LINEAR TECHNOLOGY LT3474 Technical data
Wide Input Range 1A LED Driver Powers High Brightness LEDs
with Automotive and 12VAC Supplies – Design Note 388
John Tilly and Awo Ashiabor
Introduction
Today’s ultrabright LEDs far exceed the performance
of incandescent bulbs in both effi ciency and lifetime.
Taking full advantage of these features requires a correspondingly ef fi cient and reliable LED driver, such as the
®
3474. The LT3474 is a step-down 1A LED driver that
LT
supports a variety of power sources, has a wide 4V to
36V input voltage range and is programmable to deliver
LED current from 35mA to 1A at up to 88% effi ciency. It
requires minimal external circuitry and is available in a
space saving 16-lead TSSOP package.
Automotive LED Driver
Figure 1 shows the confi guration of the LT3474 operating
from a 12V automotive bat tery input. As shown, t he circuit
can tolerate voltage swings from 4V to 36V, common
in an automotive environment. With an integrated NPN
switch, boost diode and sense resistor, the LT3474 cuts
the external component count to a minimum. The high
side sense allows a grounded cathode connection, easing
wiring constraints. Both PWM and analog dimming are
available with minor circuit modifi cation; see the LT3474
data sheet for details.
V
IN
4V TO 36V
2.2µF
80.6k
0.1µF
V
IN
SHDN
R
T
REF
V
ADJ
V
C
BOOST
SW
LT3474
BIAS
OUT
LED
GND
Figure 1. 4V-36V Input Voltage 1A LED Driver Requires
Few Components
0.22µF
10µH
2.2µF
LED1
DN388 F01
Driving LEDs from 12VAC Input
The LT3474 directly regulates LED current, maintaining constant LED current over changing V
. The wide
IN
input range of the LT3474 allows direct connection to
a rectifi ed 12V
input. Using a small input capacitor,
AC
as shown in Figure 2, minimizes size. In this case, the
LT3474 delivers nearly 1A of LED current as shown in
F igu re 3 . A d di n g m or e c ap a ci ta nc e to t he in pu t, as s ho w n
in Figure 4, holds the input voltage above the LED voltage. In this case, the LT3474 can deliver a constant LED
current even with signifi cant 120Hz ripple on the input
as shown in Figure 5.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
~
12V
AC
~
10µF
63.4k
0.22µF
V
IN
SHDN
R
T
REF
V
ADJ
V
C
BOOST
SW
LT3474
BIAS
OUT
LED
GND
Figure 2. Using a Diode Bridge Allows the LT3474 to Drive
an LED from a 12VAC Input
V
IN
10V/DIV
0
I
LED
500mA/DIV
0
2ms/DIV
Figure 3. Using a 10µF Input Capacitance, the LT3474
Delivers Nearly 1A of LED Current with Smallest Board Size
0.22µF
6.8µH
1µF
LED1
DN388 F02
DN388 F03
04/06/388
Thermal regulation
The issue of heat management is at the core of many LED
applications. A reli able solution maintains the longevit y of
the LED by keeping the LED junction temperature below
the recommended limit. One answer to this problem is
to mount massive heat sinks, wasting space and money.
Figure 6 shows a better solution. The temperature of
the LED is sensed by the thermistor mounted near the
LED and is translated into a voltage signal to the V
pin. The V
pin reduces the current through the LED
ADJ
ADJ
appropriately to meet the power derating specifi ed by the
Luxeon III Star manufacturer. Only slight modifi cations
to the resistor values are required to adjust the circuit
for use with other high brightness LEDs.
~
12V
AC
~
220µF
53.6k
10nF
V
IN
SHDN
R
T
REF
V
ADJ
V
C
BOOST
SW
LT3474
BIAS
OUT
LED
GND
Figure 4. With a 220µF Input Capacitor, the LT3474
Supplies a Constant 1A Current to the LED
0.22µF
6.8µH
1µF
LED1
DN388 F04
Conclusion
High power white LEDs are fast becoming the lighting of
choice in architectural, automotive, museum and avionic
systems due to their effi ciency, high quality light and long
lifetimes. The LT3474 makes it easy to create compact,
effi cient, robust and versatile LED drivers from a variety
of power supplies. Designers can now focus their time
on creating imaginative new LED applications, instead
of on LED drivers.
5.62k
BC847B
2.26k
REF
LT3474
V
ADJ
GND
DN388 F06
MANUFACTURERS
MAXIMUM
RECOMMENDED
CURRENT
22k
NTC
9.31k
14.7k
Figure 6. Compact, Economical Thermo-Regulating
Circuit. The NTC and NPN, Mounted Close to the LED,
Monitor the LED’s Temperature
1.2
1.0
0.8
0.6
MEASURED
LED CURRENT
V
IN
10V/DIV
0
I
LED
500mA/DIV
0
2ms/DIV
DN388 F05
Figure 5. With a 220µF Input Capacitor, the LT3474
Delivers Constant 1A LED Current with Changing Input
Voltage
Data Sheet Download
www.linear.com
Linear Technology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
●
(408) 432-1900
FAX: (408) 434-0507 ● www.linear.com
0.4
LED CURRENT (A)
0.2
0
25
45 65
35 55
CASE TEMPERATURE (°C)
85
75
95
DN388 F07
105
Figure 7. LED Current Safely Lies Within Specifi ed Limits
for the Luxeon III Star Power
For applications help,
call (408) 432-1900, Ext. 2759
dn388f LT/TP 0406 409K • PRINTED IN THE USA
© LINEAR TECHNOLOGY CORPORATION 2006
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