The devices consist of a high performance LED shining on a photocell inside a light-tight case (Figure 1). An equivalent circuit of the photocell is shown in Figure 2.

 

Figure 1

 

Figure 2

Rv can be considered as a "perfect" resistance varying with the light intensity. Cp is very low (a couple of pF) and can often be neglected at audio frequencies. Rs is also low compared to the minimum value of Rb at the maximum permissible LED current. Rb is the bulk dark resistance of the cell layer, and along with Rv shows a marked negative voltage dependency. Resistance decreases with increasing cell voltage.

Most photocells are made of either a Cadmium Sulphide (CdS) or Cadmium Selenide (CdSe) photoconductive material. Either material is suited for specific applications, but only a compound of both materials will optimize audio performance and remove significant audio performance issues.

The compound photocell responds best to a very specific wavelength and requires an LED with a matching wavelength to guarantee performance stability, both in batch and lifetime terms. The shape of the LED optics and the optical coupling of the LED to the photocell are also very important for performance. A lens on the LED will create a hotspot on the interdigital pattern of the photocell. This results in unstable resistive performance and can result in a massive dynamic range shift if the LED or cell is moved. Silonex uses an index matched coupling medium to efficiently couple the LED to the cell. The absence of, or incorrect coupling medium will result in performance changes in the finished component.

Highly developed processing of the CdS and CdSe photoconductive layer guarantees the lowest possible ohmic contact and reduces parasitic capacitance, resulting in devices with ultra low distortion.

Maximum Ratings

The data sheet for each type of coupler states absolute maximum ratings for cell voltage, cell power dissipation and LED current. It is important to adhere to these ratings. In addition, an LED will show an aging effect when operated at or near the maximum current rating. To minimize this effect in an application where the LED will be on most of the time, the optocoupler should be run at less than half of the maximum rating.

Temperature Coefficient

At a constant LED current, the cell resistance shows a marked positive temperature coefficient of approximately 1% per degree C. If temperature stability is critical to your application, it may be better to add a second coupler or temperature sensitive resistor to compensate.

Time Constants

Resistive optocouplers take a certain time to change their resistance as the LED current is varied.

Generally speaking, they are quicker to turn ON (increase conductance) than to turn OFF (decrease conductance). Both the rise and fall in conductance are essentially exponential over the majority of the range. The speed can be expressed as time constants TON to reach 68% of the ON state conductance and TOFF to fall to 37% of that level, See Figure 3.

Time constants for several audiohm couplers measured at RON = 1 KOhm are:

		TON		TOFF
NSL-28AA		2.0 msec		6.0 msec
NSL-32		6.0 msec		16.0 msec
NSL-32SR2		4.5 msec		16.0 msec
NSL-32SR3		1.0 msec		2.5 msec

The turn ON and turn OFF times observed in any application circuit will depend on the source and load resistances used.

Control Law

These couplers show a decreasing cell resistance with increased LED current. Figure 4 shows the RON vs. ILED performance for a typical NSL-32SR2 coupler. The transfer characteristic shows three regions at medium currents (3uA<ILED<75uA) the cell resistance is given by:

At low currents, Rb dominates the curve being less steep and becoming asymptotic to ROFF. At high currents saturation of the LED results in the curve becoming less steep tending towards the minimum value of RON achievable. Figure 5 shows the same measurement for the NSL-32SR3. This device has a slightly higher value for Rv and a much higher value of Rb, resulting in no flattening of the transfer characteristic at low currents.