DRV8846_15 Datasheet, PDF (15/35 Pages) Texas Instruments – DRV8846 Dual H-Bridge Stepper Motor Driver

www.ti.com
DRV8846
SLLSEK2 – JUNE 2014
1/32 STEP
117
118
119
120
121
122
123
124
125
126
127
128
1/16 STEP
59
60
61
62
63
64
Table 3. Relative Current and Step Directions (continued)
1/8 STEP
30
31
32
1/4 STEP
16
1/2 STEP
FULL STEP
70%
WINDING WINDING
CURRENT A CURRENT B
83%
–56%
86%
–51%
88%
–47%
90%
–43%
92%
–38%
94%
–34%
96%
–29%
97%
–24%
98%
–20%
99%
–15%
100%
–10%
100%
–5%
ELECTRICAL
ANGLE
326
329
332
335
338
340
343
346
349
352
354
357
8.3.3 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 reaches 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 to 30 µs through the tri-level input TOFF_SEL. After the time expires, the bridge is re-
enabled, starting another PWM cycle.
Table 4. Fixed Off-Time Selection
TOFF_SEL
0
Z
1
TOFF Duration
20 μs
10 μs
30 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. The reference voltage can be supplied by an internal
reference of 3.3 V (which requires VINT to be connected to VREF), or externally supplied to the VREF pin. The
reference voltage is then scaled first by the 3-bit torque DAC, then by the output of a sine lookup table that is
applied to a sine-weighted DAC (sine DAC). The voltage is attenuated by a factor of 6.6.
The full-scale (100%) chopping current is calculated as follows:
I_FS

VREF
6.6 K R_ISENSE
K TORQUE
where
• I_FS is the full scale regulated current
• VREF is the voltage on the VREF pin
• R_ISENSE is the resistance of the sense resistor
• TORQUE is the scaling percentage from the torque DAC.
(1)
Example: Using VREF is 3.3 V, torque DAC = 100%, and a 500-mΩ sense resistor, the full-scale chopping
current is 3.3 V / (6.6 × 500 mΩ) × 100% = 1 A.
The current for both motor windings is scaled depending on the I0 and I1 pins, which drive a 3-bit linear DAC, as
in Table 5.
Copyright © 2014, Texas Instruments Incorporated
Product Folder Links: DRV8846
Submit Documentation Feedback
15