LINEAR TECHNOLOGY LT3466 Technical data
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Dual White LED Driver Features Integrated Switches and Schottky
Diodes in 3mm × 3mm DFN Package
David Kim
Introduction
Linear Technology’s LT®3466 is a dual step-up converter
capable of driving up to 20 white LEDs with a programmable constant current. The device is also capable of
driving asymmetric LED strings with independent dimming and shutdown control for multipanel LCD backlight
applications.
The LT3466 uses a constant-current, step-up architecture
to directly regulate LED current, providing consistent light
intensity and color across all LEDs regardless of variations in their forward voltage drop. Important features
including soft-start, open LED protection, adjustable
switching frequency and DC dimming control are integrated into the part, making the LT3466 LED driver an
ideal solution for space-constrained portable devices
such as cellular phones, PDAs and digital cameras.
Li-Ion to a 2-LED and a 4-LED Display
Figure 1 shows a white LED driver to backlight two
displays, a 4-LED main and a 2-LED sub display. This
design generates a constant 20mA in each white LED
string from Li-Ion (3V ~ 4.2V) or 5V adapter input. Two
independent dimming and shutdown controls (via CTRL1
3V TO 5V
C
IN
1µF
L1
15µH
C
OUT1
1µF
R
FB1
10Ω
: TAIYO YUDEN JMK107BJ105
C
IN
: TAIYO YUDEN LMK212BJ105
C
OUT1
: TAIYO YUDEN EMK212BJ474
C
OUT2
L1, L2: MURATA LQH32CN150
Figure 1. Li-Ion to 2/4 White LEDs
SW1 SW2V
V
OUT1
LT3466
FB1
CTRL1 CTRL2
L2
15µH
IN
V
OUT2
FB2
R
T
38.3k
1%
C
OUT2
0.47µF
R
FB2
10Ω
DN1010 F01
and CTRL2) simplify power management, extending battery life. The adjustable operating frequency, integrated
Schottky diodes and optimized internal compensation
result in a circuit smaller than 0.18in
Efficiency, as shown in Figure 2, is above 80% at 20mA for
3.6V input, which is significantly higher than that of
switched-capacitor based solutions.
Figure 2. Efficiency vs LED Current, Circuit of Figure 1
Li-Ion Powered Driver for 20 White LEDs
Figure 3 highlights LT3466’s impressive input and output
voltage range. This circuit is capable of driving two sets of
ten, in-series LEDs with 12mA of constant current per
converter. As shown in Figure 3, the circuit works from
single Li-Ion (3V) battery or 5V wall adapter.
Inrush current during start-up is well controlled due to the
internal soft-start circuitry of LT3466. The circuit is also
protected with an internal 39V output clamp when the
LEDs are disconnected or fail open. Efficiency is above
80% over a wide LED current range of 5mA to 12mA,
reaching 83% at 12mA. The conversion efficiency with a
3.6V input is shown in Figure 4. To achieve the 92% duty
cycle requirement in this application, the operating frequency was set to 400kHz using a 147k resistor at R
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
– Design Note 1010
85
VIN = 3.6V
2/4 LEDs
80
75
70
65
EFFICIENCY (%)
60
55
50
510 20
0
LED CURRENT (mA)
2
.
15
DN1010 F02
pin.
T
03/05/1010
3V TO 5V
C
IN
1µF
L2
68µH
IN
V
OUT2
FB2
R
T
147k
1%
R
FB1
16.5Ω
L1
68µH
SW1 SW2V
V
C
OUT1
1µF
OUT1
FB1
CTRL1 CTRL2
: TAIYO YUDEN JMK107BJ105
C
IN
, C
C
OUT1
OUT2
L1, L2: TOKO A920CY-680M
LT3466
: TAIYO YUDEN UMK325BJ105
Figure 3. Li-Ion to 10/10 White LEDs
C
1µF
OUT2
R
FB2
16.5Ω
DN1010 F03
The LED currents are proportional to the voltages at the
CTRL1 and CTRL2 pins. DC voltage dimming is achieved
by reducing the voltage on the CTRL pin. As the CTRL
voltage decreases, the feedback voltage across the LED
current programming resistor decreases from 200mV to
0V, thus reducing the LED current from I
LED
to 0A.
Feedback voltage variation versus control voltage is shown
in Figure 5.
PWM dimming works similar to DC voltage dimming
except that the DC voltage input to the CTRL pins comes
from an RC-filtered PWM signal. The corner frequency of
R1 and C1 should be much lower than the frequency of the
PWM signal for proper filtering. The dimming control
using a filtered PWM signal is shown in Figure 6.
250
VIN = 3V
= 25°C
T
A
200
90
VIN = 3.6V
10/10 LEDs
85
80
75
EFFICIENCY (%)
70
65
60
0
48
LED CURRENT (mA)
DN1010 F04
12
Figure 4. Efficiency vs LED Current, Circuit of Figure 3
The circuit can be further optimized for efficiency and size
by programming the operating frequency over a 200kHz
to 2MHz range using a single external timing resistor.
Dimming Control
The LT3466 features single pin shutdown and dimming
control for each converter. The LED current in the two
drivers can be set independently by modulating the CTRL1
and CTRL2 pins. There are two different types of control
methods: DC voltage dimming and filtered PWM signal
dimming.
150
100
FEEDBACK VOLTAGE (mV)
50
0
0.5
0
Figure 5. Feedback Voltage vs V
PWM
10kHz TYP
1
CONTROL VOLTAGE (V)
R1
10k
C1
1µF
1.5
2
DN1010 F05
CTRL
LT3466
CTRL1,2
DN1010 F06
Figure 6. Filtered PWM Dimming
Conclusion
The LT3466 is a dual channel white LED driver capable of
driving up to 20 white LEDs from a single cell Li-Ion input.
The device features 44V internal power switches, internal
Schottky diodes, adjustable switching frequency, DC dimming control, automatic soft-start, open LED protection
and optimized internal compensation. The LT3466 is an
ideal solution for multipanel LCD backlight applications or
space constrained portable applications such as cellular
phones, PDAs and digital cameras.
Data Sheet Download
http://www.linear.com
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com
For applications help,
call (408) 432-1900, Ext. 2759
dn1010f 0305 • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2005
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