Monday, June 1, 2009

How we should drive a LED?

LED works with a linear current control or Constant current control because of the following reasons:

The Current has a non-linear relationship with the Relative intensity of the LED.
The variation of the Current will change the dominant wavelength of a LED.
So it is necessary to maintain a constant current thus to ensure constant LED colour.
The current control supply can be all SMT components; this high power resistor is eliminated. This reduces the assembly complexity. Again this will give a cost saving.
Reduce power dissipation at high temperature, high supply voltage, and lower LED forward voltage condition. (as compared to a resistor drive)

Constant current source will give better, more uniform appearance, and allow it to meet a tighter tolerance specification.

Two technologies can be considered in designing the LED driver: the first one is linear current control driver; the second is switch mode current control driver.
The Linear current control or constant current source is commonly used due to lower cost than the Switch mode current control driver.

LED Constant Current Source
A constant current source for LED is usually consisting of a small integrated circuit and several external components, the constant current configuration serves as a relatively inexpensive LED driver solution. The drive current is typically set during design by the use of an external resistor connected between ground and a single pin of the IC chip package. The circuitry is compact enough to fit directly on the circuit board along with the LED lights in the array. As the voltage fluctuates during operation, the constant current source will do the job to compensate and to maintain a steady current output for each of the LEDs within the array. Lighting systems equipped with multiple LEDs may operate from several redundant constant current sources. In such a case, the components are located upon the same circuit board and share a common power supply. Individual sub-circuits will appear to operate as one.

Junction Voltage Drop Variations
Another common cause of voltage fluctuation is the result of varying junction temperatures within the LED. As current passes through the LED, the junction temperature will begin to increase. This increase in temperature causes the forward voltage drop across the junction to decrease. Reductions in forward voltage lead to an increase of total current flowing through the LED light. The increase in current causes an increase in temperature. This continuous cycle can lead to catastrophic failure unless the current is limited. High power LEDs are extremely susceptible to variations in forward voltage. The constant current source provides an ideal solution in order to achieve a reliable circuit and prevent risk of thermal run away.

Manufacturing deviation
Imperfections during the LED manufacturing process can cause individual LED characteristics to vary somewhat drastically. The binning process relieves this dilemma by categorizing LEDs according to key characteristics including luminous intensity, spectral wavelength, and forward voltage drops. However, LEDs common to a voltage bin typically do not present identical forward voltage drops. Furthermore, the difference between the forward voltages of dissimilar bins may be dramatic. Some light emitting diode manufactures may not even offer binning according to forward voltage. As forward voltage differentials will cause drive current differentials. The solution is to apply the constant current source in order to compensate for such deviations caused by the LED manufacturing process.

Simplified Circuitry
Some engineers consider using simple resistor driven circuitry as a bad practice. This is mainly because forward currents can vary as the forward voltage across the LED and junction temperature changes. Deviations during LED manufacturing cause forward voltage drops to vary, which results in dissimilar drive currents. Varying source voltages often associated with automotive applications can also lead to current fluctuations. Although never recommended in lighting systems equipped with high power LEDs, simple resistor driven circuitry does not present such a negative impact when utilized within circuits containing traditional LEDs. However, one should be very cautions employing this technique! Typically the lower power devices such as a 100 mW LED, to not require advanced drive circuitry.

Although advanced circuitry may present numerous advantages when used in conjunction with lower power LEDs, the overall cost efficiency is typically not one of them. Heat dissipation is much less and forward currents only vary slightly in a properly designed circuit.
Considering the benefits cost efficiency are up to one hundred times greater with simplified drive circuitry, it is clear why such small variations in forward current become insignificant.
Selection of appropriate resistors and proper circuit architecture, help assure operation over a minimal current range even as the voltage variables are applied. It is very important to perform resistor calculations using the typical forward voltage drop as specified in the LED datasheets, but also while considering the minimum and maximum specified voltage drops. By doing so, it becomes easy to see how much or how little the forward current may vary between LED lots.

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