TSL238T
HIGH-SENSITIVITY
LIGHT-TO-FREQUENCY CONVERTER
TAOS080E
−
JUNE 2007
APPLICATION INFORMATION
Power-Supply Considerations
Power-supply lines must be decoupled by a 0.01-μF to 0.1-μF capacitor with short leads placed close to the
TSL238T (Figure 7). A low-noise power supply is required to minimize jitter on output pulse.
V
DD
V
DD
0.1
μF
TSL238T
OUT
Timer / Port
GND
MCU
Figure 8. Typical TSL238T Interface to a Microcontroller
Device Operational Details
The frequency at the output pin (OUT) is given by:
f
O
= f
D
+ (R
e
) (E
e
)
where:
f
O
f
D
R
e
E
e
is the output frequency
is the output frequency for dark condition (E
e
= 0)
is the device responsivity for a given wavelength of light given in kHz/(μW/cm
2
)
is the incident irradiance in
μW/cm
2
f
D
is a constant error term in the output frequency calculation resulting from leakage currents, and is
independent of light intensity. The TSL238T die is trimmed to minimize the magnitude of this dark frequency
component so that it can be neglected in the transfer function calculation. In many applications, measurement
of the actual dark frequency may be impractical due to measurement times ranging from several seconds to
several minutes, and the fact that some devices may never transition (zero dark frequency).
Output Interface
The output of the device is designed to drive a CMOS logic input over short distances. If lines greater than 12
inches in length are used on the output, a buffer or line driver is recommended.
The
LUMENOLOGY
r
Company
r
r
Copyright
E
2007, TAOS Inc.
www.taosinc.com
5
TAOS [ TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS ]
