TSL245
INFRARED LIGHT TO FREQUENCY CONVERTER
TAOS005 MAY 1999
1
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t
t
D
High-Resolution Conversion of Light
Intensity to Frequency With No External
Components
D
Communicates Directly With a
Microcontroller
D
Compact Three-Leaded Plastic Package
D
Single-Supply Operation Down to 2.7 V
D
Nonlinearity Error Typically 0.2% at 100 kHz
D
Integral Visible-Light Cutoff Filter
Description
The TSL245 infrared light-to-frequency converter combines a silicon photodiode and a current-to-frequency
converter on a single monolithic CMOS integrated circuit. The output is a square wave (50% duty cycle) with
frequency directly proportional to light intensity. Because the output is TTL compatible, it allows direct interface to a
microcontroller or other logic circuitry. The device responds over the infrared light range of 800 nm to 1100 nm and
is characterized for operation over the temperature range of 25
°
C to 70
°
C.
Functional Block Diagram
Light
Current-to-Frequency
Converter
Photodiode
Output
Absolute Maximum Ratings over operating free-air temperature range (unless otherwise noted)
Supply voltage, V
DD
(see Note 1)
6.5 V
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Operating free-air temperature range, T
A
25
°
C to 70
°
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Storage temperature range, T
stg
25
°
C to 85
°
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Lead temperature 1,6 mm (1/16 inch) from case for 10 seconds
240
°
C
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Stresses beyond those listed under "absolute maximum ratings" may cause permanent damage to the device. These are stress ratings only, and
functional operation of the device at these or any other conditions beyond those indicated under "recommended operating conditions" is not
implied. Exposure to absolute-maximum-rated conditions for extended periods may affect device reliability.
NOTE 1: All voltage values are with respect to GND.
t
t
Copyright
©
2000, TAOS Inc.
Texas Advanced Optoelectronic Solutions Inc.
800 Jupiter Road, Suite 205
S
Plano, TX 75074
S
(972) 673-0759
3
2
1
OUT
V
DD
GND
SR PACKAGE
(FRONT VIEW)
TSL245
INFRARED LIGHT TO FREQUENCY CONVERTER
TAOS005 MAY 1999
2
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Recommended Operating Conditions
MIN
NOM
MAX
UNIT
Supply voltage, V
DD
2.7
5
6
V
Operating free-air temperature range, T
A
25
70
°
C
Electrical Characteristics at V
DD
= 5 V, T
A
= 25
°
C (unless otherwise noted)
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
V
OH
High-level output voltage
I
OH
= 4 mA
4
4.3
V
V
OL
Low-level output voltage
I
OL
= 4 mA
0.17
0.26
V
I
DD
Supply current
2
3
mA
Full-scale frequency
500
kHz
k
SVS
Supply-voltage sensitivity
V
DD
= 5 V
±
10%
0.5
%/V
Full-scale frequency is the maximum operating frequency of the device without saturation.
Operating Characteristics at V
DD
= 5 V, T
A
= 25
°
C
PARAMETER
TEST CONDITIONS
MIN
TYP
MAX
UNIT
f
O
Output frequency
E
e
= 920
µ
W/cm
2
,
p
= 940 nm
200
250
300
kHz
f
O
Output frequency
E
e
= 0
0.25
10
Hz
N
li
it
§
f
O
= 0 kHz to 10 kHz
±
0.1%
%F.S.
Nonlinearity
§
f
O
= 0 kHz to 100 kHz
±
0.2%
%F.S.
Step response to full-scale step input
1 pulse of new
frequency plus 1
µ
s
Full-scale frequency is the maximum operating frequency of the device without saturation.
§
Nonlinearity is defined as the deviation of f
O
from a straight line between zero and full scale, expressed as a percent of full scale.
TSL245
INFRARED LIGHT TO FREQUENCY CONVERTER
TAOS005 MAY 1999
3
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TYPICAL CHARACTERISTICS
1
0.1
0.01
0.001
0.001
0.01
0.1
1
10
Output Frequency kHz
10
100
OUTPUT FREQUENCY
vs
IRRADIANCE
100
1 k
E
e
Irradiance
µ
W/cm
2
f O
V
DD
= 5 V
p
= 940 nm
T
A
= 25
°
C
1000
PHOTODIODE SPECTRAL RESPONSIVITY
Wavelength nm
Normalized Responsivity
0.5
0.4
0.2
0.1
0
0.9
0.3
600
700
800
900
0.7
0.6
0.8
1
1000
1100
T
A
= 25
°
C
Figure 1
Figure 2
0.1
0.01
25
0
25
50
1
DARK FREQUENCY
vs
TEMPERATURE
10
75
V
DD
= 5 V
E
e
= 0
100
Dark Frequency Hz
f O(dark)
T
A
Temperature
°
C
Wavelength of Incident Light nm
T
emperature Coefficient of Output Frequency ppm/
°
C
TEMPERATURE COEFFICIENT
OF OUTPUT FREQUENCY
vs
WAVELENGTH OF INCIDENT LIGHT
4000
0
700
800
900
1000
6000
8000
2000
V
DD
= 5 V
T
A
= 25
°
C to 70
°
C
10000
750
850
950
Figure 3
Figure 4
TSL245
INFRARED LIGHT TO FREQUENCY CONVERTER
TAOS005 MAY 1999
4
www.taosinc.com
t
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TYPICAL CHARACTERISTICS
0.998
0.997
0.996
0.995
2.5
3
3.5
4
4.5
Normalized Output Frequency
0.999
1
OUTPUT FREQUENCY
vs
SUPPLY VOLTAGE
5
5.5
V
DD
Supply Voltage V
T
A
= 25
°
C
f
O
= 500 kHz
1.001
6
1.002
1.003
1.004
1.005
Figure 5
TSL245
INFRARED LIGHT TO FREQUENCY CONVERTER
TAOS005 MAY 1999
5
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APPLICATION INFORMATION
Power-supply considerations
For optimum device performance, power-supply lines should be decoupled by a 0.01-
µ
F to 0.1-
µ
F capacitor
with short leads (see Figure 6).
Output interface
The output of the device is designed to drive a standard TTL or CMOS logic input over short distances. If lines
greater than 12 inches are used on the output, a buffer or line driver is recommended.
Measuring the frequency
The choice of interface and measurement technique depends on the desired resolution and data-acquisition
rate. For maximum data-acquisition rate, period-measurement techniques are used.
Period measurement requires the use of a fast reference clock with available resolution directly related to
reference-clock rate. The technique is employed to measure rapidly varying light levels or to make a fast
measurement of a constant light source.
Maximum resolution and accuracy may be obtained using frequency-measurement, pulse-accumulation, or
integration techniques. Frequency measurements provide the added benefit of averaging out random- or
high-frequency variations (jitter) resulting from noise in the light signal. Resolution is limited mainly by available
counter registers and allowable measurement time. Frequency measurement is well suited for slowly varying
or constant light levels and for reading average light levels over short periods of time. Integration, the
accumulation of pulses over a very long period of time, can be used to measure exposure the amount of light
present in an area over a given time period.
TSL245
Timer/Port
MCU
0.1
µ
F
V
DD
2
3
1
Figure 6. Typical TSL245 Interface to a Microcontroller
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