DN392 - High Voltage Buck Converters Drive High Power LEDs

High Voltage Buck Converters Drive High Power LEDs
Design Note 392
Keith Szolusha
Introduction
High power LEDs continue to replace traditional bulbs
in new automotive, industrial, backlight display and
architectural detail lighting systems. LEDs excel in a wide
range of performance and cost parameters, including
excellent spectral performance, long life, robustness,
falling manufacturing cost and relatively safe materials.
Linear Technology offers a large and growing family of
high voltage DC/DC converters tailored specifically to
drive high-powered LEDs.
The LT® 3474 and LT3475, for example, are high voltage,
high current, single- and dual-channel buck LED converters with wide PWM dimming ratios that can drive one
or more LEDs up to 1A and 1.5A for 80 lumens to 120
lumens per LED (or more as higher output LEDs become
available). These dedicated LED drivers have onboard
high voltage NPN power switches and internal sense
resistors to minimize board space, reduce component
count and simplify design.
With their high side sense resistors, the LT3474 and
LT3475 can drive LEDs tied to ground, an important
advantage in many systems. Current-mode control and
a precise reference voltage optimize loop dynamics for
a well regulated, low ripple constant LED current. Thermally enhanced exposed pad packages keep the junction
temperature low during high power operation in stressful
environments. A PWM pin uses the dimming MOSFET
gate signal to extend the dimming ratio of the converter
by maintaining constant output capacitor voltage and
control loop state during PWM dimming off-time. Shutdown and external analog current adjust pins provide
simple interface for further LED light and current control
flexibility in any system.
Single Buck 1A LED Driver
The LT3474 buck converter 1A LED driver shown in Figure 1 has features that suit it to automotive applications
(and other battery-powered applications) or to industrial
applications with limited board space, high voltage and
high ambient temperature. This scheme uses a high side
integrated 100mΩ sense resistor for true LED current
sensing and regulation, superior to the common and less
efficient method of biasing LEDs with a constant voltage
and a power wasting bias resistor.
The 4V to 36V input voltage range makes it ideal to use with
little-to-no input transient protection circuitry in automotive, industrial and avionic applications where long cables
from the battery result in very high input spikes.
The boosted NPN power switch results in high efficiency
for both 1- and 2-LED applications (Figure 2). The boost
diode is integrated to further reduce component count.
Driving the shutdown pin to ground turns off the LEDs
and reduces the input current to less than 2µA for battery longevity.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
True Color PWM is a trademark of Linear Technology Corporation. All other
trademarks are the property of their respective owners.
95
VIN = 12V
90
BOOST
VIN
2.2µF
0.22µF
85
10µH
EFFICIENCY (%)
VIN
5V TO 36V
SW
SHDN
LT3474
RT
BIAS
80.6k
10k
REF
OUT
VADJ
PWM
0.1µF
3.3nF
DIMMING
CONTROL
2.2µF
LED
VC
GND
TWO SERIES CONNECTED
WHITE LUXEON III STARS
80
ONE WHITE LUXEON III STAR
75
70
65
60
LED1
55
DN392 F01
0
200
400
800
600
LED CURRENT (mA)
1000
DN392 F02
Figure 1. LT3474 High Voltage Buck
LED Driver Regulates 1A
06/06/392
Figure 2. LT3474 Buck Drives Single or
Multiple LEDs with High Efficiency
LED brightness is controlled by either the 400:1 True
Color PWMTM dimming with an external MOSFET driver
or with an analog 25:1 (or filtered PWM) signal on the
VADJ pin. Applications can be optimized for highest
efficiency or smallest component size via an external
resistor that programs the switching frequency from
200kHz to 2MHz.
The maximum output voltage of the LT3474 is clamped at
13.8V which protects the LT3474 output from LED open
circuit. Short-circuit protection is the final detail that
makes the LT3474 a bulletproof converter in the case of
all types of LED failures.
Dual Buck 1.5A LED Driver
Figure 3 shows a dual-channel 1.5A buck converter LED
driver using the LT3475 which is essentially two LT3474
converters combined in a single IC with a few additional
features. This simple solution is ideal for automotive
applications where two overhead or dashboard lights are
needed in the same system. Both light channels (each
a single or a string of LEDs) have separate VADJ voltages and PWM signals for independent operation, but
a single shutdown pin further improves battery-saving
micropower operation by reducing the total battery drain
of the circuit to 2µA in shutdown.
Each of the dual outputs can be driven as high as 1.5A for
more powerful LEDs or LEDs that require higher current
and less forward voltage such as red and amber brake
VIN
5V TO 36V
CIN
4.7µF
50V
1N4448W
VIN
SHDN
1N4448W
BOOST1 BOOST2
L1 0.22µF
10µH
0.22µF
LT3475
SW1
B240A
COUT1
2.2µF
6.3V
800:1
DIMMING
CONTROL
LED 1
1.5A
B240A
OUT1
OUT2
LED1
LED2
PWM1
PWM2
VC1
LED 2
1.5A
10k
3.3nF
3.3nF
REF
RT
VADJ1 VADJ2 GND
10k
0.1µF
COUT2
2.2µF
6.3V
Compared to the LT3474, the LT3475 offers three times
the power capability, the same shutdown current, the
same switching frequency range, a slightly higher
input voltage (36V operating, 40V maximum), a higher
dimming ratio, a higher LED current and only a slightly
bigger package (20-pin versus 16-pin exposed thermal
pad TSSOP)—making it a great choice for higher power
solutions. In addition, the anti-phase switching of the
two channels in the LT3475 reduces the input ripple seen
by the source and limits the need for extra high voltage
input capacitors.
Conclusion
The LT3474 and LT3475 are excellent choices for high
voltage, high current, buck LED drivers in automotive,
industrial, backlight display and architectural display
lighting systems. The heavily integrated ICs reduce
component count and board space, while still providing
flexible features such as adjustable LED current, PWM dimming and adjustable operating frequency. Accurate LED
current regulation makes these ICs superior to other
DC/DC voltage regulators or LED drivers. Efficiency as
high as 88% combined with less than 2µA of shutdown
current save battery power and extend lifetime.
1MΩ
VC2
10k
Q1
L2
10µH
SW2
and signal LEDs. Although the maximum output voltage
is clamped at the same level as the LT3474 at 13.8V,
the maximum power output capability of the LT3475
is three times higher. The PWM dimming ratio is also
greater—1200:1 or higher with the extended dimming
ratio circuit in Figure 4. Improvements in PWM dimming techniques with lower minimum dimming on-time
requirements help this IC achieve extreme automotive
and nighttime dimming levels while maintaining the
same true color as 100% duty cycle. Independent analog
VADJ dimming ratio is 30:1 (50mA LED current) for each
channel. To reduce internal power dissipation, the boost
diode for each channel is left out of the IC.
Q2
800:1
DIMMING
CONTROL
24.3k
10k
0.1µF
L1, L2: SUMIDA CDRH8D28-100
LED1, LED2: LUMILEDS LXK2 WHITE
Q1, Q2: Si2312BDS
Figure 3. LT3475 Dual High Voltage
Buck LED Driver Regulates 1.5A
Data Sheet Download
www.linear.com
RT
220pF
PWM1
2N7002L
DN392 F03
DN392 F04
Figure 4. LT3475 Extended Dimming Range Circuit
Provides 1200:1 PWM Dimming Ratio When Added
to Figure 3, and Up to 3000:1 at 1.4MHz
For applications help,
call (408) 432-1900, Ext. 2759
Linear Technology Corporation
dn392 LT 0606 409K • PRINTED IN THE USA
FAX: (408) 434-0507 ● www.linear.com
© LINEAR TECHNOLOGY CORPORATION 2006
1630 McCarthy Blvd., Milpitas, CA 95035-7417
(408) 432-1900
●
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