Supplier eBooks

A proper circuit design will enhance LED longevity and performance. A simple transistor, an op amp, and a power resistor can provide a constant current for the LED—a linear design, which is very inexpensive but not at all efficient. The lack of efficiency can produce a lot of heat, often put out near the LED chip itself—not what you want. A lot of integrated circuits are available for driving LEDs. A search at Mouser.com finds thousands of LED Lighting Driver ICs and more than 100 channels. The linear types are simple but inefficient because ICs need to put a lot of power into a load resistor when the bus (or battery) voltage is above the required LED voltage. These chips can feature lots of safety options. Open LED detection, over-voltage protection, and short-circuit protection are readily available. Some even have a thermistor input that watches the LED heat sink temperature and adjusts to keep it below maximum. They also can have power-width modulation (PWM) lamp dimming. When using a linear current source (Figure 2), the bus voltage must always be greater than the LED forward voltage plus the voltage drops required by the driver and load resistor. So if you have one blue LED needing 3.6V to get 20ma, and the bus is 12V, you will drop 8.4V across the transistor and resistor. That's 168mw lost to the driver. This method is inefficient because, depending on the bus to LED voltage differential, the load resistor/transistor must consume considerable power. You could put a DC/DC converter in front of this linear circuit, set to just the correct voltage, and the losses could be minimized. The circuit would need to be set up for a specific LED type because forward voltages are very different (Table 1). The LED forward voltage will drop as the lamps warm. More voltage will be dropped across the load resistor, and efficiency will dip. Assuming the bus is the standard automotive 12V battery, that voltage can vary from about 7V to 15V, depending on conditions, but the DC/DC converter will regulate that. The switch-mode LED drivers are a more efficient solution. They are usually called boost or buck- boost type and are very efficient simply because they adapt to the exact voltage that the LED(s) require(s). Also, forward voltage for various colors and materials have variations (Table 1). The switch mode will adjust to any combination. As noted, the forward voltage will go down with temperature, and most circuits have three or four LEDs in a series. When looking at manufacturer selection guides, it can be exceedingly difficult to discriminate between linear and efficient switch-mode LED drivers. The converter PWM switching frequency is usually around 250kHz and must be well over the audio range. Tests have shown no degradation of LEDs because of this constant switching on and off 16 A pRopeR dRive CiRCuit Figure 2: A Simple Linear LED Drive Circuit (Source: Mouser Electronics).

• Cover