Constant Flux LED

Among all the possible applications using LEDs, those where the optical power is set high continuously over long periods of time are challenging in terms of stability over time. For instance, object sensing over long distances, Infrared illumination, therapeutic treatment or spectroscopy are optimized when the LED operates close to their maximum rated current, delivering high optical power for hours at a time, with a high repeat frequency.

The challenge comes from the fact that the rate of decay of the light output of a LED is directly correlated to the Current x Time factor (A X Hrs). The effect of that accelerated decay is a shortening of the life of the system as the light power falls under a given threshold. Note that another important factor is the ambient temperature. The baseline current should be set so that the junction maximum temperature is not exceeded. We will assume that the operating ambient temperature is constant at all time.

To address that challenge, Light in Motion has developed a single component platform, on a 3 leads TO-46 metal header, allowing the design of a servo control circuit that compensates the loss of light efficiency with an increase in the driving current.

This platform associates a light emitting element (LED) with a photodetector in one package. The photodetector receives a fraction of the emitted light, that it converts into a current that serves as the control signal of the servo loop.

The platform is designed to accommodate several options depending on the application requirements:

1-     The header can be capped by either a flat window or a lensed window. Its mechanical axis is also the optical axis of the emitting element.

2-     The emitting element can be visible or infrared LEDs of various wavelengths.

3-     The photodetector can be a photodiode or a phototransistor that provides a substantial gain, necessary for some servo loops designs.

An electrical schematic of a phototransistor option is shown on Fig 1.



                                             Fig 1: Constant Flux LED schematic

                                                  

                                

The actual performances of this device measured with the set up shown on Fig 2 are:

1-     Current limiting resistor R = 100 Ohm

IF = 34 mA

IE = 9 mA



2-     Current limiting resistor R = 200 Ohm

IF = 18 mA

IE = 4.5 mA





                                                Fig 2:   Constant Flux LED Test Circuit                                                             

                                                                  

Example of LED controlling circuit

The most straightforward design consists in building a servo loop, where the Phototransistor provides the negative feedback.

One example of such a design is shown in Fig 3.



                                           Fig 3: Constant Flux LED Control Schematic



We assume that for this specific LED type, the maximum DC current at which it can be driven is 100 mA and that the purpose of the circuit is to allow for a 30% decay of the light output over time. Then the initial driving current is set at 70 mA. Compensating for a 30% decay will need to raise the current to 100 mA. As a result the limiting current resistor R3 should be:

(Vcc - VF - Vcesat) / IF max

(12 – 1.5 - 0.3) / 0.1 = 102 Ohm (rounded to 100 Ohm).

Setting Vin at 3V (arbitrarily), and taking into consideration that the ratio IF/IE at 0 hours is about 3.5, the initial IE current is around 20 mA. As a result the Op amp is balanced for R1 = 3V/20 mA = 150 Ohm.

Vin can be fine tuned at 0 hours to result in a LED current of 70 mA.

In our example R2 has been set to 100 Ohm.

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