Part # Application Notes Power Supply And Power Management L6902 AN2129 datasheet

Part Manufacturer: ST Microelectronics

ST Microelectronics


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AN2129 APPLICATION NOTE DIMMING OF SUPER HIGH BRIGHTNESS LEDS WITH L6902D 1 IntroductionThanks to the high efficiency and reliability, super high brightness LEDs are becoming moreand more important when compared to conventional light sources. Although LEDs can be sup-plied directly from a simple voltage source (like battery with resistor), for most applications itis better to use a switching current source to get not even higher efficiency but also to get abetter light output. This paper will focus on a L6902D based DC/DC converter with dimminginterface. For more details about other converters and applications for LEDs available fromSTMicroelectronics please refer to other application notes ([1] and [2]). 2 Dimming ConceptsThere are two basic principles how the light output of the LED can be controlled. Since the lightbrightness is proportional to the current, both methods are dealing with current regulation. Thefirst and the easiest way is to control the LED current itself, with the principal sketch in Figure1, where current is changed proportionally with the dimming signal. Disadvantage of this ana-log control is that there can be a significant change of color (wavelength difference could beseveral nanometers) in deep dimming (less that 10%). This potential disadvantage is compen-sated by a very simple control circuit (usually a simple potentiometer is enough). Figure 1. Analog current control Figure 2. Average current control by PWM I I I max I average time time The second method is based on an average current control (digital control) as can be seen inFigure 2. The current is switched between zero and the nominal current with a frequency high-er than 100Hz (to avoid flickering). The change of duty cycle and hence the average currentchange will be seen as a brightness change, because human eye reaction is slow enough to"integrate" the light output and it will not be noticed as a blinking.This method avoids the color change problem, but on the other hand it needs more sophisti-cated control circuits (usually a microcontroller or another simple PWM generator). 3 L6902D DC/DC ConverterThe L6902D is a complete and simple step down switching regulator with adjustable currentand voltage feedback. Thanks to its current control loop with external sense resistor it is ableto work in a constant current mode, providing up to 1A output current with an accuracy of 5%.Among other features there can be also found general purpose 3.3Volts precise (2%) refer-ence voltage or 2.5A (typical value) internal current limit for short circuit protection. Rev. 2 AN2129/0705 1/9 AN2129 APPLICATION NOTE In Figure 3 is the internal structure of the L6902D converter, the datasheet [3] should be re-ferred for more details. Figure 3. L6902D Block diagram (see [3] for details) Vcc VOLTAGESMONITOR 3.8V Vref INHIBIT VREF SUPPLY GOOD 1.235V Current_E/A CS+ + THERMAL SHUTDOWN - TRIMMING CS- COMP PEAK TO PEAK CURRENT LIMITING Voltage_E/A + 1.235V VFB E/A - - D

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Application Notes Power Supply And Power Management L6902 AN2129.pdf Datasheet