Datasheet

Full-Scale I =
TRIP
xVREF
¾
5 R·
ISENSE
DRV8834
www.ti.com SLVSB19C –FEBRUARY 2012–REVISED JUNE 2013
With stepping motors, current control is normally used at all times. Often it is used to vary the current in the two
windings in a sinusoidal fashion to provide smooth motion. This is referred to as microstepping. The DRV8834
can provide up to 1/32 step microstepping, using internal 5-bit DACs. Finer microstepping can be implemented
using the xPHASE/xENBL signals to control the stepper motor, and varying the xVREF voltages. The current
flowing through the corresponding H-bridge varies according to the equation given below. A very high degree of
microstepping can be achieved through this technique.
With DC motors, current control can be used to limit the start-up current of the motor to less than the stall current
of the motor.
Current regulation works as follows:
When an H-bridge is enabled, current rises through the winding at a rate dependent on the supply voltage and
inductance of the winding. If the current reaches the current chopping threshold, the bridge disables the current
until the beginning of the next PWM cycle. Note that immediately after the current is enabled, the voltage on the
xISEN pin is ignored for a period of time before enabling the current sense circuitry. This blanking time also sets
the minimum on time of the PWM when operating in current chopping mode.
Note that the blanking time also sets the minimum PWM duty cycle. This can cause current control errors near
the zero current level when microstepping. To help eliminate this error, the DRV8834 has a "dynamic" t
BLANK
time. When the commanded current is low, the blanking period is reduced, which in turn lowers the minimum
duty cycle. This provides a smoother current transition across the zero crossing region of the current waveform.
The end result is smoother and quieter motor operation.
The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor
connected to the xISEN pins, with a reference voltage supplied to the AVREF and BVREF pins. In indexer mode,
the reference voltages are scaled by internal DACs to provide scaled currents used to perform microstepping.
The chopping current is calculated as follows:
(1)
Example: If xVREF is 2 V (as it would be if xVREF is connected directly to VREFO) and a 400-mΩ sense resistor
is used, the chopping current will be 2 V/5 x 400 mΩ = 1 A.
In indexer mode, this current value is scaled by between 5% and 100% by the internal DACs, as shown in the
step table in the "Microstepping Indexer" section of the datasheet.
Note that if current control is not needed, the xISEN pins may be connected directly to ground. in this case it is
also recommended to connect AVREF and BVREF directly to VREFO.
Current Recirculation and Decay Modes
During PWM current chopping, the H-bridge is enabled to drive current through the motor winding until the PWM
current chopping threshold is reached. This is shown in Figure 2 as case 1. The current flow direction shown
indicates positive current flow in the step table below for indexer mode, or the current flow with xPHASE = 1 in
phase/enable mode.
Once the chopping current threshold is reached, the drive current is interrupted, but due to the inductive nature
of the motor, the current must continue to flow. This is called recirculation current. To handle this recirculation
current, 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 through the opposing FETs. As the winding current approaches zero, the bridge
is disabled to prevent any reverse current flow. Fast decay mode is shown in Figure 2 as case 2.
In slow decay mode, winding current is re-circulated by enabling both of the low-side FETs in the bridge. This is
shown as case 3 below.
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