DRV8802-Q1_15 Datasheet, PDF (11/26 Pages) Texas Instruments – DRV8802-Q1 Automotive DC Motor-Driver IC

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DRV8802-Q1
SLVSCI2A – JUNE 2014 – REVISED JUNE 2014
7.3.4 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. See case 1 in Figure 6. The current-flow direction shown indicates the
state when the xENBL pin is high.
When the chopping current threshold is reached, the H-bridge can operate in two different states, fast decay or
slow decay.
In fast decay mode, when 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. See case 2 in Figure 6 for fast decay mode.
In slow decay mode, winding current is re-circulated by enabling both of the low-side FETs in the bridge. See
case 3 in Figure 6.
V_M
1
xOUTA
3
2
xOUTB
1 Drive
2 Fast decay with synchronous rectification
3 Low-side slow decay with synchronous rectification
xISEN
R(SENx)
Figure 6. Decay Mode
The DRV8802-Q1 device supports fast decay and slow decay mode. Slow or fast decay mode is selected by the
state of the DECAY pin. A 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.
The DECAY mode also affects the operation of the bridge when it is disabled (by taking the ENBL pin inactive).
This effect 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, allowing the motor to coast to a stop.
If the DECAY pin is low (slow decay), both low-side FETs are turned on when the xENBL pin is made inactive.
When the xENBL pin is made inactive, the inactivation essentially shorts out the back EMF of the motor, causing
the motor to brake, and stop quickly. The low-side FETs stays in the ON state even after the current reaches
zero.
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