DRV8814 Datasheet, PDF (8/16 Pages) Texas Instruments – DC MOTOR DRIVER IC 2.5-A Maximum Drive Current at 24 V, 25°C

DRV8814
SLVSAB9C – MAY 2010 – REVISED MARCH 2011
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Bridge Control
The xPHASE input pins control the direction of current flow through each H-bridge, and hence the direction of
rotation of a DC motor. The xENBL input pins enable the H-bridge outputs when active high, and can also be
used for PWM speed control of the motor. Note that the state of the DECAY pin selects the behavior of the
bridge when xENBL = 0, allowing the selection of slow decay (brake) or fast decay (coast). Table 2 shows the
logic.
DECAY
0
1
X
X
Table 2. H-Bridge Logic
xENBL
0
0
1
1
xPHASE
X
X
1
0
xOUT1
L
Z
H
L
xOUT2
L
Z
L
H
Current Regulation
The maximum current through the motor winding is regulated by a fixed-frequency PWM current regulation, or
current chopping. When the H-bridge is enabled, current rises through the winding at a rate dependent on the
DC voltage and inductance of the winding. Once the current hits the current chopping threshold, the bridge
disables the current until the beginning of the next PWM cycle.
For DC motors, current regulation is used to limit the start-up and stall current of the motor. Speed control is
typically performed by providing an external PWM signal to the ENBLx input pins.
If the current regulation feature is not needed, it can be disabled by connecting the xISENSE pins directly to
ground, and connecting the xVREF pins to V3P3.
The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor
connected to the xISEN pins, multiplied by a factor of 5, with a reference voltage. The reference voltage is input
from the xVREF pins, and is scaled by a 2-bit DAC that allows current settings of 100%, 71%, 38% of full-scale,
plus zero.
The full-scale (100%) chopping current is calculated in Equation 1.
V
I = ₅^¾ _{R CHOP}
REFX
· _ISENSE
(1)
Example:
If a 0.25-Ω sense resistor is used and the VREFx pin is 2.5 V, the full-scale (100%) chopping current will be
2.5 V / (5 x 0.25 Ω) = 2 A.
Two input pins per H-bridge (xI1 and xI0) are used to scale the current in each bridge as a percentage of the
full-scale current set by the VREF input pin and sense resistance. The function of the pins is shown in Table 3.
Table 3. H-Bridge Pin Functions
xI1
xI0
RELATIVE CURRENT
(% FULL-SCALE CHOPPING CURRENT)
1
1
0% (Bridge disabled)
1
0
38%
0
1
71%
0
0
100%
Note that when both xI bits are 1, the H-bridge is disabled and no current flows.
Example:
If a 0.25-Ω sense resistor is used and the VREF pin is 2.5 V, the chopping current will be 2 A at the 100%
setting (xI1, xI0 = 00). At the 71% setting (xI1, xI0 = 01) the current will be 2 A x 0.71 = 1.42 A, and at the
38% setting (xI1, xI0 = 10) the current will be 2 A x 0.38 = 0.76 A. If (xI1, xI0 = 11) the bridge will be disabled
and no current will flow.
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