REFX

CHOP

ISENSE

V

I

5

R

u

DRV8843

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SLVSAS8D – APRIL 2011 – REVISED NOVEMBER 2015

7.4.2 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.

For stepping motors, current regulation is normally used at all times, and can changing the current can be used

to microstep the motor. For DC motors, current regulation is used to limit the start-up and stall current of the

motor.

If the current regulation feature is not needed, it can be disabled by connecting the xISENSE pins directly to

ground and connecting the xVREF pins to V3P3.

The PWM chopping current is set by a comparator which compares the voltage across a current sense resistor

connected to the xISEN pins, multiplied by a factor of 5, with a reference voltage. The reference voltage is input

from the xVREF pins, and is scaled by a 2-bit DAC that allows current settings of 100%, 71%, 38% of full-scale,

plus zero.

The full-scale (100%) chopping current is calculated in Equation 1.

(1)

Example:

If a 0.25-

Ω sense resistor is used and the VREFx pin is 2.5 V, the full-scale (100%) chopping current will be

2.5 V / (5 x 0.25

Ω) = 2 A.

Two input pins per H-bridge (xI1 and xI0) are used to scale the current in each bridge as a percentage of the full-

scale current set by the VREF input pin and sense resistance. The xI0 and xI1 pins have internal pulldown

resistors of approximately 100 k

Ω. The function of the pins is shown in Table 3.

Table 3. H-Bridge Pin Functions

RELATIVE CURRENT

xI1

xI0

(% FULL-SCALE CHOPPING CURRENT)

1

1

0% (Bridge disabled)

1

0

38%

0

1

71%

0

0

100%

NOTE

When both xI bits are 1, the H-bridge is disabled and no current flows.

Example:

If a 0.25-

Ω sense resistor is used and the VREF pin is 2.5 V, the chopping current will

be 2 A at the 100% setting (xI1, xI0 = 00). At the 71% setting (xI1, xI0 = 01) the

current will be 2 A x 0.71 = 1.42 A, and at the 38% setting (xI1, xI0 = 10) the current

will be 2 A x 0.38 = 0.76 A. If (xI1, xI0 = 11) the bridge will be disabled and no current

will flow.

7.4.3 Decay Mode During Current Chopping

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 7 as case 1. The current flow direction shown

indicates the state when the xIN1 pin is high and the xIN2 pin is low.

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. As the winding current approaches zero, the bridge is

disabled to prevent any reverse current flow. Fast decay mode is shown in Figure 7 as case 2.

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