8

REFX

CHOP

ISENSE

V

I

R

=

·

OFF

t

R

C

=

·

1400

BLANK

t

C

=

·

DRV8811

www.ti.com

SLVS865G – SEPTEMBER 2008 – REVISED MAY 2010

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:

(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:

(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:

(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.

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