Dec 2006 - Reliable, Efficient LED Backlighting for Large LCD Displays

LINEAR TECHNOLOGY
DECEMBER 2006
IN THIS ISSUE…
Cover Article
Reliable, Efficient LED Backlighting
for Large LCD Displays .......................1
Hua (Walker) Bai
Linear Technology in the News…..........2
Design Features
Precise Current Sense Amplifiers
Operate from 4V to 60V........................6
by Jun He
Tiny, High Efficiency Monolithic
Buck Converters are Perfect for
Powering Portable Devices....................9
Phil Juang
Supply Supervisor Family Accurately
Monitors Multiple Voltages with
Independent Undervoltage and
Overvoltage Detection........................11
Scott A. Jackson
Improve that Mobile Phone Camera:
Replace the Anemic LED Flash with
a Xenon Flashlamp and a Tiny
Photoflash Capacitor Charger............14
Wei Gu
High Performance, Feature-Rich
Solutions for High Voltage
DC/DC Converters...............................16
Kevin Huang
OLED Driver Has Low Ripple, Small
Footprint and Output Disconnect.......20
Jesus Rosales
Hot Swap Controller Controls Power
to Two PCI Express Slots....................21
CY Lai
DESIGN IDEAS
.....................................................25–36
(complete list on page 25)
New Device Cameos............................37
Design Tools.......................................39
Sales Offices......................................40
VOLUME XVI NUMBER 4
Reliable, Efficient
LED Backlighting for
Large LCD Displays
Introduction
LEDs are rapidly becoming the
preferred light source for large LCD
displays in computers, TVs, navigation
systems, and various automotive and
consumer products. LEDs offer several
benefits over fluorescent tubes: high
luminous efficacy (lm/W), more vivid
colors, tunable white point, reduced
motion artifacts, low voltage operation and low EMI. However, system
engineers face certain problems
associated with driving LEDs for LCD
backlight applications, including
effectively providing sufficient power,
regulating the LED current, matching
current in multiple LED strings,
achieving high LED dimming ratios,
and fast LED current turn on/off.
All of these issues can be easily
addressed in compact and reliable
circuits that use the LT3476 high
current LED driver and LT3003 3channel ballaster.
The LT3476 is a quad output,
current mode DC/DC converter
operating as a constant current source
with up to 96% efficiency. It is ideal
for driving up to 1A of current for up
to eight-per-channel RGB or white
LEDs (such as Luxeon III) in series.
That results in a total output power
of about 100W.
The LT3003 is a 3-channel LED
current ballaster, which can be used
to triple the number of LEDs driven by
a single LT3476 channel. When LED
by Hua (Walker) Bai
strings are in parallel, special care is
required to ensure safe operation and
accurate current matching. Otherwise,
one string will almost always draw
much more current and eventually
be damaged. The LT3003 can be used
System engineers face a
number of problems when
designing LED backlights for
LCD backlight applications—
such as effectively providing
sufficient power, accurately
regulating the LED current,
matching current in multiple
LED strings, achieving high
LED dimming ratios, and
fast LED current turn on/off.
All of these issues can be
easily addressed in compact
and reliable circuits that
use the LT3476 high
current LED driver and
LT3003 3-channel ballaster.
with the LT3476 or other LED drivers
to regulate current in the LED strings.
This is one way to reduce the perLED current and increase brightness
uniformity across a large display. For
continued on page L, LT, LTC, LTM, Burst Mode, OPTI-LOOP, Over-The-Top and PolyPhase are registered trademarks of Linear Technology
Corporation. Adaptive Power, Bat-Track, BodeCAD, C-Load, DirectSense, Easy Drive, FilterCAD, Hot Swap, LinearView,
µModule, Micropower SwitcherCAD, Multimode Dimming, No Latency ΔΣ, No Latency Delta-Sigma, No RSENSE, Operational
Filter, PanelProtect, PowerPath, PowerSOT, SmartStart, SoftSpan, Stage Shedding, SwitcherCAD, ThinSOT, True Color PWM,
UltraFast and VLDO are trademarks of Linear Technology Corporation. Other product names may be trademarks of the
companies that manufacture the products.
DESIGN FEATURES L
required for instantaneous setting of
the backlight brightness according to
the image information and environment in which the device is used. A
large dimming ratio also helps reduce
motion artifacts. Without adding
components and cost, both the LT3476
and the LT3003 can achieve at least
1000:1 PWM dimming ratio with less
than 5µs rise/fall time. Additional
analog dimming is also possible.
LT3476, continued from page example, the 1A output LED current
of the LT3476 can be safely shared by
three parallel strings of LEDs when the
LT3003 is added. Each string carries
up to 350mA. The LT3003 guarantees
3% LED current matching.
Dimming ratio is defined as the
ratio between the highest and the
lowest achievable brightness of a
system. A large dimming ratio is often
R3 100k
R2 4.99k
R24 100k
R25 4.99k
R26 100k
R27 4.99k
R28 100k
R29 4.99k
C4
1µF
E1
18
7
REF
R6 0Ω
LT3476EUHF
1
VC1
R7 0Ω 38
VC2
R9 0Ω 13
VC3
R10 0Ω 12
C10
1nF
C11
1nF
4 PVN
CAP2
5
LED2
SW2 27
SW2 26
35
PWM1
34
PWM2
17
PWM3
16
PWM4
C12
1nF
VC4
C13
1nF
6
RT
R11
21k
GND
39
C1
4.7µF
50V
(OPT)
C2
2.2µF
35V
C3
2.2µF
35V
D1 DFLS140
3 PVN
CAP1
2
LED1
29
SW1
28
SW1
37
VADJ1
36
VADJ2
15
VADJ3
14
VADJ4
PWM1
PWM2
PWM3
PWM4
+
33
VIN
SHDN
High Side Current Sensing for
Versatility and Reliability
High side LED current sensing is generally more flexible than low side, in that
it supports buck, boost or buck-boost
configurations. High side sensing also
allows for “one-wire” operation. For
example, in a boost circuit with a high
side sense resistor, if the LEDs are
PVIN
33V MAX
VIN
2.8V TO 16V
C6
22nF
Features
9 PVN
CAP3
8
LED3
25
SW3
24
SW3
10 PVN
CAP4
11
LED4
23
SW4
22
SW4
N/C N/C
19-21 30-32
C5
0.33µF
R1
0.1Ω
LED
L1
10µH
LED
D2 DFLS140
C7
0.33µF
R4
0.1Ω
LED
L2
10µH
LED
D3 DFLS140
C8
0.33µF
R5
0.1Ω
LED
L3
10µH
LED
D4 DFLS140
C9
0.33µF
R8
0.1Ω
LED
L4
10µH
LED
Figure 1. The LT3476 delivers 100W in buck mode
0.98
1000
900
800
LED CURRENT (mA)
EFFICIENCY (%)
0.96
0.94
PWM
5V/DIV
0.92
0.90
0.88
0.86
200
600
400
800
LED CURRENT (mA)
1000 1200
Figure 2. Efficiency of the buck
mode circuit in Figure 1
Linear Technology Magazine • December 2006
600
500
400
300
200
ILED
500mA/
DIV
1
700
100
5µs/DIV
PWM FREQ = 100Hz
PWM PULSE WIDTH = 10µs
Figure 3. 1000:1 PWM dimming
0
0
20
60
40
DUTY CYCLE
80
100
Figure 4. Average LED current
vs PWM duty cycle
L DESIGN FEATURES
Buck, Boost or
Buck-Boost Operation
Because of the high side current sense
scheme, the LT3476 and the LT3003
support buck, boost or buck-boost
operation. In buck mode, an LT3476
circuit can achieve 96% efficiency,
generating less heat and providing
more reliability. For automotive applications where the LEDs must be
remote from the driver in some way,
such as in a hinged laptop display, the
LED current can return to the local
display ground, saving a wire in the
return path. Low side sensing requires
an extra wire, because the LED current
must return to the driver side for low
noise operation. The one wire setup
lowers cost and improves reliability,
especially as the channels multiply in
high performance displays.
PVIN
8V TO
16V
2.2µF
L1
10µH
L2
10µH
L3
10µH
D2
D3
D1
CAP1
2.2µF
6–8 LEDS
VIN
3.3V
CAP2
0.1Ω
2.2µF
LED2
350mA
2.2µF
D4
CAP4
0.1Ω
0.1Ω
LED3
350mA
SW1
SW2
CAP1-4
LED1-4
VIN
PWM1-4
SHDN
PWM1-4
SHDN
2.2µF
L4
10µH
CAP3
0.1Ω
LED1
powered from a lead-acid battery, the
LT3476 can be configured for boost
mode to drive up to eight LEDs per
channel. Furthermore, returning the
LED current in a boost configuration
to the battery enables buck-boost
operation, where the input voltage can
be higher or lower than the output
voltage. As a result, the LT3476 and
LT3003 can accept a variety of power
sources.
2.2µF
LED4
350mA
SW3
LT3476
350mA
1.05V
SW4
REF
VADJ1-4
66.5k
33.2k
VC1-4
RT
GND
1k
21k
1nF
Figure 5. The LT3476 configured into a boost circuit for automotive applications
VIN
3V TO 16V
PVIN
33V MAX
C2
2.2µF
35V
C1
1µF
18
37
3
CAP1
2
LED1
REF
R1
0.1Ω
PWM1
PWM1
6
R3
21k
LED
LED
LED
SW1
RT
SW1
GND
39
35
LED
LED
2
3
D2
20V
VIN
LED1
D1
DFLS140
5
VMAX
SHDN
LT3003EMSE
LED2
VEE
LED3
9
C5
0.33µF
10
L1
10µH
PWM OT1 OT2 GND
6
7
8
11
R4
10k
R2
0Ω
C6
1nF
LED
VADJ1
VC1
4
1
VIN
LT3476EUHF
1
C4
1µF
35V
33
SHDN
7
C3
2.2µF
35V
29
28
N/C N/C
19-21 30-32
Figure 6. The LT3476 and LT3003 in buck mode
Linear Technology Magazine • December 2006
DESIGN FEATURES L
PWM and Analog Dimming
Dedicated PWM dimming circuitry
inside the LT3476 and LT3003 allows
a 1000:1 dimming ratio. Additional
analog dimming is possible through
the VADJ pins. This allows for a significant number of hues and tones,
resulting in finer and more exact color
definition.
Small Packages
The LT3476 is available in a 5mm
× 7mm QFN package. The LT3003
comes in a small MS10 package. Both
packages are thermally enhanced with
exposed metal ground pads on the
bottom of the package.
Accurate Current
Monitoring and Matching
Each of the four LT3476 current
monitor thresholds is trimmed to
within 2.5% at the full scale of 105mV.
The LT3003 drives three separate
strings of LEDs at up to 350mA/string
with 3% accurate current matching.
Both measures result in uniform LED
brightness and intensity.
LT3476 Delivers
100W in Buck Mode
In today’s large LCD TVs with LED
backlights, the power requirement
for driving the LEDs can be a couple
hundred watts. Figure 1 shows a
circuit for a high power LED driver. It
is configured as a buck mode converter,
delivering 100W to the LEDs from a
33V supply at 96% efficiency. Two of
these circuits are enough to drive all
the LEDs for a 32-inch LCD TV. For
simplicity’s sake only channel 1 is
discussed here.
All four LT3476 channels are
independent and function in the same
way. When the internal power switch
turns on, the SW1 pin is grounded.
Wide Range of Operating
Frequencies to Match
any Application
The LT3476 frequency is adjustable
between 200kHz and 2MHz, allowing
the user to trade off between the
efficiency and the solution size.
The voltage crossing the inductor L1
is PVIN – VLED1, where the VLED1 is
the voltage drop on the LED string
at the given current. As a result,
the inductor L1 current ramps up
linearly and energy builds up. When
the power switch is off, the inductor
sees VLED1. The energy in the inductor
is discharged and transferred to the
LEDs through the catch diode D1. The
capacitor C5 filters out the inductor
current ripple. The LED current is
the average of the inductor current.
Figure 2 shows the efficiency as a
function of the LED current.
To change the maximum LED
current, adjust the R1 value or the
resistor divider values at the VADJ1
pin. The VADJ pins can be used for
white balance calibration. At 100Hz
PWM frequency, the PWM control of
this circuit allows 1000:1 dimming
as shown in Figure 3. Figure 4 shows
that the PWM dimming ratio has a
good linear relationship to the average
LED current. Faster switch on/off
time is possible if a PFET disconnect
circuit with a level shifter is in series
Applications
continued on page 33
VIN
8V TO 16V
D1
DFLS140
L1
4.7µH
13
14
1
10
9
3
7
8
5
R5
1.02M
1%
17
SW SW
FBN
6
18
N/C
19
N/C
20
N/C
VIN
IADJ1
IADJ2
SHDN
ISP2
LT3477
R6
45.3k
1%
11
R4
0.3Ω
1%
FBP
ISN2
VREF
VC
GND
GND
15
21
D2
1N4148W
R1
10k
C4
4.7µF
16
ISP1 ISN1
PWM
VMAX
SUMIDA
CDRH5D16-4R7
C1
1µF
25V
Q1
2N7002
C2
R2
22nF
0Ω
SS
4
RT
12
2
R3
6.81k
6-8
LEDs/STRING
C3
0.033µF
6
7
8
3
2
LED3
LED2
LED1
VMAX 4
LT3003
VIN
PWM
OT1
OT2
GND
11
1
SHDN
VEE
10
VMAX VIN
5
9
C3
1µF
25V
Figure 7. The LT3476 and LT3003 in boost mode
Linear Technology Magazine • December 2006
DESIGN IDEAS L
Better than Buck-Only or
Boost-Only Solutions
To avoid the cost or real estate requirements of traditional SEPIC or
cascaded boost-buck topologies, some
designers opt for buck-only or boostonly solutions. For example, in two
AA alkaline cell applications such as
MP3 players, 2.5V often serves as the
main rail since it drives both the flash
memory and the main processor I/O.
In such applications, some designers
use a synchronous boost converter
to save cost and space. The problem
is that the boost converter is very
inefficient while the battery voltage is
above 2.5V because a boost converter
incurs both the losses inherent in an
LDO and the switching losses an LDO
doesn’t have. Figure 4 shows that the
boost converter operates inefficiently
for 28% of the battery runtime (the
portion of the battery life when the
battery’s voltage declines from a fully
charged 3V to 1.8V). An LTC3530
solution results in significantly longer
battery runtimes compared to these
solo boost or buck solutions.
Conclusion
Linear Technology’s synchronous
buck-boost converter simplifies the
design of lithium-ion or 2-AA-cell powered handheld devices that require up
LT3476, continued from page with a LED string. With this PFET
disconnect circuit, the switch off time
is less than 2µs.
Boost Circuit for
Automobile Lighting
It is straightforward to use the LT3476
for a boost application given the fact
the main power switch is tied to the
ground. Figure 5 shows a boost circuit
for applications such as automotive
exterior and interior lighting. This
circuit provides 350mA to eight Luxeon
LEDs per channel from a car battery.
The efficiency is over 92% with a 16V
input.
Triple the Number of
LED Strings with the LT3003
Each LT3476 channel can be configured to drive three parallel LED
strings by adding the LT3003. In such
a configuration, each LED string uses
one third of the output current of the
channel. The LT3003 easily operates
in boost mode, or in buck mode with
an architecture that allows the power
ground (VEE) to move with the output
capacitor voltage. Figure 6 shows
LT3476 channel 1 plus a LT3003
circuit in buck mode. The stringto-string current matching is 5%,
important to maintaining uniform LED
brightness between the strings. Figure 7 shows a LT3477 and a LT3003
circuit in boost mode. The VMAX of the
Linear Technology Magazine • December 2006
Figure 8. Recommended parts
placement and layout
LT3003 should be tied to the highest
voltage in a circuit. In the buck mode,
it is PVIN. In the boost mode, it is the
cathode of D1.
Layout Considerations
For proper operation and minimum
EMI, care must be taken during
the PCB layout. Figure 8 shows the
recommended components placement
for LT3476 in buck mode for a 4-layer
board. The schematic is shown in
Figure 1. In a buck circuit, the loop
formed by the input capacitors (C2
and C3), the SW pins and the catch
diodes (D1, D2, D3 and D4) should
be as small as possible because of the
present of high di/dt pulsing current
in this loop. The second layer should
be an unbroken ground plane. The SW
nodes should be as small as possible.
From each sense resistor, the traces
to 600mA output. Programmable softstart and switching frequency, as well
as external compensation, make the
LTC3530 a flexible and compact solution. The buck-boost topology helps a
designer extend battery runtime while
the automatic Burst Mode operation
further maximizes the runtime in
applications with widely varying load
requirements. L
for
the latest information
on LTC products,
visit
www.linear.com
to the CAP pin and to the LED pin
should be a Kelvin trace pair. Those
traces should be in the third layer for
best shielding. The fourth layer should
be another ground plane.
If long wires are used to connect a
power supply to PVIN of the LT3476,
an aluminum-electrolytic capacitor
should be used to reduce input
ringing which could break down the
LT3476 internal switch. See Linear
Technology Application Note 88 for
more information.
To ensure reliable operation, good
thermal designs for both the LT3476
and the LT3003 are essential. The
exposed pads on the bottom of the
packages must be evenly soldered
to the ground plane on the PCB
so that the exposed pads act as
heat sinks. Unevenly soldered IC
package degrades the heat sinking
capability dramatically. To keep the
thermal resistance low, the ground
plane should be extended as much
as possible. For the LT3476, on the
top layer, ground can be extended
out from the pins 19, 20, 21, 30, 31
and 32. This also allows tight loop
components placement mentioned
above. The second and fourth layers
should be reserved for the ground
plane. Thermal vias under and near
the IC package helps transfer the heat
from the IC to the ground plane and
from inner layers to outer layers. L
33
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