FEATURES
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LT3478/LT3478-1
4.5A Monolithic LED
Drivers with True Color
PWM Dimming
DESCRIPTIO
The LT
®
3478/LT3478-1 are 4.5A step-up DC/DC convert-
ers designed to drive LEDs with a constant current over
a wide programmable range. Series connection of the
LEDs provides identical LED currents for uniform bright-
ness without the need for ballast resistors and expensive
factory calibration.
The LT3478-1 reduces external component count and cost
by integrating the LED current sense resistor. The LT3478
uses an external sense resistor to extend the maximum
programmable LED current beyond 1A and also to achieve
greater accuracy when programming low LED currents.
Operating frequency can be set with an external resistor
from 200kHz up to 2.25MHz. Unique circuitry allows a
PWM dimming range up to 3000:1 while maintaining
constant LED color. The LT3478/LT3478-1 are ideal for
high power LED driver applications such as automotive TFT
LCD backlights, courtesy lighting and heads-up displays.
One of two CTRL pins can be used to program maximum
LED current. The other CTRL pin can be used to program
a reduction in maximum LED current vs temperature to
maximize LED usage and improve reliability.
Additional features include inrush current protection,
programmable open LED protection and programmable
soft-start. Each part is available in a 16-pin thermally
enhanced TSSOP Package.
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True Color PWM™ Dimming Delivers Constant LED
Color with Up to 3000:1 Range
Wide Input Voltage Range: 2.8V to 36V
4.5A, 60mΩ, 42V Internal Switch
Drives LEDs in Boost, Buck-Boost or Buck Modes
Integrated Resistors for Inductor and LED Current
Sensing
Program LED Current:
100mA to 1050mA (LT3478-1)
(10mV to 105mV)/R
SENSE
(LT3478)
Program LED Current De-Rating vs Temperature
Separate Inductor Supply Input
Inrush Current Protection
Programmable Soft-Start
Fixed Frequency Operation from 200kHz to 2.25MHz
Open LED Protection (Programmable OVP)
Accurate Shutdown/UVLO Threshold with
Programmable Hysteresis
16-Pin Thermally Enhanced TSSOP Package
APPLICATIO S
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High Power LED Driver
Automotive Lighting
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Patents Pending.
TYPICAL APPLICATIO
V
IN
8V TO 16V
4.7µF
V
IN
SHDN
V
REF
45.3k
CTRL2
OVPSET
54.9k
CTRL1
130k
PWM
1µF
PWM DIMMING
CONTROL
SS
V
S
Automotive TFT LCD Backlight
10µH
L
SW
OUT
0.1Ω
R
SENSE
(LT3478)
10µF
95
EFFICIENCY (%)
100
Efficiency vs V
IN
I
LED
= 700mA
f
OSC
= 500kHz
PWM DUTY CYCLE = 100%
LT3478-1
LED
90
85
700mA
15W
6 LEDs
(WHITE)
69.8k
V
C
0.1µF
R
T
80
8
10
3478 TA01
U
U
U
6 LEDs LUXEON III (WHITE)
12
V
IN
(V)
14
16
3478 TA01b
34781f
1
LT3478/LT3478-1
ABSOLUTE
(Note 1)
AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
TOP VIEW
SW
SW
V
IN
V
S
L
V
OUT
LED
OVPSET
1
2
3
4
5
6
7
8
17
16 SS
15 R
T
14 PWM
13 CTRL2
12 CTRL1
11 SHDN
10 V
REF
9
V
C
SW ............................................................................42V
V
OUT
, LED ..................................................................42V
V
IN
, V
S
, V
L
,
⎯
S
⎯
H
⎯
D
⎯
N (Note 5) .......................................36V
PWM .........................................................................15V
CTRL1, 2 .....................................................................6V
SS, R
T
, V
C
, V
REF
, OVPSET............................................2V
Operating Junction Temperature Range
(Notes 2, 3, 4).................................... –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
T
JMAX
= 125°C,
θ
JA
= 35°C/W
EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB.
ORDER PART NUMBER
LT3478EFE
LT3478EFE-1
LT3478IFE
LT3478IFE-1
FE PART MARKING
3478FE
3478FE-1
3478FE
3478FE-1
Order Options
Tape and Reel: Add #TR
Lead Free: Add #PBF Lead Free Tape and Reel: Add #TRPBF
Lead Free Part Marking:
http://www.linear.com/leadfree/
Consult LTC Marketing for parts specified with wider operating temperature ranges.
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. SW = open, V
IN
= V
S
= L = V
OUT
=
⎯
S
⎯
H
⎯
D
⎯
N = 2.7V, LED = open, SS = open,
PWM = CTRL1, CTRL2 = 1.25V, V
REF
= open, V
C
= open, R
T
= 31.6k.
PARAMETER
Minimum Operating Voltage
Operational Input Voltage
V
IN
Quiescent Current
V
IN
Shutdown Current
⎯
S
⎯
H
⎯
D
⎯
N Pin Threshold (V
SD_µp
)
⎯
S
⎯
H
⎯
D
⎯
N Pin Threshold (V
SD_UVLO
)
⎯
SH
⎯
D
⎯
N Pin Current
⎯
V
REF
Voltage
V
REF
Line Regulation
V
REF
Load Regulation
Frequency: f
OSC
200kHz
Frequency: f
OSC
1MHz
CONDITIONS
(Rising)
V
S
V
IN
(Note 5)
V
C
= 0V (No Switching)
⎯
S
⎯
H
⎯
D
⎯
N = 0V
(Micropower)
(Switching)
⎯
S
⎯
H
⎯
D
⎯
N = V
SD_UVLO
– 50mV
⎯
S
⎯
H
⎯
D
⎯
N = V
SD_UVLO
+ 50mV
I(V
REF
) = 0µA, V
C
= 0V
I(V
REF
) = 0µA, 2.7V < V
IN
< 36V
0 < I(V
REF
) < 100µA (Max)
R
T
= 200k
R
T
= 31.6k
●
●
●
●
ELECTRICAL CHARACTERISTICS
MIN
2.8
2.8
TYP
2.4
MAX
2.8
36
36
UNITS
V
V
V
mA
µA
V
V
µA
µA
V
%/V
mV
MHz
MHz
6.1
3
0.1
1.3
8
1.213
0.4
1.4
10
0
1.240
0.005
8
0.18
0.88
0.2
6
0.7
1.5
12
1.263
0.015
12
0.22
1.12
34781f
2
U
W
U
U
W W
W
LT3478/LT3478-1
The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. SW = open, V
IN
= V
S
= L = V
OUT
=
⎯
S
⎯
H
⎯
D
⎯
N = 2.7V, LED = open, SS = open,
PWM = CTRL1, CTRL2 = 1.25V, V
REF
= open, V
C
= open, R
T
= 31.6k.
PARAMETER
Frequency: f
OSC
2.25MHz
Line Regulation f
OSC
Nominal R
T
Pin Voltage
Maximum Duty Cycle
R
T
= 31.6k
R
T
= 200k
R
T
= 9.09k
(Note 6)
(Note 6)
CTRL1 = 0.4V, V
C
= 1V
CTRL1 = 0V, V
C
= 1V
CTRL1 = 0.4V
CTRL1 = 0V
2.7V < V
S
< 36V
I
SW
= 4.5A
SW = 42V, V
C
= 0V
OVPSET = 1V
OVPSET = 0.3V
CTRL1 = V
REF
, Current Out of LED Pin
CTRL1 = 700mV, Current Out of LED Pin
CTRL1 = 350mV, Current Out of LED Pin
CTRL1 = 100mV, Current Out of LED Pin
CTRL1 = 700mV, V
SENSE
= V
VOUT
– V
LED
CTRL1 = 350mV, V
SENSE
= V
VOUT
– V
LED
CTRL1 = 100mV, V
SENSE
= V
VOUT
– V
LED
CTRL1 = 100mV, CTRL2 = 1.25V or
CTRL2 = 100mV, CTRL1 = 1.25V (Current Out of Pin)
OVPSET = 1V, V
OUT
= 41V (Current Out of Pin)
0.8
V
C
= 1V, PWM = 0
PWM = 0
I
(SS)
= 20µA
V
C
= 0V
V
C
= 0V
SS = 1V, Current Out of Pin, V
C
= 0V
SS = 0.5V, V
C
= 0V
●
●
●
●
●
ELECTRICAL CHARACTERISTICS
CONDITIONS
R
T
= 9.09k
MIN
2
TYP
2.25
0.05
0.64
MAX
2.6
0.2
UNITS
MHz
%/V
V
%
%
%
µA/A
V/A
A/V
µA
µA
V
V
V
R
T
= 31.6k, 2.7V < V
IN
< 36V
80
88
97
73
770
400
13
40
40
0.65
1.5
0.2
LED Current to V
C
Current Gain
LED Current to V
C
Voltage Gain
V
C
to Switch Current Gain
V
C
Source Current (Out of Pin)
V
C
Sink Current
V
C
Switching Threshold
V
C
High Level (V
OH
)
V
C
Low Level (V
OL
)
Inductor Current Limit
Switch Current Limit
Switch V
CE
SAT
Switch Leakage Current
V
OUT
Overvoltage Protection (OVP)
(Rising)
Full Scale LED Current (LT3478-1)
700mA LED Current (LT3478-1)
350mA LED Current (LT3478-1)
100mA LED Current (LT3478-1)
CTRL1 = 700mV, V
SENSE
(LT3478)
CTRL1 = 350mV, V
SENSE
(LT3478)
CTRL1 = 100mV, V
SENSE
(LT3478)
CTRL1, 2 Input Currents
OVPSET Input Current
PWM Switching Threshold
V
C
Pin Current in PWM Mode
OUT Pin Current in PWM Mode
SS Low Level (V
OL
)
SS Reset Threshold
SS High Level (V
OH
)
Soft-Start (SS) Pin Charge Current
Soft-Start (SS) Pin Discharge Current
4.5
4.5
6
6.3
270
1
41
12.3
6.8
7.5
A
A
mV
µA
V
V
1010
655
325
70
101
67
33
7
1050
700
350
100
105
70.5
35.5
10
40
200
1
1
1
0.15
0.25
1.5
12
350
1090
730
375
130
109
74
38
13
mA
mA
mA
mA
mV
mV
mV
mV
nA
nA
Full Scale LED Current V
SENSE
(LT3478) CTRL1 = V
REF
, V
SENSE
= V
VOUT
– V
LED
1.2
50
100
V
nA
nA
V
V
V
µA
µA
34781f
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LT3478/LT3478-1
ELECTRICAL CHARACTERISTICS
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:
The LT3478EFE/LT3478EFE-1 are guaranteed to meet performance
specifications from 0°C to 125°C junction temperature. Specifications over
the –40°C to 125°C operating junction temperature range are assured by
design, characterization and correlation with statistical process controls.
The LT3478IFE/LT3478IFE-1 are guaranteed over the full –40°C to 125°C
operating junction temperature range.
Note 3:
This IC includes over-temperature protection that is intended
to protect the device during momentary overload conditions. Junction
temperature will exceed 125°C when over-temperature protection is active.
Continuous operation above the specified maximum operating junction
temperature may impair device reliability.
Note 4:
For maximum operating ambient temperature, see the “Thermal
Calculations” section in the Applications Information section.
Note 5:
The maximum operational voltage for V
IN
is limited by thermal and
efficiency considerations. Power switch base current is delivered from V
IN
and should therefore be driven from the lowest available power supply in
the system. See “Thermal Calculations” in the Applications Information
section.
Note 6:
For LT3478, parameter scales • (R
SENSE
/0.1Ω).
TYPICAL PERFOR A CE CHARACTERISTICS
LED Current vs CTRL1
1400
T
A
= 25°C
CTRL2 = V
REF
(FOR LT3478 SCALE BY 0.1Ω/R
SENSE
)
LED CURRENT (mA)
1400
LED CURRENT (mA)
LT3478-1
700
LT3478-1
700
LED CURRENT (mA)
1050
350
V
REF
0
0
0.35
0.70
CTRL1 (V)
1.05
1.40
3478 G01
CTRL1 Pin Current vs
Temperature
50
CTRL1 PIN CURRENT X (–1) (nA)
CTRL1 = 0.1V
SWITCH V
CE
(SAT) (mV)
240
40
CURRENT LIMIT (A)
30
CTRL1 = 0.35V
20 CTRL2 = V
REF
CTRL1 AND CTRL2 PINS
INTERCHANGEABLE
10
CTRL1 = 0.7V
CTRL1 = 0.9V
0
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
4
U W
LED Current vs Temperature
(FOR LT3478 SCALE BY 0.1Ω/R
SENSE
)
I
LED
= 1050mA, CTRL1 = CTRL2 = V
REF
1050
1000
LED Current vs PWM Duty Cycle
Wide PWM Dimming Range
(3000:1)
T
A
= 25°C
V
IN
= V
S
= 12V
6 LEDS AT 500mA
PWM FREQ = 100Hz
100
CTRL1 = 0.5V
CTRL2 = V
REF
F
OSC
= 1.6MHz
L = 2.2µH
10
350
I
LED
= 100mA, CTRL1 = 100mV,
CTRL2 = V
REF
0
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
125
1
0
0.01
0.1
1
10
PWM DUTY CYCLE (%)
100
3478 G03
3478 G02
Switch V
CE
(SAT) vs Switch
Current
T
A
= 25°C
7.0
Switch and Inductor Peak Current
Limits vs Temperature
210
6.5
SWITCH
INDUCTOR
5.5
180
6.0
120
60
5.0
0
125
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
SWITCH CURRENT (A)
3478 G05
4.5
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
125
3478 G04
3478 G06
34781f
LT3478/LT3478-1
TYPICAL PERFOR A CE CHARACTERISTICS
V
REF
vs Temperature
1.28
1.60
1.50
SHDN (V)
SHDN PIN CURRENT (µA)
1.26
V
REF
(V)
1.24
1.22
1.20
1.18
–50 –25
0
25
50
75 100
JUNCTION TEMPERATURE (°C)
V
IN
Shutdown Current vs
Temperature
50
SHDN = 0V
14
12
40
V
IN
CURRENT (mA)
30
V
IN
= 36V
V
IN
= 20V
10
V
IN
= 2.8V
0
–50 –25
0
25
50
75 100
JUNCTION TEMPERATURE (°C)
125
8
6
4
2
0
0
3
6
V
IN
CURRENT (mA)
V
IN
CURRENT (µA)
20
V
S
, L, SW Shutdown Currents vs
Temperature
4
SWITCH PEAK CURRENT LIMIT (A)
SHDN = 0V
V
S
= L = SW = 36V
7
6
5
4
3
2
1
0
125
PIN CURRENT (µA)
2
I(V
S
PIN) = I(L PIN)
0
–50 –25
0
25
50
75 100
JUNCTION TEMPERATURE (°C)
U W
⎯
S
⎯
H
⎯
D
⎯
N Threshold vs Temperature
15
⎯
S
⎯
H
⎯
D
⎯
N Pin (Hysteresis) Current vs
Temperature
JUST BEFORE PART TURNS ON
10
1.40
5
1.30
125
1.20
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
125
AFTER PART TURNS ON
0
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
125
3478 G07
3478 G08
3478 G09
V
IN
Quiescent Current vs V
IN
14
12
10
8
6
4
2
V
IN
Quiescent Current vs
Temperature
10
T
A
= 25°C
V
C
= 0V
9 12 15 18 21 24 27 30 33 36
V
IN
(V)
3478 G11
0
–50 –25
0
25
50
75 100
JUNCTION TEMPERATURE (°C)
V
IN
= 2.8V
V
C
= 0V
125
3478 G10
3478 G12
Switch Peak Current Limit
vs Duty Cycle
I(SW PIN)
T
A
= 25°C
0
20
40
60
DUTY CYCLE (%)
80
100
3478 G19
3478 G18
34781f
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