AD5398

Rev. A | Page 13 of 16

APPLICATIONS

The AD5398 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 20

shows the transfer curve of a typical spring preloaded linear

motor for autofocus. The key points of this transfer function are

displacement or stroke, which is the actual distance the lens

moves in mm, and the current through the motor in mA.

A start current is associated with spring preloaded linear

motors, which is effectively 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 AD5398 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 AD5398 the ideal solution

for these applications is the monotonicity of the device, which

ensures that lens positioning is repeatable for the application of

a given digital word.

Figure 21 shows a typical application circuit for the AD5398.

0.5

0.4

0.3

0.2

0.1

0

10

20

30

40

START

CURRENT

50

60

70

80

90

100

110

120

0

SINK CURRENT (mA)

Figure 20. Spring Preloaded Voice Coil Stroke vs. Sink Current

POWER-ON

RESET

3

7

2

6

4

1

POWER-DOWN

RESET

8

VOICE

COIL

ACTUATOR

3.3

Ω

INTERFACE

10-BIT

CURRENT

OUTPUT DAC

REFERENCE

R

AD5398

5

SCL

SDA

DEVICE

DEVICE

DEVICE

0.1

μF

10

μF

+

0.1

μF

10

μF

+

Figure 21. Typical Application Circuit