DN376 - Monolithic Converter Drives High Power LEDs

Monolithic Converter Drives High Power LEDs – Design Note 376
Keith Szolusha
Introduction
High power LEDs are quickly expanding their reach as
the light source of choice for flat panel computer and
TV monitors, TV projection, signage, portable lights
and automotive interior, trim and brake lighting. The
input voltage and LED voltage combinations across
these applications are as diverse as the applications
themselves, precluding the ability for a single topology
to satisfy the needs of them all. Nevertheless, all LED
drivers, whether buck, boost, buck-boost or SEPIC must
regulate a constant LED current, regardless of input and
output voltages. Now a single switching regulator with
the ability to be configured in a large variety of topologies for high power constant LED current is available.
Efficiencies of up to 91% can be achieved in typical
applications. The LT3477 features a programmable
soft-start function to limit inductor current during
start-up. Both inputs of the error amplifier are available externally, allowing positive and negative output
voltages. The switching frequency is programmable
from 200kHz to 3.5MHz through an external resistor.
It comes in two thermally enhanced packages: a 20-pin
(4mm × 4mm) QFN and a 20-pin TSSOP.
Boost Driver
The LT3477’s internal ground-referenced 3A NPN power
switch is most commonly used for boost applications.
Figure 1 shows a 5V to four 1W LED boost converter with
open LED protection and 330mA constant LED current.
The constant current is regulated using a current sense
amplifier and a 0.3Ω sense resistor. The feedback voltage amplifier is only used for overvoltage protection in
case the LEDs are removed from the circuit.
The LT®3477 can drive high power LEDs at constant
current in any of the topologies stated above. It is a
current mode, 3A DC/DC step-up converter that incorporates dual rail-to-rail current sense amplifiers and an
internal 3A, 42V switch. It combines a traditional voltage
feedback loop and two unique current feedback loops to
operate as a constant-current and/or constant-voltage
source. The floating rail-to-rail current sense amplifiers
allow for both ground-referenced and floating LED
solutions in different topologies, along with the added
benefit of inrush current or short-circuit protection.
The forward voltage of the four LEDs ranges from 12V to
16V. The input voltage range is 2.5V (minimum LT3477
input voltage) up to just below the LEDs’ forward voltage. The LT3477 can drive more LEDs as long as the
peak switch current remains below 3A.
L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks
of Linear Technology Corporation. All other trademarks are the property of their
respective owners.
Both current sense voltages are 100mV and can be
adjusted independently using the IADJ1 and IADJ2 pins.
VIN
5V
L1
10μH
C1
3.3μF
ISP1
ISN1
D1
VIN
IADJ1
IADJ2
SHDN
SHDN
C2
3.3μF
R1
200k
SW
FBN
R2
10k
LT3477
ISP2
R5
0.3Ω
VC
ISN2
VREF
R4
1k
C1: TAIYO-YUDEN LMK316BJ335ML
C2: TAIYO-YUDEN TMK325BJ335MN
D1: DIODES INC. DFLS120L
L1: TOKO A915AY-100M
R4
4.7nF
FBP
RT
GND
SS
330mA
C3
33nF
R3
22k
DN376 F01
Figure 1. 330mA Boost LED Driver with Open LED Protection
11/05/376_conv
PVIN
32V
C1
2.2μF
R1
0.1Ω
D2
t
t
t
1A
D5
LED
STRING C2
1μF
D3
L1
33μH
ISP1
VIN
3.3V
ISN1
D1
C3
3.3μF
SHDN
SHDN
FBN
R6
10k
LT3477
ISP2
VC
FBP
C5
4.7nF
SHDN
ISN1
LT3477
ISN2
VREF
RT
FBP
C4
33nF
ISP2
VC
SS
R3
22k
DN376 F02
C1: NIPPON UNITED CHEMICON NTS40X5R1H225M
C2: TAIYO YUDEN GMK316BJ105ML
C3: TAIYO YUDEN LMK316BJ475
L1: TOKO D1OFA814AY-330M
D1: DIODES INC DFLS140
R5
200k
SW1
FBN
SHDN
RT
GND
ISP1
R1
0.16Ω
D1
VIN
IADJ1
IADJ2
LED BRIGHTNESS
CONTROL
0mV TO 650mV
ISN2
VREF
R2
1k
C1
3.3μF
R5
309k
SW
VIN
IADJ1
IADJ2
L1
4.7μH
VIN
3V TO 16V
600mA
D2
GND
SS
C4
33nF
C3
10nF
R3
18k
C1: TAIYO YUDEN LMK316BJ335ML
C2: MURATA GRM31CR71E475KA88L
D1: DIODES, INC. B320A
L1: TOKO FDV0630-4R7M
R6
10k
C2
4.7μF
DN376 F03
Figure 2. 1A Buck LED Driver
Figure 3. Buck-Boost LED Driver
Buck Driver
LEDs are best driven with a constant current source,
but unlike most system loads, they do not have to be
ground referenced. Therefore, the LT3477 with its floating current sense amplifiers can be converted into a buck
LED driver with low LED ripple current. Figure 2 shows
a 32V input voltage buck converter driving a string of
1A LEDs. The LEDs are tied to the input source through
a sense resistor and the typical output Schottky catch
diode is tied back to VIN, converting the boost IC to a
monolithic buck LED driver. Once again, the feedback
voltage amplifier is only used for overvoltage protection—this time on the input—to prevent damage to
the 42V switch. In buck mode, the VIN pin need only
be tied to a 3.3V or 5V source for maximum efficiency,
and to keep it below its absolute maximum rating of
25V. The dimming IADJ pin functions work the same in
buck mode as boost mode, reducing the LED current
proportionally.
Buck-Boost Driver
If the battery range lies both above and below the forward voltage range of the LEDs, the LT3477 can simply
be converted into a buck-boost converter as floating
current sense amplifiers allow the LED string and its
series sense resistor to be tied anywhere in the circuit.
By altering the typical boost LED driver application so
that the LED string returns to VIN as opposed to ground,
step-up/step-down capability is provided.
Data Sheet Download
www.linear.com
Linear Technology Corporation
Conclusion
The LT3477 is a versatile, monolithic boost, buck, and
buck-boost or SEPIC LED driver with a high power
3A, 42V switch. It can also be used for boost or SEPIC
voltage converters requiring inrush or short-circuit
protection. Two floating current sense amplifiers and a
ground-referenced voltage feedback amplifier help give
the LT3477 its high level of versatility. The externally
programmable switching frequency, a shutdown pin,
LED current dimming adjustment and a single soft-start
capacitor satisfy the additional requirements of LED
drivers and boost/SEPIC regulators.
For applications help,
call (408) 432-1900
dn376f_conv LT/TP 1105 409K • 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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