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-糉 to 0.1-糉 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/(糤/cm
2
)
E
e
is the incident irradiance in 糤/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.