De-rating

The light-emitted diode like many others components has a de-rating curve. Most of LEDs have a de-rating curve based on ambient temperature. You can generally find this curve in the datasheet of your LED. It has a maximum forward current and this maximum current will be lower when operating at high ambient temperature. In figure 1, you have an example of a de-rating curve for the QTLP690C LED from Fairchild Semiconductor.

Figure 1 : Forward Current (mA) vs Ambient Temperature (Celsius) for the QTLP690C (Fairchild Semiconductor) surface mount LED

For this particular LED, there is two curves since the yellow light-emitted diode has a different forward current than the others colors. If we look at the curve for the red LED, the maximum forward current starts dropping at an ambient temperature of 25°C. At 50°C, the maximum forward current is about 21mA. This LED will burn or explodes eventually if you drive this LED at 30mA in an environment with an ambient temperature of 50°C. You must take this in consideration when designing circuits. 50°C seems like a lot but this ambient temperature is easily attainable in real life applications. A server room is an example, some server cabinet can go up to 50°C ambient temperature. There is a small drawback if you reduce the forward current to stay within the limit of the LED. You will get less light from your LED since you are driving it with a lower current. In figure 2, we have the curve for the relative intensity vs the forward current. You can see that the relative intensity of the LED drop linearly with the current. It is a small drawback but a necessary one if you want a functional LED in high ambient temperature applications.

Figure 2 : Relative Intensity vs Forward Current (mA) for the QTLP690C (Fairchild Semiconductor) surface mount LED