TSL238 datasheet(6/12 Pages) TAOS

6 / 12 page

TSL238

HIGH-SENSITIVITY

LIGHT-TO-FREQUENCY CONVERTER

TAOS073F − OCTOBER 2007

The LUMENOLOGY r Company

www.taosinc.com

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

TSL238 (Figure 7). A low-noise power supply is required to minimize jitter on output pulse.

TSL238

Timer / Port

MCU

0.1 μF

VDD

OUT

GND

Figure 7. Typical TSL238 Interface to a Microcontroller

Device Operational Details

The frequency at the output pin (OUT) is given by:

fO = fD + (Re) (Ee)

where:

is the output frequency

is the output frequency for dark condition (Ee = 0)

is the device responsivity for a given wavelength of light given in kHz/(μW/cm2)

is the incident irradiance in μW/cm2

fD is a constant error term in the output frequency calculation resulting from leakage currents, and is

independent of light intensity. The TSL238 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.

TSL238 Datasheet(PDF) 6 Page - TEXAS ADVANCED OPTOELECTRONIC SOLUTIONS