DRV8843_15 Datasheet, PDF (10/20 Pages) Texas Instruments – DUAL H-BRIDGE DRIVER IC

DRV8843
SLVSAS8C – APRIL 2011 – REVISED AUGUST 2013
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Current Regulation
The current through the motor windings is regulated by a fixed-frequency PWM current regulation, or current
chopping. When an 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 stepping motors, current regulation is normally used at all times, and can changing the current can be used
to microstep the motor. For DC motors, current regulation is used to limit the start-up and stall current of the
motor.
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 xI0 and xI1 pins have internal pulldown
resistors of approximately 100 kΩ. The function of the pins is shown in Table 4.
Table 4. 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.
Decay Mode During Current Chopping
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 3 as case 1. The current flow direction shown
indicates the state when the xIN1 pin is high and the xIN2 pin is low.
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 3 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 3 as case 3.
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