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| 型号： | EL7900ILCZ-EVALZ |
| 厂商： | Intersil |
| 文件页数： | 5/8页 |
| 文件大小： | 0K |
| 描述： | BOARD EVALUATION FOR EL7900ILCZ |
| 标准包装： | 1 |
| 传感器类型： | 灯光，电流输出 |
| 传感范围： | 8000Lux |
| 接口： | 模拟 |
| 电源电压： | 2.7 V ~ 5.5 V |
| 嵌入式： | 否 |
| 已供物品： | 板 |
| 已用 IC / 零件： | EL7900 |
EL7900
Application Information
Product Description
The EL7900 is a light-to-current optical sensor combining
photodiodes and current amplifiers on a single monolithic IC.
The photodiodes are temperature-compensated and their
spectrum resembles the human eye response. The output
current is directly proportional to the intensity of light falling
on the photodiodes. For 100lux of input fluorescent light, the
EL7900 has an output current of 60μA.
The EL7900 is housed in an ultra-compact surface mount
clear plastic package.
Light-to-Current and Voltage Conversion
The EL7900 has a responsiveness that is directly
proportional to the intensity of light intercepted by the
photodiodes. Although the conversion rate varies depending
on the light sources (fluorescent light, incandescent light or
direct sunlight), in general for a fluorescent light, the light-to-
current conversion is:
Resistor Output R LOAD Selection
The resistor output, R LOAD , determines the voltage transfer
function of the device. The device converts light into current
then R LOAD converts the output current to an output voltage.
R LOAD can range from 10 Ω to 10M Ω depending on the input
lux levels. The table below lists R LOAD values to maximize
output swing for typical lux range levels. A careful balance of
dynamic swing and fast response has to be considered
when choosing R LOAD . For faster response, choose a
smaller value R LOAD to shunt stray capacitances that may
slow down response time. For maximum dynamic range or
swing, choose a higher value R LOAD . Although finite, the
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 LOAD is 10M Ω .
The output current must never exceed 6mA. When using
load resistances less than 800 Ω , care must be taken when
lux go as high as 10,000lux because the output current rises
I OUT = ? ------------------- ? × L INPUT
60 μ A
? 100lux ?
(EQ. 1)
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
Here, I OUT is the output current in μA, and L INPUT is the
input light in lux.
voltage reaches voltage compliance.
TABLE 1. V DD = 5V, MAXIMUM OUTPUT VOLTAGE = 4.7V
For some applications, a load resistor is added between the
output and the ground as shown in Figure 1. The output
voltage can be expressed in Equation 2:
ILLUMINATION RANGE
(lux)
0 to 10
RLOAD
(k Ω )
783
CURRENT OUT
(μA)
0 to 6
V OUT = I OUT × R LOAD = ? ------------------- ? × L INPUT × R LOAD
60 μ A
? 100lux ?
(EQ. 2)
0 to 200
0 to 500
39.2
15.7
0 to 120
0 to 300
Here, V OUT is the output voltage and R LOAD is the value of
the load resistor added. The compliance of the EL7900's
output circuit may result in premature saturation of the
0 to 1,000
0 to 10,000
7.8
0.78
0 to 600
0 to 6,000
output current and voltage when an excessively large
R LOAD is used. The output compliance voltage is 300mV
below the supply voltage as listed in V O(MAX) of the
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
Equation 3:
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
( V SUP – 0.3V ) 100lux
R LOAD = --------------------------------------- × -----------------------
L RANGE
(EQ. 3)
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.
Here, V SUP is the supply voltage, and L RANGE is the
specific input light range for an application. For example, an
indoor light ranges typically from 0lux to 1,000lux. A resistor
value of 4.5k Ω 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.
5
The example in Figure 10 shows a fully automatic dimming
solution with no user interaction. Choose R 1 and R 2 values
for any desired minimum brightness and slope. Choose C 1
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
FN7377.8
September 18, 2009
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