Temperature Compensation Circuit for Constant LED Intensity - App Note

The luminous intensity of an LED has a temperature dependence of exp [ k ( T - TA ) ] where k is on the order of -0.01 / °C. This means that as temperature linearly increases or decreases, the light intensity of an LED exponentially decreases or increases respectively. This characteristic is a liability in outdoor applications where the LED becomes dimmer during the warmer daytime when sunlight viewability is of concern. The circuit configuration shown in Figure 1 is an example of how to minimize the intensity dependency on temperature problems in a voltage sourced application where the LED current is set with the LED in series with a current limiting resistor (Rlim). This configuration can also be used as a discrete circuit whose output is referred to other LED biasing networks (that have the same type of LED as the discrete) under the same ambient temperature condition. If large enough quiescent voltage levels (i.e. Vb < -5 Volts) are used to bias the LED under nominal temperature conditions, then any pn junction potential variations (i.e. silicon = -2 mV/ °C is a small linear value) can be treated as negligible in a circuit trying to compensate for the exponential dependence of light intensity on temperature.