DRV8814

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SLVSAB9D – MAY 2010 – REVISED AUGUST 2013

Decay Mode and Braking

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 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 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 2 as case 3.

Figure 2. Decay Mode

The DRV8814 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. Note that the DECAY pin

sets the decay mode for both H-bridges.

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.

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