I

=

CHOP

V

REFX

5

¾

DRV8812

SLVS997F – OCTOBER 2009 – REVISED AUGUST 2013

www.ti.com

Bridge Control

The xPHASE input pins control the direction of current flow through each H-bridge. The xENBL input pins enable

the H-bridge outputs when active high. Table 2 shows the logic.

Table 2. H-Bridge Logic

xENBL

xPHASE

xOUT1

xOUT2

0

X

Z

Z

1

1

H

L

1

0

L

H

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.

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 0.5-

Ω sense resistor is used and the VREFx pin is 3.3 V, the full-scale (100%) chopping current will be

3.3 V / (5 x 0.5

Ω) = 1.32 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 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 that when both xI bits are 1, the H-bridge is disabled and no current flows.

Example:

If a 0.5-

Ω sense resistor is used and the VREF pin is 3.3 V, the chopping current will be 1.32 A at the 100%

setting (xI1, xI0 = 00). At the 71% setting (xI1, xI0 = 01) the current will be 1.32 A x 0.71 = 0.937 A, and at

the 38% setting (xI1, xI0 = 10) the current will be 1.32 A x 0.38 = 0.502 A. If (xI1, xI0 = 11) the bridge will be

disabled and no current will flow.

8

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