AD5821

Rev. 0 | Page 14 of 16

APPLICATIONS INFORMATION

The AD5821 is designed to drive both spring-preloaded and

nonspring linear motors used in applications such as lens auto-

focus, image stabilization, or optical zoom. The operation principle

of the spring-preloaded motor is that the lens position is controlled

by the balancing of a voice coil and spring. Figure 25 shows the

transfer curve of a typical spring-preloaded linear motor for

autofocus. The key points of this transfer function are displace-

ment or stroke, which is the actual distance the lens moves in

millimeters (mm) and the current through the motor, measured

in milliamps (mA).

A start current is associated with spring-preloaded linear

motors, which is a threshold current that must be exceeded for

any displacement in the lens to occur. The start current is usually

20 mA or greater; the rated stroke or displacement is usually

0.25 mm to 0.4 mm; and the slope of the transfer curve is

approximately 10 μm/mA or less.

The AD5821 is designed to sink up to 120 mA, which is more

than adequate for available commercial linear motors or voice

coils. Another factor that makes the AD5821 the ideal solution

for these applications is the monotonicity of the device, ensuring

that lens positioning is repeatable for the application of a given

digital word.

Figure 26 shows a typical application circuit for the AD5821.

10

50

60

80

70

90 100

40

30

20

110 120

0.1

0.5

0.4

0.3

0.2

START

CURRENT

SINK CURRENT (mA)

Figure 25. Spring-Preloaded Voice Coil Stroke vs. Sink Current

7

2

1

XSHUTDOWN

8

VOICE

COIL

REFERENCE

R

AD5821

5

SCL

SDA

DEVICE

DEVICE

D1

0.1µF

INTERFACE

3

4

6

POWER-ON

RESET

0.1µF

10µF

10µF

+

+

10-BIT

CURRENT

OUTPUT DAC

Figure 26. Typical Application Circuit