AN3668| Application Note
Maxim/Dallas > App Notes > AMPLIFIER AND COMPARATOR CIRCUITS POWER-SUPPLY CIRCUITS
DISPLAY DRIVERS
Keywords: cree , nichia, ledtronics, Toyoda, agilent, xlamp, ostar led, high-power led, buck boost led driver, general lighting led driver, HB LED, HB LED driver, HB LEDs, HB-LED, white led lighting, white leds , luxeon star, hbled driver, buck led driver, boost led driver, high
Oct 10, 2005
APPLICATION NOTE 3668
High-Efficiency Current Drive for High-Brightness LEDs
This article describes a simple circuit for regulating the current through high-brightness LEDs. The design includes an off-the-shelf, highly integrated step-down switching regulator (the MAX5035), which allows accurate control of the LED current. The MAX5035 DC/DC converter operates on a fixed frequency of 125kHz with wide 7.5V to 76V input-voltage range especially useful for automotive applications. For brightness control, this converter implements either analog (linear dimming) or low-frequency duty-cycle modulation (PWM dimming).
Background on High-Brightness LEDs
In recent years, high-brightness (HB) LEDs have gained prominence as the lighting source for a variety of applications. HB LEDs are rugged and reliable semiconductor devices capable of several tens of thousands of cycles--up to 100,000 hours of operation. That performance represents an operating life that is orders of magnitude longer than conventional incandescent and halogen lamps. Thus, HB-LED applications can be found in automotive lighting, public and commercial signage, and architectural lighting. HB LEDs are PN-junction devices especially processed to produce white, red, green, and blue light when forward biased. (Amber and a few other colors are possible as well.) As PN-junction devices, LEDs exhibit V-I characteristics similar to those of conventional diodes, but with higher voltage drops across their junctions. Little current passes through an LED until the forward voltage reaches VF, which varies from 2.5V for red LEDs to about 4.5V for blue LEDs. When VF is reached, the current increases very rap
idly (as in conventional diodes). Consequently, the designer must employ current limiting to prevent possible damage. Current limiting can be implemented with three basic methods, each with advantages and disadvantages (Table 1). Table 1. Comparison of Current-Limiting Methods Current-Limit Method Power Loss in Series Device1
Advantages
Disadvantages
Resistor
q q
Inexpensive Only one, large component
q It cannot control current accurately. q Current varies to some extent 2.8W with supply voltage. q High power dissipation in the resistor; must be sized accordingly.
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