Light modulation photo IC

S4289-61

CONSTANT

VOLTAGE

CIRCUIT

BUFFER

SIGNAL

PROCESSING

CIRCUIT

TIMING

GENERATOR

OSCILLATOR

OUTPUT

CIRCUIT

GND

CATHODE

(LED)

Vout

Vcc

COMPARATOR

Vref

PREAMP

PD

TRUTH TABLE

INPUT

OUTPUT LEVEL

LIGHT ON

LIGHT OFF

LOW

HIGH

s Block diagram and internal functions

(a) Oscillator and timing signal generator

The oscillator produces a reference oscillation output by charg-

ing and discharging the built-in capacitor with constant current.

The oscillation output is fed to the timing signal generator, which

then creates LED drive pulses and various timing pulses for

digital signal processing.

(b) Photodiode and preamplifier circuit

The photodiode is formed on the same monolithic chip. A photo-

current generated in the photodiode is converted to a voltage by

a preamplifier circuit. The preamplifier circuit uses an AC amplifier

to expand the dynamic range versus DC or low-frequency back-

ground light, without impairing signal detection sensitivity.

(c) Capacitivecoupling,bufferamplifierandreferencevoltagegenerator

Capacitive coupling removes low-frequency noise and also can-

cels the DC offset in the preamplifier. The buffer amplifier boosts

the signal up to the comparator level, and the reference voltage

generator produces a comparator level voltage.

(d) Comparator circuit

The comparator circuit has a hysteresis function to prevent chat-

tering caused by small fluctuations in the input light.

(e) Signal processing circuit

The signal processing circuit consists of a digital integrator circuit

that eliminates non-continuous, synchronous background light.

(f) Output circuit

This circuit serves as an output buffer for the signal processing

circuit and outputs the signal to an external circuit.

Note: The cathode (LED) terminal should be grounded.

KPICC0040EA

s Handling precautions

Synchronous mode acquires signals in synchronization with the LED emission timing. In asynchronous mode, a time window

is provided which opens at a certain cycle to acquire signals, and the LED frequency is set so that at least one signal enters the

time window while open. Since this time window for signal acquisition is wider when compared with synchronous mode

operation, the asynchronous mode is less capable of eliminating disturbance background light at high frequencies or in

pulsed form (noise light). However, asynchronous mode has the advantage of not requiring any wiring in order to synchronize

the LED with the photo IC.

S4289-61 exhibits the same characteristics as S7136 series light modulation photo IC except for synchronous detection. To

use S4289-61, the cathode (LED) terminal should be grounded and the LED operated by an external driver circuit.