DRV8711 Datasheet, PDF (13/34 Pages) Texas Instruments – STEPPER MOTOR CONTROLLER IC

DRV8711
www.ti.com
SLVSC40 – JUNE 2013
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
and inductance of the winding and the magnitude of the back EMF present. Once the current hits the current
chopping threshold, the bridge disables the current for a fixed period of time, which is programmable between
500 nS and 128 µS by writing to the TOFF bits in the OFF register. After the off time expires, the bridge is re-
enabled, starting another PWM cycle.
In stepping motors, current regulation is used to vary the current in the two windings in a 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 xISENx pins, multiplied by the gain of the current sense amplifier, with a reference voltage. The
current sense amplifier is programmable in the CTRL register.
Registers
PWM
logic
Gate
Drive
&
OCP
Gate
Drive
&
OCP
VM
x1HS
xOUT1
x1LS
x2HS
VM
xOUT2
x2LS
Indexer
2.75 V
SIN
DAC
1V
TORQUE
Torque
DAC
Comp
Comp
X
ISEN
amp
ISGAIN
xISENP
xISENN
R_ISENSE
Figure 2. PWM Chopping Current
To generate the reference voltage for the current chopping comparator, the output of a sine lookup table is
multiplied by the value of the bits in the TORQUE register. This result is applied to a sine-weighted DAC, whose
full-scale output voltage is 2.75 V.
Therefore, the full-scale (100%) chopping current is calculated as follows:
IFS = ^{2.75V · TORQUE}
256 · ISGAIN · RISENSE
(2)
Where TORQUE is the setting of the TORQUE bits, and ISGAIN is the programmed gain of the ISENSE
amplifiers (5, 10, 20, or 40).
Copyright © 2013, Texas Instruments Incorporated
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