Datasheet

AD5398
Rev. 0 | Page 13 of 16
APPLICATIONS INFORMATION
The AD5398A is designed to drive both spring preloaded
and nonspring linear motors used in applications such as lens
autofocus, image stabilization, or optical zoom. The operating
principle of the spring-preloaded motor is that the lens position
is controlled by the balancing of a voice coil and a spring. Figure 21
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 millimeters (mm), and the current through the motor
in milliamps (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 approxi-
mately 10 µm/mA or less.
The AD5398A 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 AD5398A the ideal
solution for these applications is the monotonicity of the device,
which ensures that lens positioning is repeatable for the applica-
tion of a given digital word.
Figure 22 shows a typical application circuit for the AD5398A.
07795-020
0.5
0.4
0.3
0.2
0.1
0
10 20 30 40
START
CURRENT
50 60 70 80 90 100 110 1200
SINK CURRENT (mA)
STROKE (mm)
Figure 21. Spring Preloaded Voice Coil Stroke vs. Sink Current
0
7795-022
POWER-ON
RESET
D1
POWER-DOWN
RESET
VOICE
COIL
ACTUATOR
R
SENSE
3.3Ω
V
DD
I
2
C SERIAL
INTERFACE
10-BIT
CURRENT
OUTPUT DAC
REFERENCE
R
AD5398A
SCL
SDA
I
2
C MASTER
DEVICE
I
2
C SLAVE
DEVICE
I
2
C SLAVE
DEVICE
R
P
R
P
V
DD
V
DD
V
CC
0.1µF 10µF
+
0.1µF10µF
+
SDA
AGNDDGND
V
DD
SCL
PD
I
SINK
DGND
Figure 22. Typical Application Circuit