DRV8821_15 Datasheet, PDF (9/23 Pages) Texas Instruments – DUAL STEPPER MOTOR CONTROLLER/DRIVER

DRV8821
www.ti.com
SLVS912I – JANUARY 2009 – REVISED JANUARY 2014
Current Regulation
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 pin.
The full-scale (100%) chopping current is calculated as follows:
5
(1)
Example:
If a 0.5-Ω sense resistor is used and the VREFx pin is 2.5 V, the full-scale (100%) chopping current is
2.5 V/(5 * 0.5 Ω) = 1 A.
The reference voltage is also scaled by an internal DAC that allows torque control for 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 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 in the step table below.
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.
VM
1
xOUT1
3
2
xOUT2
1 Drive current
2 Fast decay (reverse)
3 Slow decay (brake)
Figure 3. Decay Mode
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