LM9831CCVJDX datasheet(24/41 Pages) NSC

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the 2 bit DAC, shown in Figure 9.

4.2 Microstep Mode

Microstepping is a technique of driving the stepper motor with a

staircase approximation of a sine wave, as shown in Figure 10.

This technique maximizes the torque of a given motor, resulting in

a higher maximum speed. In addition, it increases the resolution

of the stepper motor. If a stepper motor moves 3.6° per full step,

microstepping can create positions inside the 3.6°: 1.8°, 0.9°, or

0.45°, for example. This increases the maximum vertical resolu-

tion of the scanner. Microstepping also results in quieter motor

movement.

The amplitude of the microstepped sine wave is controlled by the

output of the stepper motor DAC (Figure 11). The current in the

stepper motor winding is measured as a voltage across the sense

resistor, and the transistor drive signals are pulse width modu-

lated (PWM) to force the average current through the winding

equal to VDAC/RSENSE. Register 56 controls the frequency of the

PWM, and Register 57 controls the minimum time the driver is on

every period. Register 57 should be set as short as possible, the

driver only needs to be on long enough to mask any transient

noise generated by the driver transistor turning on.

Figure 12 shows the LM9831’s DAC voltages. The peak current

through the stepper motor winding will be 0.465V/RSENSE. The

table index is incremented every microstep (StepSize pixel peri-

ods).

4.3 Pause Behavior - Non-Reversing Mode

When the Full Steps to Reverse When Buffer is Full register is

0, the stepper motor simply stops moving when the Pause signal

is received, as shown in Figure 13. The line of data currently

being processed (section “a” in Figure 13) will continue to be pro-

cessed and stored in DRAM. Additional lines may be digitized

and stored as well, depending on the number programmed in the

Lines to Process After Pause Scan Signal register (Figure 14).

This value is different for different scanner designs and should be

empirically set to the value that minimizes the spatial distortion

created by the motor slowing down and stopping.

Scan Mode

DAC Voltage

Starting from

a dead stop

0.465V for number of steps specified

in Kickstart Steps register (0-7). If

current-movement begins at 0.325V.

Scanning

0.325V

Stopped

0.110V for number of steps specified

in Hold Current Timeout register (1 -

31), 0V after time out.

Figure 9: Full Step Current Control

Figure 10: Bipolar Microstepping Waveform

1 microstep

Table

Index

A (B)

DAC

Voltage

N/A

0.175V

0.325V

0.425V

0.465V

-0

N/A

-1

0.175V

-2

0.325V

-3

0.425V

-4

0.465V

Figure 12: Microstepping Current Control

Figure 11: Stepper Motor Waveform - LM9831 Signals

DAC

Figure 13: Stepper Motor Stopping

Microstep

Pulse

Pause

Scanning

Signal

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