The maximum current through the motor winding is regulated by a fixed off-time PWM current regulation or current chopping. When an half-bridge is enabled, current rises through the winding at a rate dependent on the DC voltage and inductance of the winding. When the current hits the current chopping threshold, the bridge enters a brake (low-side slow decay) mode until the toff time expires.
Immediately after the current is enabled, the voltage on the SP pin is ignored for a period (t(BLANK)) before enabling the current-sense circuitry.
The PWM chopping current is set by a comparator that compares the voltage across a current-sense resistor connected to the SP pin, multiplied by a factor of AV, with a reference voltage from the VREF pin. The factor AV is the shunt-amplifier gain, which is 19.8 V/V for the DRV8702D-Q1 device or configurable to 10, 19.8, 39.4, or 78 V/V for the DRV8703D-Q1 device.
Use Equation 1 to calculate the chopping current (ICHOP).
For example, if a 50-mΩ sense resistor and a VREF value of 3.3 V are selected, the full-scale chopping current is 3.28 A. The AV is 19.8 V/V and VIO is assumed to be 50 mV in this example.
If the load is connected between the SH pin and VM and current regulation is enabled (MODE pin is Hi-Z), the low-side FET is switched on when the current flowing through the load exceeds the ICHOP threshold. This result in an adverse effect by driving the load at 100% duty cycle because the maximum current flows through the load as the low-side FET remains switched on for the tOFF duration. Texas Instruments recommends using the PWM control interface without current regulation (MODE pin is 1) for this configuration to drive the load.
For DC motors, current regulation is used to limit the start-up and stall current of the motor. If the current regulation feature is not needed, it can be disabled by tying the VREF pin directly to the AVDD pin. If the PWM control-interface mode without current regulation (MODE pin is 1) is selected for operation, the device does not perform PWM current regulation or current chopping.