LED Driver
Drivers placed in proximity to the anode, referred to as the high side LED driver, sources current directly from the positive power rail. Drivers connected to the cathode, referred to as low side LED driver, sink current directly to ground. When used in combination the LED low side / high side circuitry permits multiplexing, very useful for LED display applications. The multiplexed output can greatly reduce the overall number of components, and promote savings. Consider an array of seven-segment LED displays, containing ten digits. Including the decimal point, the ten displays contain eighty total power inputs. Rather than provide an output from a microcontroller for each of these eighty inputs, the light emitting diode segments are subdivide into groups. All eight inputs corresponding with each display connect together, and tie to seven output pins on the microcontroller. Additionally, all ten common grounds connect to ten additional output lines on the microcontroller. As a result, the microcontroller can still select any one of the eighty light emitting diode segments, but while utilizing only seventeen output lines. A multiplexed LED driver configuration is not always beneficial in high power illumination applications. Multiplexing results in a reduction of the total luminous output since the LED lights are pulse driven, and not all light emitting diodes within the array can illuminate simultaneously.
LED Constant Current Source
A constant current source is always the recommended LED driver solution for custom LED lighting designs equipped with high power LEDs. 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 integrated circuit package. Altogether, the circuitry is compact enough to fit directly on the circuit board along with the LED lights in the array. Numerous factors can lead to voltage fluctuations within circuitry. As the voltage fluctuates during operation, the constant current source will compensate to maintain a steady current output for each of the light emitting diodes 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.
Source Voltage Variation
One factor leading to voltage fluctuations is the power supply. A power supply voltage may vary for reasons such as external devices inducing a load on the circuitry. External devices or other elements of the custom LED lighting design can cause the source voltage to decrease as current drawn is increased. In an automotive or aeronautical application, the alternator activity will actually cause the source voltage to increase. As the alternation produces an electrical current for battery maintenance, the system voltage will typically increase by as much as several volts. Yet another case of source voltage variation found in all battery dependant devices can result in source voltage reductions. As the battery begins to deplete, the voltage will steadily decrease. In the previous examples, cases of voltage reduction can lead to dimming of LEDs and a loss of luminous output. Cases when voltage increases can cause an overdriven state that can permanently damage LEDs, also leading to a loss of luminous output.
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 Imperfections
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 explained in the previous paragraph, 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 Architecture
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 temperatures change. 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 three previously discussed 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.