faults, etc., the circuit might experience overheating or system failure. To prevent such conditions, an overcurrent detection is used. Overcurrent detection is implemented by monitoring the bus current of the 3-phase MOSFET driver. Resistor RSHUNT is tapped at the low side of the motor driver. The voltage across RSHUNT represents the current that flows through the driver, since the voltage is directly proportional to the current across the resistor. The bus current can be computed using Equation 5-1.

Figure 1. Bus Current Calculation

The shunt resistor’s voltage is continuously compared to the reference voltage provided by the MCU. During motor runtime, the bus current is sampled by the comparator (CMP) input. Every 50 ms, the comparator output voltage is monitored, to verify whether it exceeds the reference voltage set in the DAC. If the comparator output is high, the CWG peripheral will be disabled and stop the motor. During motor start-up, a high current is drawn by the motor before it stabilizes. This might cause a false trigger and stop the motor. To prevent this situation, a 500 ms delay after start-up is inserted before enabling the fault detection unit.