TSL230RD, TSL230ARD, TSL230BRD
PROGRAMMABLE LIGHT-TO-FREQUENCY CONVERTERS
TAOS054P − OCTOBER 2007
6
r
r
Copyright E 2007, TAOS Inc.
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
TSL230RD device package. A low-noise power supply is required to minimize jitter on output pulses.
Device Operational Details
The frequency at the output pin (OUT) is given by:
f
O
= f
D
+ (R
e
) (E
e
)
where:
f
O
is the output frequency
f
D
is the output frequency for dark condition (E
e
= 0)
R
e
is the device responsivity for a given wavelength of light given in kHz/(μW/cm
2
)
E
e
is the incident irradiance in μW/cm
2
f
D
is an output frequency resulting from leakage currents. As shown in the equation above, this frequency
represents a light-independent term in the total output frequency f
O
. At very low light levels, this dark frequency
can be a significant portion of f
O
. The dark frequency is temperature dependent. For optimum performance of
any given device over the full output range, the value of f
D
should be measured (in the absence of light) and
later subtracted from subsequent light measurement (see Figure 1).
Input Interface
A low-impedance electrical connection between the device OE pin and the device GND pin is required for
improved noise immunity.
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.
Sensitivity Adjustment
Sensitivity is controlled by two logic inputs, S0 and S1. Sensitivity is adjusted using an electronic iris technique
— effectively an aperture control — to change the response of the device to a given amount of light. The
sensitivity can be set to one of three levels: 1×, 10×, or 100×, providing two decades of adjustment. This allows
the responsivity of the device to be optimized to a given light level while preserving the full-scale
output-frequency range. Changing of sensitivity also changes the effective photodiode area by the same factor.
