DN1010 - Dual White LED Driver Features Integrated Switches and Schottky Diodes in 3mm × 3mm DFN Package

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Dual White LED Driver Features Integrated Switches and Schottky
Diodes in 3mm × 3mm DFN Package – Design Note 1010
David Kim
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
CIN
1µF
L2
15µH
L1
15µH
SW1
COUT1
1µF
VIN
LT3466
FB1
RFB1
10Ω
SW2
VOUT2
VOUT1
CTRL1
COUT2
0.47µF
FB2
RT
CTRL2
38.3k
1%
CIN: TAIYO YUDEN JMK107BJ105
COUT1: TAIYO YUDEN LMK212BJ105
COUT2: TAIYO YUDEN EMK212BJ474
L1, L2: MURATA LQH32CN150
Figure 1. Li-Ion to 2/4 White LEDs
03/05/1010
RFB2
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 0.18in2.
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.
85
VIN = 3.6V
2/4 LEDs
80
75
EFFICIENCY (%)
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.
70
65
60
55
50
0
5
10
15
20
LED CURRENT (mA)
DN1010 F02
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 RT pin.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
3V TO 5V
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 ILED to 0A.
Feedback voltage variation versus control voltage is shown
in Figure 5.
CIN
1µF
L2
68µH
SW1
VIN
SW2
VOUT2
VOUT1
COUT1
1µF
LT3466
FB1
COUT2
1µF
FB2
CTRL1
RT
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.
CTRL2
147k
1%
RFB1
16.5Ω
CIN: TAIYO YUDEN JMK107BJ105
COUT1, COUT2: TAIYO YUDEN UMK325BJ105
L1, L2: TOKO A920CY-680M
RFB2
16.5Ω
Figure 3. Li-Ion to 10/10 White LEDs
90
VIN = 3.6V
10/10 LEDs
85
EFFICIENCY (%)
250
VIN = 3V
TA = 25°C
DN1010 F03
80
FEEDBACK VOLTAGE (mV)
L1
68µH
200
150
100
50
75
0
70
0
1
0.5
1.5
CONTROL VOLTAGE (V)
2
DN1010 F05
65
Figure 5. Feedback Voltage vs VCTRL
60
4
0
8
12
LED CURRENT (mA)
DN1010 F04
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.
PWM
10kHz TYP
LT3466
R1
10k
CTRL1,2
C1
1µF
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
For applications help,
call (408) 432-1900, Ext. 2759
dn1010f 0305 • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2005
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