Photo IC for laser beam synchronous detection

What is a photo IC for laser beam synchronous detection?

This photo IC was developed to detect the beam passage timing of the write laser used for laser printers and digital copiers, etc. It contains a high-speed photodiode and high-speed signal processing circuit to provide stable detection timing.


How is the external gain resistance determined?

The analog amplitude (voltage change generated by passage of laser beam) at the Ro terminal is expressed as Vro=Current gain x Photosensitivity x External gain resistance. External gain resistance is selected so that Vro is set to 2 to 3 V (resistance is selected in a range from 1 to 10 kilohms). Current gains of 4, 6, and 20 times are provided and selectable to match the laser beam power. In the case of dual-element type, use a resistor of the same resistance for connection to Ro1 and Ro2.


Are there any rules for the laser scanning direction?

Single-element types can be scanned in either left or right direction, but dual-element types should be scanned in the direction from Ch 1 to Ch 2.


What is the minimum detectable voltage of the analog amplitude DVro at the gain resistance terminal of a dual-element type? (threshold input power)

The minimum detectable voltage is 0.5 V.


The datasheet recommends determining the gain resistance that will set the amplitude at the Ro terminal to 2 to 3 V. What is the reason for this figure?

To utilize an advantage offered by dual-element types, which is timing stability versus power and temperature fluctuations, the Ro1 and Ro2 terminal voltage waveforms must form a cross (X) shape at the center of the dual-element photodiode. As a calculated figure, the analog amplitude that can be obtained as output is 1.5 to 8 V. However, setting the analog amplitude too high may cause faulty operation due to stray light inside the package (stray light occurring at about 6% of the ΔVRo). Setting the upper limit somewhat larger than 3 V (up to about 6 V) will not cause problems, but an excessively large analog amplitude may make it difficult to obtain a stable output, so up to 3 V is recommended.


Will an analog signal amplitude of less than 2 V cause problems?

An amplitude level greater than or equal to 1.5 V will not cause problems. However, a level below 1.5 V will prevent obtaining a cross waveform from the gain resistance terminals, so the output will no longer be stable during power and temperature fluctuations.


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