AN-40 Datasheet, PDF (1/3 Pages) Micrel Semiconductor – Application Note 40

Application Note 40
Micrel
Application Note 40
LEDs vs. CCFL
Final
General Description
LEDs vs CCFL
White LEDs (Light Emitting Diodes) are becoming more
popular choice for backlighting color LCDs (Liquid Crystal
Displays) in cellular phones, PDAs (Personal Digital Assis-
tants), digital cameras and other handheld, portable devices.
Blue LEDs are emerging as a popular backlight for mono-
chrome LCDs. The advantages of white and blue LEDs over
other light sources like CCFLs (Cold Cathode Fluorescent
Lamps) are that the power source is simple to implement, the
life span is longer, the size is smaller and the cost is lower.
CCFL requires an AC power source ranging from 2kVAC at
30kHz-40kHz for operation. This requires DC-AC power
source conversion for portable devices, which translates to
bulky transformer-based circuits. A single white or blue LED
requires a 3.6V to 4.2V DC power source. This means simple
inductor-based circuits can be used to power the LEDs.
Because of the low voltage power, the white and blue LEDs
are ideal for Li-Ion battery applications. The life span of the
CCFL is around 30kHrs to 35kHrs. For the LEDs, the life span
is over 50kHrs. The only advantage to using the CCFL over
the white and blue LEDs is that the CCFL is brighter but much
larger and more expensive to implement.
Overview
This application note describes and shows various applica-
tion circuits to power white and blue LEDs using the MIC2142
and MIC2145. The MIC2142 is a micropower, boost-switch-
ing regulator housed in a SOT-23-5 package. The input
voltage range is between 2.2V to 16V with an adjustable
output voltage up to 22V. In addition, the 330kHz operation
allows small surface mount external components to be used.
The MIC2145 is an MSOP-8 boost-switching regulator with
programmable peak switch current to allow the user to set the
maximum efficiency to occur in the applications. The input
voltage range for the MIC2145 is 2.4V to 16V with an
adjustable output voltage up to 16V.
Series Configuration
Figure 1 shows one of the most common applications for the
white and blue LEDs, which is to configure them in series. In
this particular circuit, three white LEDs are used. The
advantage of this configuration is that the feedback voltage of
the MIC2142, which is a tightly controlled parameter, and R1
set the LED's current. Equal amount of current goes through
the series string of white LEDs. Because the LED’s current
is proportional to its brightness, the result is uniform bright-
ness for all the LEDs.
A Zener diode in parallel with the LEDs is recommended to
protect the MIC2142 if one of the white LEDs opens for any
reason, causing the switch voltage to increase above the
maximum rating of the switch pin.
The challenge of the series configuration is that the MIC2142
has to boost the output voltage up to the sum of all the forward
drop of the white LEDs plus the feedback voltage.
+VIN
3.0V to 4.2V
Li-Ion Batt.
C1
10µF/6.3V
Taiyo Yuden
JMK212BJ106MG
L1
22µH
Murata
LQH32CN220K1
SHDN
R2
1M
U1 MIC2142
5
EN
3
SW
VCC ¹
2 _{GND FB} 4
GND
D1
MBR0530
R3
1k
C2
1µF
25V
White LEDs
Z1
16V
R1
86.6Ω
Figure 1. Constant-Current LED Driver
C3
10µF/16V
Murata
GRM42-6X5R106K16
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 [phone] • fax + 1 [phone] • http://www.micrel.com
August 2002
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Application Note 40