DRV8824-Q1_15 Datasheet, PDF (12/30 Pages) Texas Instruments – DRV8824-Q1 Automotive Motor Controller IC

DRV8824-Q1
SLVSCH0 – APRIL 2014
www.ti.com
Feature Description (continued)
8.3.2 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 terminals, multiplied by a factor of 5, with a reference voltage. The reference voltage is
input from the xVREF terminals.
The full-scale (100%) chopping current is calculated in Equation 1.
V
I = ₅^¾ _{R CHOP}
REFX
· _ISENSE
(1)
Example:
If a 0.5-Ω sense resistor is used and the VREFx terminal is 3.3 V, the full-scale (100%) chopping current will
be 3.3 V / (5 x 0.5 Ω) = 1.32 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.
8.3.3 Blanking Time
After the current is enabled in an H-bridge, the voltage on the xISEN terminal is ignored for a fixed period of time
before enabling the current sense circuitry. This blanking time is fixed at 3.75 μs. Note that the blanking time also
sets the minimum on time of the PWM.
8.3.4 Microstepping Indexer
Built-in indexer logic in the DRV8824-Q1 allows a number of different stepping configurations. The MODE0 -
MODE2 terminals are used to configure the stepping format as shown in .
MODE2
0
0
0
0
1
1
1
1
MODE1
0
0
1
1
0
0
1
1
Table 1. Stepping Format
MODE0
0
1
0
1
0
1
0
1
STEP MODE
Full step (2-phase excitation) with 71% current
1/2 step (1-2 phase excitation)
1/4 step (W1-2 phase excitation)
8 microsteps / step
16 microsteps / step
32 microsteps / step
32 microsteps / step
32 microsteps / step
Table 2 shows the relative current and step directions for different settings of MODEx. At each rising edge of the
STEP input, the indexer travels to the next state in the table. The direction is shown with the DIR terminal high; if
the DIR terminal is low the sequence is reversed. Positive current is defined as xOUT1 = positive with respect to
xOUT2.
Note that if the step mode is changed while stepping, the indexer will advance to the next valid state for the new
MODEx setting at the rising edge of STEP.
The home state is 45°. This state is entered at power-up or application of nRESET. This is shown in Table 2 by
the shaded cells. The logic inputs DIR, STEP, nRESET and MODEx have an internal pulldown resistors of
100 kΩ
12
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