DRV8818 Datasheet, PDF (8/20 Pages) Texas Instruments – STEPPER MOTOR CONTROLLER IC

DRV8818
SLVSAX9A – SEPTEMBER 2011 – REVISED FEBRUARY 2012
www.ti.com
Current Regulation
The PWM chopping current is set by a comparator, which compares the voltage across a current sense resistor,
multiplied by a factor of 8, with a reference voltage. The reference voltage is input from the VREF pin. The
full-scale (100%) chopping current is calculated as follows:
V
I
CHOP
=
REFX
8
·
R
ISENSE
(1)
Example:
If a 0.22-Ω sense resistor is used and the VREFx pin is 3.3 V, the full-scale (100%) chopping current is
3.3 V/(8 * 0.22 Ω) = 1.875 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.
When a winding is activated, the current through it rises until it reaches the chopping current threshold described
above, then the current is switched off for a fixed off time. The off time is determined by the values of a resistor
and capacitor connected to the RCA (for bridge A) and RCB (for bridge B) pins. The off time is approximated by:
t
OFF
= R ·C
(2)
To avoid falsely tripping on transient currents when the winding is first activated, a blanking period is used
immediately after turning on the FETs, during which the state of the current sense comparator is ignored. The
blanking time is determined by the value of the capacitor connected to the RCx pin and is approximated by:
t
BLANK
= 1400 ·C
(3)
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 2, Item 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. If synchronous rectification is enabled (SRn pin logic low), the
opposite FETs are turned on; as the winding current approaches zero, the bridge is disabled to prevent any
reverse current flow. If SRn is high, current is recirculated through the body diodes, or through external Schottky
diodes. Fast-decay mode is shown in Figure 2, Item 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 2, Item 3.
If SRn is high, current is recirculated only through the body diodes, or through external Schottky diodes. In this
case fast decay is always used.
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