The current through the motor windings is regulated by an adjustable, off-time PWM current-regulation circuit. When an H-bridge is enabled, current rises through the winding at a rate dependent on the DC voltage, inductance of the winding, and the magnitude of the back EMF present. When the current hits the current regulation threshold (as shown in Figure 8-6, Item 1), the bridge enters a decay mode determined by the DECAY pin setting to decrease the current. After the off-time expires, the bridge is re-enabled, starting another PWM cycle.

Figure 8-5 Current Chopping Waveform

Once the chopping current threshold is reached, the H-bridge can operate in two different states, fast decay or slow decay.

• In fast decay mode, as soon as the PWM chopping current level is reached, the H-bridge reverses state by switching on the opposite arm MOSFETs to allow the winding current to flow in the opposite direction. As the winding current approaches zero, the H-bridge is disabled to prevent further reverse current flow. Fast decay mode is shown in Figure 8-6, item 2.

• In slow decay mode, the winding current is re-circulated by enabling both low-side MOSFETs in the H-bridge. This is shown in Figure 8-6, Item 3.

Figure 8-6 Decay Modes

The PWM regulation current is set by a comparator which monitors the voltage across the current sense MOSFETs in parallel with the low-side power MOSFETs. The current sense MOSFETs are biased with a reference current that is the output of a current-mode sine-weighted DAC whose full-scale reference current is set by the voltage at the VREF pins.

The full-scale regulation current (I_FS) can be calculated as -

I_FS (A) = V_REFx (V) / K_V (V/A) = V_REFx (V) / 2.2 (V/A).

The decay mode of the DRV8849 is selected by the DECAY pins as shown in Table 8-6. The decay modes can be changed on the fly. After a decay mode change, the new decay mode is applied after a 2 µs de-glitch time.