DRV8880_15 Datasheet, PDF (18/48 Pages) Texas Instruments – DRV8880 2-A Stepper Motor Driver With AutoTune™

DRV8880
SLVSD18A – JUNE 2015 – REVISED JULY 2015
www.ti.com
7.3.4 Current Regulation
The current through the motor windings is regulated by an adjustable fixed-off-time PWM current regulation
circuit. When an H-bridge is enabled, current rises through the winding at a rate dependent on the DC voltage,
inductance of the winding, and the magnitude of the back EMF present. After the current hits the current
chopping threshold, the bridge enters a decay mode for a fixed period of time to decrease the current, which is
configurable between 10 and 30 µs through the tri-level input TOFF. After the off time expires, the bridge is re-
enabled, starting another PWM cycle.
Table 4. Off-Time Settings
TOFF
0
1
Z
OFF-TIME t_OFF
20 µs
30 µs
10 µs
The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor
connected to the xISEN pin with a reference voltage. To generate the reference voltage for the current chopping
comparator, the output of a sine lookup table is applied to a sine-weighted DAC, whose full-scale output voltage
is set by VREF. This voltage is attenuated by a factor of Av. In addition, the TRQx pins further scale the
reference.
VM
Device
Logic
PWM
Logic
Gate
VM
Drive
xOUT1
xOUT2
+
Step
Motor
-
+-
+
-
VREF
TRQ[1:0]
4
SINE DAC
1/Av
xISEN
RSENSE
Figure 14. Current Regulation Block Diagram
The full-scale (100%) chopping current is calculated as follows:
I _FS (A)
VREF (V) u TRQ (%)
A_V u R_SENSE (:)
VREF (V) u TRQ (%)
6.6 u R_SENSE (:)
(1)
The TRQx pins are the inputs to a Torque DAC used to scale the output current. The current scalar value for
different inputs is shown below.
TRQ1
1
1
0
0
Table 5. Torque DAC Settings
TRQ0
1
0
1
0
CURRENT SCALAR
(TRQ)
25%
50%
75%
100%
EFFECTIVE
ATTENUATION
26.4 V/V
13.2 V/V
8.8 V/V
6.6 V/V
18
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