During PWM current chopping, the H-bridge is enabled to drive current through the motor winding until the PWM current chopping threshold is reached. This is shown in Figure 7-2 as case 1. The current flow direction shown indicates the state when the xENBL pin is high.

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, once the PWM chopping current level has been reached, the H-bridge reverses state to allow winding current to flow in a reverse direction. As the winding current approaches zero, the bridge is disabled to prevent any reverse current flow. Fast decay mode is shown in Figure 7-2 as case 2.

In slow decay mode, winding current is re-circulated by enabling both of the low-side FETs in the bridge. This is shown in Figure 7-2 as case 3.

Figure 7-2 Decay Mode

The DRV8840 device supports fast decay and slow decay mode. Slow or fast decay mode is selected by the state of the DECAY pin - logic low selects slow decay, and logic high sets fast decay mode. The DECAY pin has both an internal pullup resistor of approximately 130 kΩ and an internal pulldown resistor of approximately 80 kΩ. This sets the mixed decay mode if the pin is left open or undriven.

DECAY mode also affects the operation of the bridge when it is disabled (by taking the ENBL pin inactive). This applies if the ENABLE input is being used for PWM speed control of the motor, or if it is simply being used to start and stop motor rotation.

If the DECAY pin is high (fast decay), when the bridge is disabled fast decay mode will be entered until the current through the bridge reaches zero. Once the current is at zero, the bridge is disabled to prevent the motor from reversing direction. This allows the motor to coast to a stop.

If the DECAY pin is low (slow decay), both low-side FETs will be turned on when ENBL is made inactive. This essentially shorts out the back EMF of the motor, causing the motor to brake, and stop quickly. The low-side FETs will stay in the ON state even after the current reaches zero.