The DRV8251A device integrates current regulation using a fixed off-time current chopping scheme. This allows the devices to limit the output current in case of motor stall, high torque, or other high current load events without involvement from the external controller as shown in Figure 8-5.

Figure 8-5 Off-Time Current-Regulation

The current chopping threshold (I_TRIP) is set through a combination of the VREF voltage (V_VREF) and IPROPI output resistor (R_IPROPI). This is done by comparing the voltage drop across the external R_IPROPI resistor to V_VREF with an internal comparator.

For example, if V_VREF = 3.3 V, R_IPROPI = 1310 ?, and A_IPROPI = 1575 μA/A, then I_TRIP will be approximately 1.6 A.

The fixed off-time current chopping scheme supports up to 100% duty cycle current regulation since the H-bridge automatically enables after the t_OFF period and does not require a new control input edge on the INx pins to reset the outputs. When the motor current exceeds the I_TRIP threshold, the outputs will enter a current chopping mode with a fixed off time (t_OFF). During t_OFF, the H-bridge enters a brake/low-side slow decay state (both low-side MOSFETs ON) for t_OFF duration after I_OUT exceeds I_TRIP. After t_OFF, the outputs re-enable according to the control inputs if I_OUT is less than I_TRIP. If I_OUT is still greater than I_TRIP, the H-bridge enters another period of brake/low-side slow decay for t_OFF. If the state of the INx control pins changes during the t_OFF time, the remainder of the t_OFF time is ignored, and the outputs will again follow the inputs.

The I_TRIP comparator has both a blanking time (t_BLK) and a deglitch time (t_DEG). The internal blanking time helps to prevent voltage and current transients during output switching from effecting the current regulation. These transients may be caused by a capacitor inside the motor or on the connections to the motor terminals. The internal deglitch time ensures that transient conditions do not prematurely trigger the current regulation. In certain cases where the transient conditions are longer than the deglitch time, placing a 10-nF capacitor on the IPROPI pin, close to the device, will help filter the transients on IPROPI output so current regulation does not prematurely trigger. The capacitor value can be adjusted as needed, however large capacitor values may slow down the response time of the current regulation circuitry.

The internal current regulation and current feedback can be disabled by tying IPROPI to GND and setting the VREF pin voltage greater than GND. If current feedback is required and current regulation is not required, set V_VREF and R_IPROPI such that V_IPROPI never reaches the V_VREF threshold. For proper operation of the current regulation circuit, V_VREF must be within the range of the VREF pin voltages specified in the Recommended Operating Conditions table.