el7900 Intersil Corporation, el7900 Datasheet - Page 5

• Manufacturer Part Number: el7900
• Description: Ambient Light Photo Detect Ic
• Manufacturer: Intersil Corporation

In order to have the linear relationship between the input
light and the output current and voltage, a proper resistor
value (i.e., gain) should be picked for a specific input light
range. The resistor value can be picked according to the
following equation:
Here, V
specific input light range for an application. For example, an
indoor light ranges typically from 0 lux to 1,000 lux. A resistor
value of 4.5kΩ for 3V supply voltage can be used. For a
small light range, a large resistor value should be used to
achieve better sensitivity; for a large light range, a small
resistor value should be used to prevent non-linear output
current and voltage.
Resistor Output R
The resistor output, R
function of the device. The device converts light into current
then R
R
lux levels. The table below lists R
output swing for typical lux range levels. A careful balance of
dynamic swing and fast response has to be considered
when choosing R
smaller value R
slow down response time. For maximum dynamic range or
swing, choose a higher value R
output impedance of the device is considerably large.
Hence, the light-to-current conversion deviation because of
resistor loading is infinitesimal. The recommended maximum
R
The output current must never exceed 6mA. When using
load resistances less than 800Ω, care must be taken when
lux go as high as 10,000 lux because the output current rises
above 6mA before reaching the device’s output compliance.
The output compliance of the device is 300mV below the
supply. The output current stops ramping when the output
voltage reaches voltage compliance.
R
ILLUMINATION RANGE
LOAD
LOAD
LOAD
TABLE 1. V
LOAD
0 to 10,000
0 to 1,000
SUP
can range from 10Ω to 10MΩ depending on the input
=
is 10MΩ.
0 to 200
0 to 500
0 to 10
(lux)
(
-------------------------------------- -
V
SUP
is the supply voltage, and L
converts the output current to an output voltage.
60μA
DD
LOAD
–
LOAD
= 5V, MAXIMUM OUTPUT VOLTAGE = 4.7V
0.3V
LOAD
to shunt stray capacitances that may
)
. For faster response, choose a
×
-----------------------
L
100 lux
LOAD
RANGE
, determines the voltage transfer
RLOAD
5
(kΩ)
39.2
15.7
0.78
783
7.8
LOAD
LOAD
Selection
. Although finite, the
values to maximize
RANGE
CURRENT OUT
0 to 6,000
0 to 120
0 to 300
0 to 600
is the
0 to 6
(µA)
(EQ. 3)
EL7900
Application Examples
The following examples present from fully automatic to fully
manual override implementations. These guidelines are
applicable to a wide variety of potential light control
applications. The EL7900 can be used to control the
brightness input of CCFL inverters. Likewise, it can interface
well with LED drivers. In each specific application, it is
important to recognize the target environment and its
ambient light conditions. The mechanical mounting of the
sensor, light aperture hole size and use of a light pipe or
bezel are critical in determining the response of the EL7900
for a given exposure of light.
The example in Figure 10 shows a fully automatic dimming
solution with no user interaction. Choose R
for any desired minimum brightness and slope. Choose C
to adjust response time and to filter 50/60Hz room lighting.
For example, suppose you wish to generate an output
voltage from 0.25V to 1.25V to drive the input of an LED
driver controller. The 0.25V represents the minimum LED
brightness and 1.25V represents the maximum. The first
step would be to determine the ratio of R
Next, the value of R
maximum output current coming from the EL7900 under the
application's maximum light exposure. Suppose the current
has been determined to be about 125µA. Thus, R
calculated approximately as follows:
and
R
R
R
1
2
1
=
=
=
SUPPLY VOLTAGE
R
EN
⎛
⎝
11
----------------- -
125μA
1.25V
2
FIGURE 10. AUTOMATIC DIMMING SOLUTION
×
×
3V TO 5V
EL7900
R
⎛
⎝
--------------- - 1
0.25V
VDO
GND
2
3.0V
⎞
⎠
=
=
110kΩ
10kΩ
–
OUT
⎞
⎠
2
can be calculated based on the
=
11 R
3V DC VOLTAGE
×
R
10k
2
2
R
110k
1
C
25µF
INPUT OR LED DRIVER
TO INVERTER BRIGHTNESS
CONTROLLER
1
1
and R
1
and R
2
:
2
August 1, 2007
2
can be
values
FN7377.6
(EQ. 4)
(EQ. 5)
(EQ. 6)
1