點晶科技股½有限公司
SILICON TOUCH TECHNOLOGY INC.
Application Notes
The 11-MD115 is constant current control for use in Auto-Focus. The range of
supply voltage of 11-MD115, VDD is from 2.0V to 5.5V. The 11-MD115 digital
control pin, PD, its input range is defined that logic “H” is from 0.8*VDD to
VDD+0.4V and logic “L” is from –0.4V to 0.2*VDD. The input pin, IN, is VCM
constant current setting pin.
The power down pin (PD) is the enable pin of 11-MD115, which logic high level
(PD = H) is for IC operation. On the other hand, its logic low level (PD = L) puts
the chip into standby mode for power saving. Therefore, it is easy to switch the
working status by controlling PD pin, and it is recommended that keeps PD at low
level (PD = L) before operation to reach the maximum efficiency of power saving,
especially in the application of portable device.
Constant current operation of 11-MD115 provides the current, which can be
V
IN
evaluated by the formula
I
=
(
A
)
. It is obviously that constant current
10*
R
S
(
Ω
)
not only depends on the resistance load on the current sense terminal, IM, but
the amount of input voltage level, V
IN
. Therefore, by adjusting the resistance of R
A
,
R
B
, and the capacitance C
IN
, with appropriately PWM frequency will get the
suitable and stable input voltage level, V
IN
, for setting constant current in the
output.
In the application circuit diagram, the signal PWM, V
PWM
, is filtered by a low pass
filter which consists of R
A
, R
B
, and C
IN
. The -3dB frequency
ω
3dB
and V
IN
are
given by
ω
3
dB
=
1
(
R
A
//
R
B
)
C
IN
V
IN
=
R
B
D
%
×
V
PWM
R
A
+
R
B
(
V
)
,
,
which D% is the duty ratio of PWM frequency. The corresponding of constant
current at the output could be changed by setting different duty ratio of PWM, In
order to confirm the accuracy and stable value of constant current, the amount of
SP-MD115-A.005.doc
Version
：
A.005
Page
：
7
SITI [ SILICON TOUCH TECHNOLOGY INC. ]
