Regardless of efficiency or longevity, today's ultra-bright LEDs far exceed the performance of incandescent bulbs. Taking full advantage of these features requires a correspondingly efficient and reliable LED driver, such as the LT3474.
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The LT3474 is a step-down 1A LED driver that supports multiple power supplies with a wide input voltage range of 4V to 36V and can be programmed to deliver 35mA to 1A of LED current with up to 88% efficiency. The device requires very few external circuitry and is available in a space-saving 16-pin TSSOP package.
Car LED driver
Figure 1 shows the configuration of the LT3474 with a 12V automotive battery input as the operating power supply. As shown, the circuit is capable of allowing voltage swings of 4V to 36V that are common in automotive environments. With an integrated NPN switch, boost diode and sense resistor, the LT3474 minimizes the number of external components.
High-end inspection provides a grounded negative connection that relaxes wiring constraints. PWM and analog dimming can be achieved with only minor modifications to the circuit. See the LT3474 data sheet for details.
Drive LED from 12V AC input
The LT3474 directly regulates the LED current, thereby maintaining a constant LED current with VIN variations. The wide input range of the LT3474 allows it to be directly connected to a rectified 12V AC input.
As shown in Figure 2, a small input capacitor is used to minimize the form factor. In this case, the LT3474 can provide LED current close to 1A, as shown in Figure 3. Adding more capacitors to the input (as shown in Figure 4) will keep the input voltage above the LED voltage. In this case, the LT3474 can provide a constant LED current even if there is significant 120Hz ripple on the input, as shown in Figure 5.
Thermal regulation
Thermal management is a core issue for many LED applications. A reliable solution maintains LED life by keeping the LED junction temperature below the recommended limits. One of the countermeasures for this problem is to use a large number of radiators, but this is a waste of space and money. Figure 6 shows a better solution. The temperature of the LED is sensed by a thermistor mounted near the LED and converted into a voltage signal that is sent to the VADJ pin. The VADJ pin appropriately reduces the current flowing through the LED to meet the power derating requirements specified by the Luxeon III Star manufacturer. The circuit can be adjusted by simply modifying the resistance of the resistor for use by other high-brightness LEDs.
in conclusion
With its high efficiency, high quality lighting and long life, high power white LEDs are rapidly becoming the lighting of choice for buildings, automobiles, museums and aerospace systems. The LT3474 makes it easy to implement compact, efficient, rugged and versatile LED drivers from a variety of power sources. Designers can now concentrate on creating imaginative new LED applications instead of spending time on LED drivers.
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