DRV8825 Datasheet, PDF (9/22 Pages) Texas Instruments – STEPPER MOTOR CONTROLLER IC

DRV8825
www.ti.com
SLVSA73C – APRIL 2010 – REVISED MAY 2011
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.
In stepping motors, current regulation is used to vary the current in the two windings in a semi-sinusoidal fashion
to provide smooth motion.
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.
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.
The reference voltage is scaled by an internal DAC that allows fractional stepping of a bipolar stepper motor, as
described in the microstepping indexer section below.
Decay Mode
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 positive current flow.
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.
Figure 3. Decay Mode
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