DRV8802 Datasheet, PDF (9/18 Pages) Texas Instruments – DC MOTOR DRIVER IC

DRV8802
www.ti.com
SLVSAM9A – APRIL 2011 – REVISED JUNE 2011
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 DRV8802 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, all FETs are turned off and decay current
flows through the body diodes. 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.
Copyright © 2011, Texas Instruments Incorporated
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