Dec 2005 Multi-Output Supply Drives White LEDs, Provides LCD or OLED Bias in a 3mm x 3mm DFN Package

DESIGN IDEAS
Multi-Output Supply Drives White
LEDs, Provides LCD or OLED Bias
in a 3mm × 3mm DFN Package
by Gurjit Thandi
Introduction
Many of today’s cell phones, PDAs
and digital still cameras contain a
high-resolution TFT-LCD display and
sometimes an additional secondary
OLED (Organic Light-Emitting Diode)
display. OLED displays are fast becoming the secondary display of choice
because they are brighter, thinner
and more responsive than equivalent
LCDs. The LT3466-1 is a dual switching regulator designed to meet the
power supply requirements of small
displays, including LCD-bias, white
LED backlight and OLED displays.
The LT3466-1 integrates a full
featured white LED driver and a boost
converter in a low profile 3mm × 3mm
DFN package. It provides space and
component savings with integrated
44V power switches and Schottky
diodes. The LED driver can be configured to drive up to 10 white LEDs
in series from a single Li-Ion battery.
The white LED driver features a low
200mV reference for programming
the LED current, thereby minimizing
the power loss in the current setting
resistor for better efficiency. The boost
converter can be used for generating
the main LCD bias voltages, or for
providing the OLED bias supply. The
boost converter achieves ±1.5% out-
3V TO 5V
L1
33µH
6 LEDs
COUT1
1µF
EFFICIENCY (%)
VIN
SW2
VOUT1
LT3466-1
VOUT2
FB2
SHUTDOWN
AND DIMMING
CONTROL 1
RT GND
63.4k
CIN
1µF
COUT1
1µF
VIN
5
10
15
20
25
30
OUTPUT CURRENT (mA)
Figure 2. Efficiency versus load
current for the circuit in Figure 1
Linear Technology Magazine • December 2005
–15V
10mA
COUT3
1µF
15V
10mA
VOUT2
LT3466-1
OFF ON
D1
SW2
VOUT1
CTRL1 RT
RFB1
13.3Ω
C1
0.1µF
L2
33µH
L1
33µH
70
0
a 200kHz to 2MHz range. Additional
features include output overvoltage
protection, internal compensation and
internal soft-start. The LT3466-1 operates from a wide input voltage range
of 2.7V to 24V, making it suitable for
a variety of applications.
FB1
VIN = 3.6V
6 LEDs
VOUT2 = 16V
R2
24.9k
3V TO 5V
BOOST CONVERTER
55
R1
475k
CTRL2
SHUTDOWN
AND DIMMING
CONTROL 2
put voltage accuracy by the use of an
internal precision 0.8V reference.
The LT3466-1 also provides independent dimming and shutdown
control of the two converters. The
operating frequency of LT3466-1 can
be set with an external resistor over
LED DRIVER
60
16V
30mA
Figure 1. The LT3466-1 powers a main LCD backlight and a secondary OLED display. It provides
a 20mA drive for the six-white-LED LCD backlight and a 16V output for the OLED display.
SW1
65
COUT2
1µF
CIN: TAIYO YUDEN JMK107BJ105
COUT1, COUT2: TAIYO YUDEN GMK316BJ105
L1, L2: 33µH TOKO D52LC
75
50
SW1
CTRL1
RFB1
10Ω
8 LEDs
80
1µF
FB1
90
85
L2
33µH
475k
COUT2
1µF
FB2
CTRL2
63.4k
26.7k
OFF ON
CIN: TAIYO YUDEN JMK107BJ105
COUT1, COUT2, COUT3: TAIYO YUDEN GMK316BJ105
C1: TAIYO YUDEN UMK212BJ104
L1, L2: 33µH TOKO D52LC
D1: PHILIPS BAT54S
Figure 3. High efficiency, Li-Ion powered complete TFT-LCD supply (bias
and backlighting). The LT3466-1 drives eight white LEDs at 15mA to
provide the backlight and generates dual, ±15V, outputs for the LCD-bias.
39
DESIGN IDEAS
Dual Display Power Supply
for Cell Phones
A typical application for the LT3466-1
is as a driver for dual displays in cell
phones. Present day, clam-shell cell
phones typically use a color TFT-LCD
main display and a secondary OLED
display. Figure 1 shows the LT34661 powering the main LCD backlight
and the secondary OLED display. The
86
VIN = 3.6V
8 LEDs
+15V/10mA
–15V/10mA
EFFICIENCY (%)
84
Low Cost, Complete
LCD Bias and White LED
Backlighting Solution for
Small TFT Displays
82
80
Small, active-matrix, TFT-LCD displays, used in cell phones, PDAs
and other handheld devices generally require four to ten white LEDs
for providing the backlight and fixed
+15V and –15V supply voltages to bias
the LCD. Figure 3 shows LT3466-1
powered complete TFT-LCD supply
with minimal external components
78
76
74
0
2.5
LT3466-1 drives 6 white LEDs at 20mA
for backlighting the main LCD panel
and generates 16V output for powering the OLED. The LT3466-1 allows
for independent dimming control of
the main and secondary displays via
the respective CTRL1 and CTRL2 pins.
Figure 2 shows the efficiency versus
output current for both the LED driver
and the boost converter. The typical
efficiency at 3.6V input supply is 84%
with the white LEDs and the OLED
driven at 20mA.
5
7.5
10
LED CURRENT (mA)
12.5
15
Figure 4. Efficiency versus LED current for
the circuit in Figure 3. The circuit achieves
greater than 83% efficiency driving eight
LEDs at 15mA from 3.6V input.
and high efficiency. The LT3466-1
drives eight white LEDs at 15mA and
generates 15V boost output powered
from a single Li-Ion supply. A discrete
charge pump produces the secondary
output of –15V. As seen in Figure 4,
the circuit achieves greater than 83%
efficiency driving eight LEDs at 15mA
from 3.6V input.
Conclusion
The LT3466-1 integrates a full featured
white LED driver and a boost converter
in a space saving 3mm × 3mm DFN
package. Integrated power switches
and Schottky diodes reduce the overall system cost and size making it an
excellent fit for handheld applications.
Features like internal compensation,
soft-start, Open LED protection enables LT3466-1 to provide complete
TFT-LCD supply (bias and white LED
backlight) for handheld devices with
minimal external components and
high efficiency.
LT3477, continued from page 27
C1
3.3µF
ISP1
ISN1
SW
VIN
IADJ1
IADJ2
SHDN
SHDN
D1
R2
402k
LT3477
ISP2
75
RLOAD
R4
0.2Ω
ISN2
VREF
FBP
–5V
FBN
VC
4.75k
L2
10µH
EF(FICIENCY (%)
R1
0.05Ω
VIN
5V
85
C2
0.47µF
L1
10µH
C5
3.3µF
55
45
RT
GND
65
SS
0
100
200
300
400
LOAD CURRENT (mA)
500
Figure 11. Efficiency of the Cuk converter.
C3
22nF
R3
100k
100pF
C4
33nF
R5
18.2k
R6
10k
C1, C5: TAIYO YUDEN LMK316BJ335ML
D1: DIODES INC. DFL5120L
L1, L2: TOKO A915AY-100M (D53LC SERIES)
Figure 10. Negative output voltage Cuk converter.
using a Cuk topology for 5V to –5V
conversion. The first current sense
amplifier is used for input current
limit, and the second current sense
amplifier is used for ground rail current sense to accurately limit the load
current at 500mA. Even though the
two current sense amplifiers are used,
40
efficiency up to 81% at 500mA output
load can still be achieved. Figure 11
shows the efficiency.
Conclusion
The rail-to-rail constant-current/constant-voltage operation of the LT3477
makes the device an ideal choice for
a variety of constant-current designs,
including negative outputs. The dual
current-sense amplifiers allow flexible
configuration for input current limit,
constant output current and fail-safe
protection, along with excellent output
voltage regulation. A wide input voltage range and the ability to produce
outputs up to 42V make the LT3477
extremely versatile.
Authors can be contacted
at (408) 432-1900
Linear Technology Magazine • December 2005
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