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.
L1
22µH
Murata
LQH32CN220K1
R2
1M
U1
MIC2142
5
D1
MBR0530
White LEDs
SHDN
C1
10µF/6.3V
Taiyo Yuden
JMK212BJ106MG
EN
SW
VCC
3
1
4
Z1
16V
R3
1k
C2
1µF
25V
R1
86.6Ω
C3
10µF/16V
Murata
GRM42-6X5R106K16
2
GND
FB
GND
Figure 1. Constant-Current LED Driver
Micrel, Inc. • 1849 Fortune Drive • San Jose, CA 95131 • USA • tel + 1 (408) 944-0800 • fax + 1 (408) 944-0970 • http://www.micrel.com
August 2002
1
Application Note 40
Application Note 40
85
NP04S B220M
84
EFFICIENCY (%)
VIN = 3.0V
3 White LEDs
L = 22µH
Micrel
In shutdown mode, due to the nature of the boost architec-
ture, the output voltage will equal the input voltage minus the
voltage drop of the Schottky diode. This will cause the white
LEDs to slightly turn on if the voltage of the battery is high
enough. A small switch between the battery and VIN can be
used to disconnect the white LEDs from the battery.
The advantage of the parallel configuration is that the MIC2145
only has to boost the output voltage up to one forward drop
plus the feedback voltage. This allows the MIC2145 to drive
at least 10 white LEDs in parallel with efficiency above 80%
using low profile components.
Brightness Control
The brightness of the LEDs can be easily controlled by
injecting a PWM (Pulse Width Modulation) signal into the
enable pin of the regulator, using a logic control, or applying
a DC voltage from the DAC. Figure 4 shows a 400Hz PWM
signal can be used to control the LEDs’ current by varying the
duty cycle of the PWM signal. Figure 5 shows a logic signal
and a transistor can be used to switch the LEDs current from
8mA to 1mA and vice versa. Figure 6 shows a DAC can be
used to modulate the LED's current.
CLQ4D10-220
83
A914BYW-220M
82
81
80
0
LDR655312T-220W
LQH32CN220K1
5
10
15
LED CURRENT (mA)
20
Figure 2. Typical Efficiency vs. LED current
Figure 2 shows the efficiency of the constant-current LED
driver for various inductor manufacturers. With a constant-
current LED driver, efficiencies above 80% can be achieved
with low profile inductor and capacitors.
Parallel Configuration
Figure 3 shows the parallel LED configuration with the
MIC2145 as the LED driver. Similar to the series configura-
tion, the feedback voltage of the MIC2145 and R3 set the
current for one of the white LEDs. By using the same value
resistor for each of the white LEDs, the rest of the white LEDs
should have similar current. The current for each LED will not
match perfectly due to the resistor tolerance, so there will be
a tiny variation. This variation may cause the LED's bright-
ness to be uneven as compared to the series configuration.
L1
10µH
Sumida
CR43-100
+V
IN
3.0V to 4.2V
C1
10µF/6.3V
Murata
GRM40 X5R 106K 6.3
D1
On Semiconductor V
OUT
MBR0530T1
5V/150mA
VDD
JP1
EN
SW
C3
10µF/6.3V
Murata
GRM40 X5R 106K 6.3
R3
LED1
LED10
PGND
MIC2145
FB
SGND
RSET
SS
R1
100k
GND
R2
10k
C2
0.01µF
R3
Figure 3. Parallel Configuration
+VIN
3.0V to 4.2V
L1
22µH
Murata
LQH32CN220K1
D1
MBR0530
Blue LEDs
PWM
C1
10µF/6.3V
Taiyo Yuden
JMK212BJ106MG
U1
MIC2142
5
EN
SW
VCC
3
1
4
Z1
16V
C3
10µF/16V
Murata
GRM42-6X5R106K16
C2
1µF
25V
R1
86.6Ω
2
GND
FB
GND
Figure 4. Series White LED Driver with PWM Brightness Control
Application Note 40
2
August 2002
Application Note 40
L1
47µH
Murata
LQH32C470K34
R2
1M
U1
MIC2142
5
Micrel
D1
MBR0530
White LEDs
+VIN
3.0V to 4.2V
SHDN
C1
10µF/6.3V
Taiyo Yuden
JMK212BJ106MG
EN
SW
VCC
3
1
4
Z1
16V
R3
1k
C2
1µF
25V
R1
1.27k
174Ω
C3
10µF/16V
Murata
GRM42-6X5R106K16
2
GND
FB
GND
Figure 5. Dual Mode
L1
22µH
Murata
LQH32CN220K1
R2
1M
U1
MIC2142
5
D1
MBR0530
White LEDs
+VIN
3.0V to 4.2V
SHDN
C1
10µF/6.3V
Taiyo Yuden
JMK212BJ106MG
EN
SW
VCC
3
1
4
Z1
16V
R3
1k
C2
1µF
25V
R1
86.6Ω
C3
10µF/16V
Murata
GRM42-6X5R106K16
2
GND
FB
GND
DAC
R5
R4
Figure 6. Series White LED Driver with Analog Dimming Control
Summary
The demand of white and blue LEDs will dramatically in-
crease over the year due to the their low voltage, small size,
simplicity and long life-span. Micrel’s LED drivers will allow
designer to easily power these white and blue LEDs with high
efficiency, low profile components, and minimum component
counts.
MICREL, INC. 1849 FORTUNE DRIVE
TEL
SAN JOSE, CA 95131
WEB
USA
+ 1 (408) 944-0800
FAX
+ 1 (408) 944-0970
http://www.micrel.com
This information is believed to be accurate and reliable, however no responsibility is assumed by Micrel for its use nor for any infringement of patents or
other rights of third parties resulting from its use. No license is granted by implication or otherwise under any patent or patent right of Micrel, Inc.
© 2002 Micrel, Incorporated
August 2002
3
Application Note 40
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