AN1427 Datasheet, PDF (1/12 Pages) Microchip Technology – High-Efficiency Solutions for Portable LED Lighting

AN1427
High-Efficiency Solutions for Portable LED Lighting
Authors: Mihnea Rosu
Microchip Technology Inc.
INTRODUCTION
As the LED manufacturing technology advances, parts
with higher luminous flux and higher lumen per watt
characteristics appear on the market. Applications
include: street and roadway lighting, outdoor lighting,
indoor commercial and industrial lighting, portable light-
ing. The drivers used in these kinds of applications
need to be cost-effective, but efficiency is also critical.
Depending on the type of LED (single die or multiple
die), either high voltage or high current is needed. For
example, a single die 10W LED will require 3A at 3.3V,
and a 40W multi-die LED will require 1A at 40V.
The purpose of this application note is to demonstrate
a high-efficiency design for a 10W security type flash-
light. The power source is very important, since it dic-
tates the power converter topology. Four AA type
batteries can provide loaded voltages ranging from
6.4V (Ni-Zn), 6.0V (Ultimate Lithium and Alkalines)
down to 4.8V (Ni-MH). A multi-die LED will require
higher voltage and a boost topology, which will usually
require a separate controller chip, while a single die
LED requires higher current, but a buck converter
topology can be easily adapted for this low voltage
application.
LED luminous flux is always characterized using
forward current, and varies in a linear fashion with
current (Figure 1).
FIGURE 1:
LED LUMINOUS FLUX VERSUS CURRENT FOR A CREE XM-L LED
This becomes very important when trying to achieve a
small flux variation on the proposed line of products
(flashlights). Using Voltage mode and current limiting
resistors is not recommended for several reasons. First
of all, LED forward voltages may vary significantly, so
the current will vary significantly. Also, using resistors
at the required forward current will severly compromise
efficiency (100 mOhms dissipates 0.9W at 3A). Current
mode converters will achieve the highest efficiency
when driving LEDs, and will keep luminous flux
constant.
 2012 Microchip Technology Inc.
DS01427A-page 1