LT3474/LT3474-1
Step-Down
1A LED Driver
FEATURES
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DESCRIPTION
The LT
®
3474/LT3474-1 are fixed frequency step-down
DC/DC converters designed to operate as constant-current
sources. An internal sense resistor monitors the output
current allowing accurate current regulation, ideal for
driving high current LEDs. High output current accuracy
is maintained over a wide current range, from 35mA to
1A, allowing a wide dimming range.
Unique PWM circuitry allows a dimming range of 400:1,
avoiding the color shift normally associated with LED
current dimming.
The high switching frequency offers several advantages,
permitting the use of small inductors and ceramic capaci-
tors. Small inductors combined with the 16-lead TSSOP
surface mount package save space and cost versus
alternative solutions. The constant switching frequency
combined with low-impedance ceramic capacitors result
in low, predictable output ripple.
With their wide input range of 4V to 36V, the LT3474/
LT3474-1 regulate a broad array of power sources, from
5V logic rails to unregulated wall transformers, lead acid
batteries and distributed power supplies. A current mode
PWM architecture provides fast transient response and
cycle-by-cycle current limiting. Frequency foldback and
thermal shutdown provide additional protection.
True Color PWM™ Delivers Constant Color with
400:1 Dimming Range
Wide Input Range: 4V to 36V
Up to 1A LED Current
Adjustable 200kHz–2MHz Switching Frequency
Adjustable Control of LED Current
Integrated Boost Diode
High Output Current Accuracy is Maintained
Over a Wide Range from 35mA to 1A
Open LED (LT3474) and Short-Circuit Protection
High Side Sense Allows Grounded
Cathode Connection
Uses Small Inductors and Ceramic Capacitors
LT3474-1 Drives LED Strings Up to 26V
Compact 16-Lead TSSOP Thermally Enhanced
Surface Mount Package
APPLICATIONS
■
■
■
■
Automotive and Avionic Lighting
Architectural Detail Lighting
Display Backlighting
Constant Current Sources
L,
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. Patent Pending
TYPICAL APPLICATION
Step-Down 1A LED Driver
V
IN
5V TO 36V
2.2μF
V
IN
SHDN
LT3474
R
T
REF
80.6k
0.1μF
V
ADJ
V
C
GND
LED1
55
*SEE APPLICATIONS SECTION FOR DETAILS
3474 TA01a
Efficiency
95
0.22μF
EFFICIENCY (%)
10μH
90
85
80
ONE WHITE 1A LED
75
70
65
60
0
200
800
600
LED CURRENT (mA)
400
1000
3474 G02
V
IN
= 12V
TWO SERIES CONNECTED
WHITE 1A LEDS
BOOST
SW
BIAS
OUT
PWM
LED
DIMMING*
CONTROL
2.2μF
3474fd
1
LT3474/LT3474-1
ABSOLUTE MAXIMUM RATINGS
(Note 1)
PIN CONFIGURATION
TOP VIEW
DNC*
OUT
LED
V
IN
SW
BOOST
BIAS
GND
1
2
3
4
5
6
7
8
17
16 DNC*
15 GND
14 PWM
13 V
ADJ
12 V
C
11 REF
10 SHDN
9
R
T
V
IN
Pin ........................................................(–0.3V), 36V
BIAS Pin....................................................................25V
BOOST Pin Voltage ...................................................51V
BOOST above SW Pin ...............................................25V
OUT, LED Pins (LT3474) ............................................15V
OUT, LED Pins (LT3474-1).........................................26V
PWM Pin ...................................................................10V
V
ADJ
Pin .....................................................................6V
,
V
C
, REF R
T
Pins ..........................................................3V
SHDN Pin ...................................................................V
IN
BIAS Pin Current .........................................................1A
Maximum Junction Temperature (Note 2)............. 125°C
Operating Temperature Range (Note 3)
LT3474E, LT3474E-1 ............................ –40°C to 85°C
LT3474I, LT3474I-1 ............................ –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 sec) .................. 300°C
FE PACKAGE
16-LEAD PLASTIC TSSOP
θ
JC
= 8°C/W,
θ
JA
= 40°C/W
EXPOSED PAD (PIN 17) IS GND, MUST BE SOLDERED TO PCB
*DO NOT CONNECT EXTERNAL CIRCUITRY TO THESE PINS.
ORDER INFORMATION
LEAD FREE FINISH
LT3474EFE#PBF
LT3474IFE#PBF
LT3474EFE-1#PBF
LT3474IFE-1#PBF
TAPE AND REEL
LT3474EFE#TRPBF
LT3474IFE#TRPBF
LT3474EFE-1#TRPBF
LT3474IFE-1#TRPBF
PART MARKING
3474EFE
3474IFE
3474EFE-1
3474IFE-1
PACKAGE DESCRIPTION
16-Lead TSSOP
16-Lead TSSOP
16-Lead TSSOP
16-Lead TSSOP
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 125°C
–40°C to 85°C
–40°C to 125°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
Consult LTC Marketing for information on non-standard lead based finish parts.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
Input Quiescent Current
Shutdown Current
LED Pin Current
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V, V
BOOST
= 16V, V
OUT
= 4V unless otherwise noted (Note 3).
CONDITIONS
l
MIN
TYP
3.5
2.6
0.01
MAX
4
4
2
1.02
1.025
0.207
0.210
1.265
UNITS
V
mA
μA
A
A
A
A
V
Not Switching
SHDN = 0.3V, V
BOOST
= 0V, V
OUT
= 0V
V
ADJ
Tied to V
REF
V
ADJ
Tied to V
REF
/5
l
l
l
0.98
0.968
0.193
0.186
1.23
1
0.2
1.25
REF Voltage
3474fd
2
LT3474/LT3474-1
ELECTRICAL CHARACTERISTICS
PARAMETER
Reference Voltage Line Regulation
Reference Voltage Load Regulation
V
ADJ
Pin Bias Current (Note 4)
Switching Frequency
Maximum Duty Cycle
R
T
= 80.6k
R
T
= 80.6k
R
T
= 10k
R
T
= 232k
R
T
= 80.6k, V
OUT
= 0V
2.6
V
SHDN
= SHDN Threshold
8.3
0.4
V
C
= 1V
V
C
= 1V
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 12V, V
BOOST
= 16V, V
OUT
= 4V unless otherwise noted (Note 3).
CONDITIONS
5V < V
IN
< 36V
0 < I
REF
< 250μA
l
l
l
MIN
TYP
0.01
0.0002
20
MAX
UNITS
%/V
%/μA
400
530
540
nA
kHz
kHz
%
%
%
kHz
470
450
90
500
95
76
98
70
2.65
10.3
0.9
0.8
100
100
1.5
1
2
1.9
Foldback Frequency
SHDN Threshold (to Switch)
SHDN Pin Current (Note 5)
PWM Threshold
V
C
Switching Threshold
V
C
Source Current
V
C
Sink Current
LED to V
C
Current Gain
LED to V
C
Transresistance
V
C
to Switch Current Gain
V
C
Clamp Voltage
V
C
Pin Current in PWM Mode
OUT Pin Clamp Voltage (LT3474)
OUT Pin Current in PWM Mode
Switch Current Limit (Note 6)
Switch V
CESAT
Boost Pin Current
Switch Leakage Current
Minimum Boost Voltage (Note 7)
Boost Diode Forward Voltage
2.7
12.3
1.2
V
μA
V
V
μA
μA
μA/mA
V/mA
A/V
V
V
C
= 1V, V
PWM
= 0.3V
V
OUT
= 4V, V
PWM
= 0.3V
–40°C to 85°C
LT3474I, LT3474I-1 at 125°C
I
SW
= 1A
I
SW
= 1A
l
0.01
13.2
13.8
0.1
1.6
1.5
2.1
380
30
0.01
1.9
1
14.5
10
3.2
3.2
500
50
1
2.5
μA
V
μA
A
A
mV
mA
μA
V
mV
l
l
I
DIO
= 100mA
600
Note 1:
Stresses beyond those listed under Absolute Maximum Ratings
may cause permanent damage to the device. Exposure to any Absolute
Maximum Rating condition for extended periods may affect device
reliability and lifetime.
Note 2:
This IC includes overtemperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when overtemperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 3:
The LT3474E and LT3474E-1 are guaranteed to meet performance
specifications from 0°C to 70°C. Specifications over the –40°C to 85°C
operating temperature range are assured by design, characterization and
correlation with statistical process controls. The LT3474I and LT3474I-1
are guaranteed to meet performance specifications over the –40°C to
125°C operating temperature range.
Note 4:
Current flows out of pin.
Note 5:
Current flows into pin.
Note 6:
Current limit is guaranteed by design and/or correlation to static
test. Slope compensation reduces current limit at higher duty cycles.
Note 7:
This is the minimum voltage across the boost capacitor needed to
guarantee full saturation of the internal power switch.
3474fd
3
LT3474/LT3474-1
TYPICAL PERFORMANCE CHARACTERISTICS
LED Current vs V
ADJ
1000
T
A
= 25°C
800
1000
1200
V
ADJ
= V
REF
LED Current vs Temperature
700
600
Switch Voltage Drop
T
A
= 25°C
SWITCH VOLTAGE DROP (mV)
100
125
LED CURRENT (mA)
LED CURRENT (mA)
800
600
400
200
0
–50 –25
V
ADJ
= V
REF
/5
500
400
300
200
100
0
0
600
400
200
0
0
0.25
0.75
0.5
V
ADJ
(V)
1
1.25
3474 GO3
50
25
75
0
TEMPERATURE (°C)
1000
500
SWITCH CURRENT (mA)
1500
3474 G05
3474 G04
Current Limit vs Duty Cycle
2.5
Switch Current Limit vs
Temperature
2.5
2.5
Current Limit vs Output Voltage
T
A
= 25°C
2
2
CURRENT LIMIT (A)
2
MINIMUM (85°C)
CURRENT LIMIT (A)
1.5
1.5
CURRENT LIMIT (A)
TYPICAL
1.5
MINIMUM (125°C)
1
1
1
0.5
0.5
0.5
0
0
20
60
40
DUTY CYCLE (%)
80
100
3474 G06
0
–50
0
–25
50
25
0
75
TEMPERATURE (°C)
100
125
0
2
4
6
V
OUT
(V)
8
10
12
3474 G08
3474 G07
Oscillator Frequency vs R
T
600
T
A
= 25°C
Oscillator Frequency vs
Temperature
R
T
= 80.6k
Oscillator Frequency Foldback
600
T
A
= 25°C
R
T
= 80.6k
OSCILLATOR FREQUENCY (kHz)
OSCILLATOR FREQUENCY (kHz)
550
1000
OSCILLATOR FREQUENCY (kHz)
–25
75
0
25
50
TEMPERATURE (°C)
100
125
500
400
300
200
100
0
0
500
450
100
10
R
T
(kΩ)
3474 G09
100
400
–50
0.5
1
1.5
V
OUT
(V)
2
2.5
3474 G11
3474 G10
3474fd
4
LT3474/LT3474-1
TYPICAL PERFORMANCE CHARACTERISTICS
Boost Pin Current
60
T
A
= 25°C
50
BOOST PIN CURRENT (mA)
INPUT CURRENT (mA)
Quiescent Current
3.0
T
A
= 25°C
2.5
2.0
1.5
1.0
0.5
0
Reference Voltage
1.260
1.255
40
30
20
10
0
V
REF
(V)
1.250
1.245
1.240
0
250
500
750 1000 1250
SWITCH CURRENT (mA)
1500
0
6
12
18
V
IN
(V)
24
30
36
3474 G13
1.235
–50
–25
50
25
0
75
TEMPERATURE (°C)
100
125
3473 G12
3474 G14
Schottky Reverse Leakage
20
500
Schottky Forward Voltage Drop
T
A
= 25°C
Open-Circuit Output Voltage and
Input Current
60
T
A
= 25°C
50
OUTPUT VOLTAGE (V)
V
R
= 5V
8
INPUT CURRENT
LT3474-1
7
INPUT CURRENT (mA)
REVERSE CURRENT (μA)
15
FORWARD CURRENT (mA)
400
6
5
40
LT3474
30
20
10
0
LT3474-1
LT3474
OUTPUT VOLTAGE
300
10
4
3
2
1
0
0
10
20
V
IN
(V)
30
40
3474 G16
200
5
100
0
–50
–25
75
0
25
50
TEMPERATURE (°C)
100
125
0
0
200
600
800
400
FORWARD VOLTAGE (mV)
1000
3474 G19
3474 G15
Minimum Input Voltage,
One White Luxeon III Star
6
5
4
TO RUN
V
IN
(V)
T
A
= 25°C
TO START
10
Minimum Input Voltage,
Two Series Connected White
Luxeon III Stars
T
A
= 25°C
9
V
IN
(V)
LED VOLTAGE
3
2
1
0
0
200
400
600
800
LED CURRENT (mA)
1000
3474 G17
8
TO START
7 TO RUN
LED VOLTAGE
6
5
0
200
600
800
400
LED CURRENT (mA)
1000
3474 G18
3474fd
5
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