LT3478-1 [Linear]
4.5A Monolithic LED Drivers with True Color PWM Dimming; 4.5A单片LED驱动器,提供真正彩色PWM调光型号: | LT3478-1 |
厂家: | Linear |
描述: | 4.5A Monolithic LED Drivers with True Color PWM Dimming |
文件: | 总24页 (文件大小:280K) |
中文: | 中文翻译 | 下载: | 下载PDF数据表文档文件 |
LT3478/LT3478-1
4.5A Monolithic LED
Drivers with True Color
PWM Dimming
U
DESCRIPTIO
FEATURES
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.
■
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:
TheLT3478-1reducesexternalcomponentcountandcost
by integrating the LED current sense resistor. The LT3478
uses an external sense resistor to extend the maximum
programmableLEDcurrentbeyond1Aandalsotoachieve
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
highpowerLEDdriverapplicationssuchasautomotiveTFT
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.
■
100mA to 1050mA (LT3478-1)
(10mV to 105mV)/R
Program LED Current De-Rating vs Temperature
Separate Inductor Supply Input
Inrush Current Protection
Programmable Soft-Start
(LT3478)
SENSE
■
■
■
■
■
■
■
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
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APPLICATIO S
■
High Power LED Driver
Automotive Lighting
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.
■
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Patents Pending.
U
TYPICAL APPLICATIO
Automotive TFT LCD Backlight
Efficiency vs V
IN
100
95
90
85
80
10µH
V
IN
8V TO 16V
I
f
= 700mA
= 500kHz
PWM DUTY CYCLE = 100%
LED
OSC
10µF
4.7µF
V
IN
V
S
L
SW
SHDN
OUT
V
REF
0.1Ω
SENSE
(LT3478)
R
45.3k
CTRL2
LT3478-1
LED
OVPSET
54.9k
130k
CTRL1
PWM
700mA
15W
SS
V
R
T
C
6 LEDs LUXEON III (WHITE)
12 14 16
(V)
6 LEDs
(WHITE)
8
10
1µF
0.1µF
69.8k
V
IN
3478 TA01b
PWM DIMMING
CONTROL
3478 TA01
34781f
1
LT3478/LT3478-1
W W U W
U
W
U
ABSOLUTE AXI U RATI GS
PACKAGE/ORDER I FOR ATIO
(Note 1)
TOP VIEW
SW............................................................................42V
SW
SW
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
SS
V
IN
, LED..................................................................42V
OUT
R
T
V , V , V , SHDN (Note 5).......................................36V
S
L
V
IN
PWM
PWM.........................................................................15V
V
S
CTRL2
CTRL1
SHDN
17
CTRL1, 2.....................................................................6V
L
SS, R , V , V , OVPSET............................................2V
T
C
REF
V
OUT
Operating Junction Temperature Range
LED
V
REF
(Notes 2, 3, 4).................................... –40°C to 125°C
Storage Temperature Range................... –65°C to 150°C
Lead Temperature (Soldering, 10 Sec).................. 300°C
OVPSET
V
C
FE PACKAGE
16-LEAD PLASTIC TSSOP
= 125°C, θ = 35°C/W
EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB.
T
JMAX
JA
ORDER PART NUMBER
FE PART MARKING
LT3478EFE
LT3478EFE-1
LT3478IFE
3478FE
3478FE-1
3478FE
LT3478IFE-1
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.
ELECTRICAL CHARACTERISTICS The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25°C. SW = open, V = V = L = V = SHDN = 2.7V, LED = open, SS = open,
A
IN
S
OUT
PWM = CTRL1, CTRL2 = 1.25V, V
= open, V = open, R = 31.6k.
REF
C
CONDITIONS
(Rising)
T
PARAMETER
MIN
TYP
MAX
UNITS
●
Minimum Operating Voltage
Operational Input Voltage
2.4
2.8
V
V
S
V
2.8
2.8
36
36
V
V
(Note 5)
IN
V
V
Quiescent Current
Shutdown Current
V = 0V (No Switching)
6.1
3
mA
µA
V
IN
C
SHDN = 0V
(Micropower)
(Switching)
6
IN
●
●
SHDN Pin Threshold (V
SHDN Pin Threshold (V
SHDN Pin Current
)
0.1
1.3
8
0.4
1.4
0.7
1.5
12
SD_µp
)
V
SD_UVLO
SHDN = V
SHDN = V
– 50mV
+ 50mV
10
0
µA
µA
SD_UVLO
SD_UVLO
V
V
V
Voltage
I(V ) = 0µA, V = 0V
1.213
1.240
0.005
8
1.263
0.015
12
V
%/V
mV
REF
REF
REF
REF
C
Line Regulation
Load Regulation
I(V ) = 0µA, 2.7V < V < 36V
REF IN
0 < I(V ) < 100µA (Max)
REF
Frequency: f
Frequency: f
200kHz
R = 200k
0.18
0.88
0.2
0.22
1.12
MHz
MHz
OSC
OSC
T
●
1MHz
R = 31.6k
T
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LT3478/LT3478-1
ELECTRICAL CHARACTERISTICS The
●
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T = 25°C. SW = open, V = V = L = V = SHDN = 2.7V, LED = open, SS = open,
A
IN
S
OUT
PWM = CTRL1, CTRL2 = 1.25V, V
= open, V = open, R = 31.6k.
REF
C
T
PARAMETER
CONDITIONS
MIN
TYP
2.25
0.05
0.64
MAX
2.6
UNITS
MHz
%/V
V
Frequency: f
2.25MHz
R = 9.09k
T
2
OSC
Line Regulation f
R = 31.6k, 2.7V < V < 36V
T
0.2
OSC
IN
Nominal R Pin Voltage
T
●
Maximum Duty Cycle
R = 31.6k
80
88
97
73
%
%
%
T
R = 200k
T
R = 9.09k
T
LED Current to V Current Gain
(Note 6)
(Note 6)
770
400
13
µA/A
V/A
A/V
µA
µA
V
C
LED Current to V Voltage Gain
C
V to Switch Current Gain
C
V Source Current (Out of Pin)
C
CTRL1 = 0.4V, V = 1V
40
C
V Sink Current
C
CTRL1 = 0V, V = 1V
40
C
V Switching Threshold
C
0.65
1.5
0.2
6
V High Level (V
)
OH
CTRL1 = 0.4V
CTRL1 = 0V
V
C
V Low Level (V
)
OL
V
C
●
●
Inductor Current Limit
Switch Current Limit
2.7V < V < 36V
4.5
4.5
6.8
7.5
A
S
6.3
270
1
A
Switch V SAT
I
= 4.5A
SW
mV
µA
CE
Switch Leakage Current
SW = 42V, V = 0V
C
V
Overvoltage Protection (OVP)
OVPSET = 1V
OVPSET = 0.3V
41
12.3
V
V
OUT
(Rising)
Full Scale LED Current (LT3478-1)
700mA LED Current (LT3478-1)
350mA LED Current (LT3478-1)
100mA LED Current (LT3478-1)
CTRL1 = V , Current Out of LED Pin
1010
655
325
70
1050
700
350
100
105
70.5
35.5
10
1090
730
375
130
109
74
mA
mA
mA
mA
mV
mV
mV
mV
nA
REF
●
●
CTRL1 = 700mV, Current Out of LED Pin
CTRL1 = 350mV, Current Out of LED Pin
CTRL1 = 100mV, Current Out of LED Pin
Full Scale LED Current V
(LT3478) CTRL1 = V , V
= V
– V
LED
101
67
SENSE
REF SENSE
VOUT
CTRL1 = 700mV, V
CTRL1 = 350mV, V
CTRL1 = 100mV, V
(LT3478)
CTRL1 = 700mV, V
CTRL1 = 350mV, V
CTRL1 = 100mV, V
= V
– V
SENSE
SENSE
SENSE
SENSE
SENSE
SENSE
VOUT
VOUT
VOUT
LED
LED
LED
(LT3478)
(LT3478)
= V
= V
– V
– V
33
38
7
13
CTRL1, 2 Input Currents
CTRL1 = 100mV, CTRL2 = 1.25V or
CTRL2 = 100mV, CTRL1 = 1.25V (Current Out of Pin)
40
OVPSET Input Current
OVPSET = 1V, V = 41V (Current Out of Pin)
200
1
nA
V
OUT
PWM Switching Threshold
0.8
1.2
50
V Pin Current in PWM Mode
C
V = 1V, PWM = 0
C
1
nA
nA
V
OUT Pin Current in PWM Mode
PWM = 0
1
100
SS Low Level (V
)
OL
I
= 20µA
0.15
0.25
1.5
12
(SS)
SS Reset Threshold
SS High Level (V
V = 0V
C
V
)
OH
V = 0V
C
V
Soft-Start (SS) Pin Charge Current
Soft-Start (SS) Pin Discharge Current
SS = 1V, Current Out of Pin, V = 0V
µA
µA
C
SS = 0.5V, V = 0V
350
C
34781f
3
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.
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
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.
Calculations” section in the Applications Information section.
Note 5: The maximum operational voltage for V is limited by thermal and
IN
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 3: This IC includes over-temperature protection that is intended
to protect the device during momentary overload conditions. Junction
Note 6: For LT3478, parameter scales • (R /0.1Ω).
SENSE
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TYPICAL PERFOR A CE CHARACTERISTICS
LED Current vs PWM Duty Cycle
Wide PWM Dimming Range
(3000:1)
LED Current vs CTRL1
LED Current vs Temperature
1000
100
10
1
1400
1050
700
350
0
1400
1050
700
350
0
T
= 25°C
T
= 25°C
(FOR LT3478 SCALE BY 0.1Ω/R
)
A
V
A
SENSE
= V = 12V
CTRL2 = V
IN
S
REF
6 LEDS AT 500mA
PWM FREQ = 100Hz
CTRL1 = 0.5V
(FOR LT3478 SCALE BY 0.1Ω/R
)
SENSE
I
= 1050mA, CTRL1 = CTRL2 = V
REF
LED
CTRL2 = V
REF
LT3478-1
F
= 1.6MHz
OSC
LT3478-1
L = 2.2µH
I
= 100mA, CTRL1 = 100mV,
LED
V
CTRL2 = V
REF
REF
0
0.01
0.1
1
10
100
–50 –25
0
25
50
75 100 125
0
0.35
0.70
1.05
1.40
PWM DUTY CYCLE (%)
JUNCTION TEMPERATURE (°C)
CTRL1 (V)
3478 G03
3478 G01
3478 G02
CTRL1 Pin Current vs
Temperature
Switch V (SAT) vs Switch
Switch and Inductor Peak Current
Limits vs Temperature
CE
Current
240
210
180
120
60
7.0
6.5
6.0
5.5
5.0
4.5
50
40
30
20
10
0
T
= 25°C
A
CTRL1 = 0.1V
SWITCH
INDUCTOR
CTRL1 = 0.35V
CTRL1 = 0.7V
CTRL2 = V
CTRL1 AND CTRL2 PINS
INTERCHANGEABLE
REF
CTRL1 = 0.9V
0
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75 100 125
JUNCTION TEMPERATURE (°C)
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
SWITCH CURRENT (A)
JUNCTION TEMPERATURE (°C)
3478 G04
3478 G05
3478 G06
34781f
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LT3478/LT3478-1
U W
TYPICAL PERFOR A CE CHARACTERISTICS
SHDN Pin (Hysteresis) Current vs
Temperature
V
vs Temperature
SHDN Threshold vs Temperature
REF
1.28
1.26
1.24
1.22
1.20
1.18
1.60
1.50
1.40
1.30
1.20
15
JUST BEFORE PART TURNS ON
10
5
AFTER PART TURNS ON
25 50 75 100 125
0
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75 100 125
–50 –25
0
JUNCTION TEMPERATURE (°C)
JUNCTION TEMPERATURE (°C)
JUNCTION TEMPERATURE (°C)
3478 G07
3478 G08
3478 G09
V
Shutdown Current vs
V
Quiescent Current vs
IN
IN
Temperature
V
IN
Quiescent Current vs V
Temperature
IN
50
40
30
20
10
0
14
12
10
8
14
SHDN = 0V
12
10
8
6
6
V
V
= 36V
= 20V
IN
IN
4
4
2
2
V
= 2.8V
IN
T = 25°C
C
V
V
= 2.8V
= 0V
A
V
IN
C
= 0V
0
0
–50 –25
0
25
50
75 100 125
0
3
6
9
12 15 18 21 24 27 30 33 36
(V)
–50 –25
0
25
50
75 100 125
JUNCTION TEMPERATURE (°C)
V
JUNCTION TEMPERATURE (°C)
IN
3478 G10
3478 G11
3478 G12
V , L, SW Shutdown Currents vs
Switch Peak Current Limit
vs Duty Cycle
S
Temperature
4
2
0
7
6
5
4
3
2
1
0
SHDN = 0V
S
V
= L = SW = 36V
I(V PIN) = I(L PIN)
S
I(SW PIN)
T = 25°C
A
–50 –25
0
25
50
75 100 125
0
20
40
60
80
100
JUNCTION TEMPERATURE (°C)
DUTY CYCLE (%)
3478 G19
3478 G18
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LT3478/LT3478-1
U W
TYPICAL PERFOR A CE CHARACTERISTICS
Switching Frequency vs
Temperature
Open-Circuit Output Clamp
Voltage vs Temperature
Switching Frequency vs R
T
10000
1000
100
1.20
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
43.0
42.5
42.0
41.5
41.0
40.5
40.0
39.5
39.0
R
= 31.6k
OVPSET = 1V
T
= 25°C
T
A
–50 –25
0
25
50
75 100 125
–50 –25
0
25
50
75 100 125
1
10
100
1000
JUNCTION TEMPERATURE (°C)
JUNCTION TEMPERATURE (°C)
R
(kΩ)
T
3478 G13
3478 G14
3478 G15
SS Pin Charge Current vs
Temperature
V Pin Active and Clamp Voltages
C
vs Temperature
14
13
12
11
10
1.8
1.5
1.2
0.9
0.6
0.3
0
V
CLAMP
C
V
ACTIVE THRESHOLD
C
0
–50 –25
0
25
50
75 100 125
–50 –25
25
50
75 100 125
JUNCTION TEMPERATURE (°C)
JUNCTION TEMPERATURE (°C)
3478 G16
3478 G17
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LT3478/LT3478-1
U
U
U
PI FU CTIO S
SW (Pins 1, 2): Switch Pin. Collector of the internal NPN
power switch. Both pins are fused together inside the IC.
Connect the inductor and diode here and minimize the
metal trace area connected to this pin to minimize EMI.
hysteresis allows programming of undervoltage lockout
(UVLO) hysteresis. SHDN above 1.4V turns the part on
and removes a 10µA sink current from the pin. SHDN = 0V
reducesV current<3µA.SHDNcanbedirectlyconnected
IN
to V . If left open circuit the part will be turned off.
IN
V (Pin 3): Input Supply. Must be locally bypassed with
IN
a capacitor to ground.
CTRL1 (Pin 12): CTRL1 pin voltage is used to program
maximum LED current (CTRL2 = V ). CTRL1 voltage
REF
V (Pin 4): Inductor Supply. Must be locally bypassed
S
can be set by a resistor divider from V
or an external
REF
with a capacitor to ground. Can be shorted to V if only
IN
voltage source. Maximum LED current is given by:
one supply is available (see L (Pin 5) function).
(LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps
L (Pin 5): Inductor Pin. An internal resistor between V
S
and L pins monitors inductor current to protect against
inrush current. Exceeding 6A immediately turns off the
internal NPN power switch and discharges the soft-start
pin. Input current monitoring can be disabled by connect-
ing the inductor power supply directly to the L pin and
(LT3478)Max LED Current =
0.1
RSENSE
Min(CTRL, 1.05)•
Amps
(linear for 0.1V < CTRL1< 0.95V ; CTRL2 = V ) For maxi-
mum LED current, short CTRL1 and CTRL2 pins to V
REF
leaving the V pin open (requires local bypass capacitor
S
.
REF
to GND on L pin; not V pin).
S
CTRL2 (Pin 13): The CTRL2 pin is available for program-
ming a decrease in LED current versus temperature
(setting temperature breakpoint and slope). This feature
allows the output LED(s) to be programmed for maximum
allowablecurrentwithoutdamageathighertemperatures.
This maximizes LED usage and increases reliability. A
CTRL2 voltage with negative temperature coefficient is
V
(Pin 6): Output voltage of the converter. Connect a
OUT
capacitor from this pin to ground. Internal circuitry moni-
tors V for protection against open LED faults.
OUT
LED (Pin 7): Connect the LED string from this pin to
ground.Aninternal(LT3478-1)/external(LT3478)resistor
between the V
accurate control.
and LED pins senses LED current for
OUT
created using an external resistor divider from V with
REF
temperature dependant resistance. If not used, CTRL2
OVPSET (Pin 8): Programs V
overvoltage protection
OUT
should be tied to V
.
REF
level (OVP) to protect against open LED faults. OVP =
(OVPSET • 41)V. OVPSET range is 0.3V to 1V for an OVP
range of typically 12.3V to 41V.
PWM(Pin14):InputpinforPWMdimmingcontrol.Above
1V allows converter switching and below 1V disables
switching with V pin level maintained. With an external
C
V (Pin 9): Output of the transconductance error amplifier
C
MOSFET placed in series with the ground side of the LED
string, a PWM signal driving the PWM pin and MOSFET
gate provides accurate dimming control. The PWM signal
can be driven from 0V to 15V. If unused, the pin should
and compensation pin for the converter regulation loop.
V
REF
(Pin 10): Bandgap Voltage Reference. This pin can
supply up to 100µA. Can be used to program CTRL1,
CTRL2, OVPSET pin voltages using resistor dividers to
ground.
be connected to V
.
REF
R (Pin 15): A resistor to ground programs switching
T
SHDN (Pin 11): The SHDN pin has an accurate 1.4V
threshold and can be used to program an undervoltage
lockout (UVLO) threshold for system input supply using a
resistor divider from supply to ground. A 10µA pin current
frequency between 200kHz and 2.25MHz.
SS (Pin 16): Soft-Start Pin. Placing a capacitor here pro-
grams soft-start timing to limit inductor inrush current
duringstart-upduetotheconverter.Wheninductorcurrent
34781f
7
LT3478/LT3478-1
U
U
U
PI FU CTIO S
exceeds 6A or V
exceeds OVP, an internal soft-start
Exposed Pad (Pin 17): The ground for the IC and the con-
verter.TheFEpackagehasanExposedPadunderneaththe
ICwhichisthebestpathforheatoutofthepackage. Pin17
should be soldered to a continuous copper ground plane
under the device to reduce die temperature and increase
the power capability of the LT3478/LT3478-1.
OUT
latch is set, the power NPN is immediately turned off and
the SS pin is discharged. The soft-start latch is also set
if V and/or SHDN do not meet their turn on thresholds.
IN
The SS pin only recharges when all faults are removed
and the pin has been discharged below 0.25V.
W
BLOCK DIAGRA
SHDN
V
S
L
SS
16
SW
1, 2
11
4
5
V
10µA
OUT
V
C
6
OVERVOLTAGE
DETECT
–
+
9.5mΩ
–
+
+
–
57mV
OVPSET
1.4V
100Ω
R
R
SENSE
(EXTERNAL FOR
LT3478)
SENSE
0.1Ω
INRUSH
SOFT-START
(INTERNAL FOR
LT3478-1)
CURRENT
V
IN
PROTECTION
UVLO
REF
1.24V
3
LED
7
PWM
DETECT
V
REF
OSC
S
Q
R
Q1
10
LED
LED
LED
LED
PWM
CTRL1
CTRL2
1.05V
+
12
13
SLOPE
COMP
+
+
–
GM
Q2
–
+
1V
Σ
PWM
14
+
–
1000Ω
R
S
TO OVERVOLTAGE
DETECT CIRCUIT
8
15
17
9
3478 F01
OVPSET
R
T
EXPOSED PAD
(GND)
V
C
Figure 1
34781f
8
LT3478/LT3478-1
U
OPERATIO
The LT3478/LT3478-1 are high powered LED drivers with
a 42V, 4.5A internal switch and the ability to drive LEDs
withupto1050mAforLT3478-1andupto105mV/R
for LT3478.
the V voltage controls the peak switch current limit and
C
hence the inductor current available to the output LED(s).
As with all current mode converters, slope compensation
is added to the control path to ensure stability.
SENSE
The LT3478/LT3478-1 work similarly to a conventional
currentmodeboostconverterbutuseLEDcurrent(instead
of output voltage) as feedback for the control loop. The
Block Diagram in Figure 1 shows the major functions of
the LT3478/LT3478-1.
The CTRL1 pin is used to program maximum LED current
via Q2. The CTRL2 pin can be used to program a decrease
inLEDcurrentversustemperatureformaximumreliability
andutilizationoftheLED(s).ACTRL2voltagewithnegative
temperature coefficient can be created using an external
resistor divider from V
with temperature dependant
REF
For the part to turn on, the V pin must exceed 2.8V and
IN
resistance. Unused CTRL2 is tied to V
.
REF
the SHDN pin must exceed 1.4V. The SHDN pin threshold
allows programming of an undervoltage lockout (UVLO)
threshold for the system input supply using a simple
resistor divider. A 10µA current flows into the SHDN pin
before part turn on and is removed after part turn on. This
current hysteresis allows programming of hysteresis for
theUVLOthreshold.See“ShutdownPinandProgramming
Undervoltage Lockout” in the Applications Information
Section. For micropower shutdown the SHDN pin at 0V
For True Color PWM dimming, the LT3478/LT3478-1
provide up to a 3000:1 wide PWM dimming range by al-
lowing the duty cycle of the PWM pin (connected to the
IC and an external N-channel MOSFET in series with the
LED(s)) to be reduced from 100% to as low as 0.033%
for a PWM frequency of 100Hz. Dimming by PWM duty
cycle, allows for constant LED color to be maintained over
the entire dimming range.
reduces V supply current to approximately 3µA.
IN
Forrobustoperation,theLT3478/LT3478-1monitorsystem
Each LED driver is a current mode step-up switch-
ing regulator. A regulation point is achieved when the
performance for any of the following faults : V or SHDN
IN
pin voltages too low and/or inductor current too high
and/or boosted output voltage too high. On detection of
any of these faults, the LT3478/LT3478-1 stop switching
immediately and a soft-start latch is set discharging the
SS pin (see Timing Diagram for SS pin in Figure 11). All
faults are detected internally and do not require external
components. When all faults no longer exist, an internal
12µAsupplychargestheSSpinwithatimingprogrammed
using a single external capacitor. A gradual ramp up of SS
pin voltage limits switch current during startup.
boosted output voltage V
across the output LED(s) is
OUT
high enough to create current in the LED(s) equal to the
programmed LED current. A sense resistor connected in
series with the LED(s) provides feedback of LED current
to the converter loop.
Thebasicloopusesapulsefromaninternaloscillatortoset
the RS flip-flop and turn on the internal power NPN switch
Q1 connected between the switch pin, SW, and ground.
Current increases in the external inductor until switch
current limit is exceeded or until the oscillator reaches
its maximum duty cycle. The switch is then turned off,
causing inductor current to lift the SW pin and turn on an
external Schottky diode connected to the output. Inductor
current flows via the Schottky diode charging the output
capacitor. The switch is turned back on at the next reset
cycle of the internal oscillator. During normal operation
For optimum component sizing, duty cycle range and ef-
ficiencytheLT3478/LT3478-1allowforaseparateinductor
supply V and for switching frequency to be programmed
S
from 200kHz up to 2.25MHz using a resistor from the R
T
pintoground.Theadvantagesoftheseoptionsarecovered
in the Applications Informations section.
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Inductor Selection
Capacitor Selection
Low ESR (equivalent series resistance) ceramic capaci-
tors should be used at the output to minimize the output
ripple voltage. Use only X5R or X7R dielectrics, as these
materials retain their capacitance over wider voltage and
temperature ranges than other dielectrics. A 4.7µF to
10µF output capacitor is sufficient for most high output
current designs. Some suggested manufacturers are
listed in Table 2.
SeveralinductorsthatworkwellwiththeLT3478/LT3478-1
are listed in Table 1. However, there are many other manu-
facturers and inductors that can be used. Consult each
manufacturerformoredetailedinformationandtheirentire
range of parts. Ferrite cores should be used to obtain the
best efficiency. Choose an inductor that can handle the
necessary peak current without saturating. Also ensure
that the inductor has a low DCR (copper-wire resistance)
2
to minimize I R power losses. Values between 4.7µH and
Diode Selection
22µH will suffice for most applications.
Schottky diodes, with their low forward voltage drop and
fast switching speed, are ideal for LT3478/LT3478-1 ap-
plications. Table 3 lists several Schottky diodes that work
well. The diode’s average current rating must exceed the
application’saverageoutputcurrent.Thediode’smaximum
reverse voltage must exceed the application’s output volt-
age. A 4.5A diode is sufficient for most designs. For PWM
dimming applications, be aware of the reverse leakage
current of the diode. Lower leakage current will drain the
output capacitor less, allowing for higher dimming range.
The companies below offer Schottky diodes with high
voltage and current ratings.
Inductor manufacturers specify the maximum current
rating as the current where inductance falls by a given
percentage of its nominal value. An inductor can pass a
current greater than its rated value without damaging it.
Aggressive designs where board space is precious will
exceedthemaximumcurrentratingoftheinductortosave
space. Consult each manufacturer to determine how the
maximum inductor current is measured and how much
more current the inductor can reliably conduct.
Table 1. Suggested Inductors
MANUFACTURER PART NUMBER
IDC (A)
INDUCTANCE (µH)
MAX DCR (mΩ)
L × W × H (mm)
MANUFACTURER
CDRH104R-100NC
CDRH103RNP-4R7NC-B
CDRH124R-100MC
CDRH104R-5R2NC
3.8
10
4.7
10
35
30
28
22
10.5 × 10.3 × 4.0
10.5 × 10.3 × 3.1
12.3 × 12.3 × 4.5
10.5 × 10.3 × 4.0
Sumida
www.sumida.com
4
4.5
5.5
5.2
FDV0630-4R7M
4.2
4.7
49
7.0 × 7.7 × 3.0
Toko
www.toko.com
UP4B-220
7.6
22
34
22 × 15 × 7.9
Cooper
www.cooperet.com
Table 2. Ceramic Capacitor Manufacturers
MANUFACTURER
Taiyo Yuden
AVX
PHONE NUMBER
WEB
(408) 573-4150
(803) 448-9411
(714) 852-2001
www.t-yuden.com
www.avxcorp.com
www.murata.com
Murata
Table 3. Suggested Diodes
MANUFACTURER PART NUMBER
UPS340
MAX CURRENT (A)
MAX REVERSE VOLTAGE
WEB
3
40
Microsemi
www.microsemi.com
B520C
B530C
B340A
B540C
PDS560
5
5
3
5
5
30
30
40
40
60
Diodes, Inc.
www.diodes.com
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Shutdown and Programming Undervoltage Lockout
Programming Switching Frequency
Theswitchingfrequencyisprogrammedusinganexternal
resistor(R )connectedbetweentheR pinandground.The
The LT3478/LT3478-1 have an accurate 1.4V shutdown
threshold at the SHDN pin. This threshold can be used in
conjunction with a resistor divider from the system input
supply to define an accurate undervoltage lockout (UVLO)
threshold for the system (Figure 2). SHDN pin current
hysteresis allows programming of hysteresis voltage for
this UVLO threshold. Just before part turn on, 10µA flows
into the SHDN pin. After part turn on, 0µA flows from the
SHDNpin.Calculationoftheon/offthresholdsforasystem
input supply using the LT3478/LT3478-1 SHDN pin can
be made as follows:
T
T
internal free-running oscillator is programmable between
200kHz and 2.25MHz. Table 4 shows the typical R values
T
required for a range of switching frequencies.
Selecting the optimum switching frequency depends
on several factors. Inductor size is reduced with higher
frequency but efficiency drops due to higher switching
losses.Inaddition,someapplicationsrequireveryhighduty
cycles to drive a large number of LEDs from a low supply.
Lowswitchingfrequencyallowsagreateroperationalduty
cycle and hence a greater number of LEDs to be driven.
In each case the switching frequency can be tailored to
provide the optimum solution. When programming the
switching frequency the total power losses within the IC
should be considered. See “Thermal Calculations” in the
Applications Information section.
V
V
OFF = 1.4 [1 + R1/R2)]
SUPPLY
SUPPLY
ON = V
OFF + (10µA • R1)
SUPPLY
An open drain transistor can be added to the resistor
divider network at the SHDN pin to independently control
the turn off of the LT3478/LT3478-1.
V
SUPPLY
10000
T
= 25°C
A
R1
R2
SHDN
11
–
+
1.4V
1000
100
OFF ON
10µA
3478 F02
1
10
100
1000
Figure 2. Programming Undervoltage Lockout (UVLO)
with Hysteresis
R
(kΩ)
T
3478 F03
Figure 3. Switching Frequency vs R Resistor Value
T
With the SHDN pin connected directly to the V pin, an
IN
internal undervoltage lockout threshold exists for the V
IN
Table 4. Switching Frequencies vs R Values
T
pin (2.8V max). This prevents the converter from operat-
ing in an erratic mode when supply voltage is too low.
The LT3478/LT3478-1 provide a soft-start function when
SWITCHING FREQUENCY (MHz)
R (kΩ)
T
2.25
1
9.09
31.6
200
recovering from such faults as SHDN <1.4V and/or V
IN
0.2
<2.8V. See details in the Applications Information section
“Soft-Start”.
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Programming Maximum LED current
maximum allowed LED current versus temperature to
warn against exceeding this current limit and damaging
the LED (Figure 6).
MaximumLEDcurrentcanbeprogrammedusingtheCTRL1
pin with CTRL2 tied to the V pin (see Figures 4 and 5).
REF
The maximum allowed LED current is defined as:
Luxeon V (Maximum) and LT3478-1
(Programmed) Current Derating
Curves vs Temperature
(LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps
900
800
(LT3478)Max LED Current =
0.1
RSENSE
Min(CTRL1, 1.05)•
Amps
700
600
500
400
300
200
100
0
LUXEON V EMITTER
CURRENT DERATING
CURVE
LED current vs CTRL1 is linear for approximately
0.1V < CTRL1 < 0.95V
EXAMPLE
LT3478-1
PROGRAMMED LED
CURRENT DERATING CURVE
For maximum possible LED current, connect CTRL1 and
CTRL2 to the V pin.
REF
0
25
50
75
100
1400
T
= 25°C
A
T
AMBIENT TEMPERATURE (°C)
A
CTRL2 = V
REF
(FOR LT3478 SCALE
BY 0.1Ω/R
LUXEON V EMITTER
)
(GREEN, CYAN, BLUE, ROYAL BLUE)
SENSE
1050
700
350
0
θ
= 20°C/W
JA
3478 F06
LT3478-1
Figure 6. LED Current Derating Curve vs Ambient Temperature
Without the ability to back off LED current as temperature
increases, many LED drivers are limited to driving the
LED(s)atonly50%orlessoftheirmaximumratedcurrents.
This limitation requires more LEDs to obtain the intended
brightness for the application. The LT3478/LT3478-1 al-
low the output LED(s) to be programmed for maximum
allowable current while still protecting the LED(s) from
excessive currents at high temperature. This is achieved
by programming a voltage at the CTRL2 pin with a nega-
tive temperature coefficient using a resistor divider with
temperature dependent resistance (Figures 7 and 8).
CTRL2voltageisprogrammedhigherthanCTRL1voltage.
This allows initial LED current to be defined by CTRL1.
As temperature increases, CTRL2 voltage will fall below
CTRL1 voltage causing LED currents to be controlled by
CTRL2 pin voltage. The choice of resistor ratios and use
of temperature dependent resistance in the divider for the
CTRL2 pin will define the LED current curve breakpoint
and slope versus temperature (Figure 8).
V
REF
0
0.35
0.70
CTRL1 (V)
1.05
1.40
3478 F04
Figure 4. LED Current vs CTRL1 Voltage
LT3478/LT3478-1
(LT3478)
10
13
12
V
V
OUT
REF
R2
R1
R
SENSE
CTRL2
CTRL1
LED
3478 F05
Figure 5. Programming LED Current
Programming LED Current Derating vs Temperature
A useful feature of the LT3478/LT3478-1 is the ability
to program a derating curve for maximum LED current
versus temperature. LED data sheets provide curves of
AvarietyofresistornetworksandNTCresistorswithdiffer-
enttemperaturecoefficientscanbeusedforprogramming
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CTRL2toachievethedesiredCTRL2curvevstemperature.
The current derating curve shown in Figure 6 uses the
resistor network shown in option C of Figure 7.
to obtain a resistor’s exact values over temperature from
the manufacturer. Hand calculations of CTRL2 voltage
can then be performed at each given temperature and the
resultingCTRL2curveplottedversustemperature.Several
iterations of resistor value calculations may be required
to achieve the desired breakpoint and slope of the LED
current derating curve.
10
V
REF
R2
R1
R4
LT3478/LT3478-1
13
12
CTRL2
CTRL1
Table 5. NTC Resistor Manufacturers/Distributors
MANUFACTURER
R3
OPTION A TO D
Murata Electronics North America
TDK Corporation
www.murata.com
www.tdk.com
R
R
Y
Y
Digi-key
www.digikey.com
R
R
R
R
R
R
X
NTC
NTC
X
NTC
NTC
If calculation of CTRL2 voltage at various temperatures
gives a downward slope that is too strong, alternative
resistor networks can be chosen (B, C, D in Figure 7)
which use temperature independent resistance to reduce
the effects of the NTC resistor over temperature.
A
B
C
D
3478 F07
Figure 7. Programming LED Current Derating Curve
vs Temperature (R
Located on LEDs PCB)
NTC
1100
1000
900
800
700
600
500
400
300
200
Murata Electronics provides a selection of NTC resistors
with complete data over a wide range of temperatures. In
addition, a software tool is available which allows the user
toselectfromdifferentresistornetworksandNTCresistor
values and then simulate the exact output voltage curve
(CTRL2 behavior) over temperature. Referred to as the
‘Murata Chip NTC Thermistor Output Voltage Simulator’,
userscanlogontowww.murata.com/designlibanddown-
load the software followed by instructions for creating an
CTRL1
CTRL2
LED CURRENT = MINIMUM
OF CTRL1, CTRL2
R3 = OPTION C
100
0
output voltage V
(CTRL2) from a specified V supply
OUT
CC
0
25
50
75
100
(V ). At any time during selection of circuit parameters
T
AMBIENT TEMPERATURE (°C)
REF
A
3478 F08
the user can access data on the chosen NTC resistor by
clicking on a link to the Murata catalog.
Figure 8. CTRL1, 2 Programmed Voltages vs Temperature
The following example uses hand calculations to derive
the resistor values required for CTRL1 and CTRL2 pin
voltages to achieve a given LED current derating curve.
The resistor values obtained using the Murata simulation
tool are also provided and were used to create the derating
curve shown in Figure 6. The simulation tool illustrates
the non-linear nature of the NTC resistor temperature
coefficient at temperatures exceeding 50°C ambient. In
addition, the resistor divider technique using an NTC
resistor to derive CTRL2 voltage inherently has a flatten-
ing characteristic (reduced downward slope) at higher
Table 5 shows a list of manufacturers/distributors of NTC
resistors. There are several other manufacturers available
and the chosen supplier should be contacted for more
detailed information. To use an NTC resistor to indicate
LED temperature it is only effective if the resistor is con-
nected as close as possible to the LED(s). LED derating
curves shown by manufacturers are listed for ambient
temperature. The NTC resistor should be submitted to
the same ambient temperature as the LED(s). Since the
temperature dependency of an NTC resistor can be non-
linear over a wide range of temperatures it is important
temperatures.ToavoidLEDcurrentexceedingamaximum
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–1.026
allowed level at higher temperatures, the CTRL2 voltage
curve may require a greater downward slope between
25°C and 50°C to compensate for that loss of slope at
higher temperatures.
R
R
R
(50°C) = R
(25°C).e
NTC
NTC
NTC
NTC
(50°C) = 22k • 0.358
(50°C) = 7.9k
CTRL2(50°C) = 1.24/(1 + 16.9/7.9) = 395mV
Example:Calculatetheresistorvaluesrequiredforgenerat-
ing CTRL1 and CTRL2 from V based on the following
REF
CTRL2 slope (25°C to 50°C) = [CTRL2(50°C)
– CTRL2(25°C)]/25°C
requirements:
(a) I
(b) I
(c) I
= 700mA at 25°C
LED
LED
LED
= (395 – 701)/25
= –306mV/25°C
derating curve breakpoint occurs at 25°C
derating curve has a slope of –200mA/25°C be-
tween 25°C and 50°C ambient temperature
I
slope = –306mA/25°C
LED
The required I
slope is –200mA/25°C. To reduce the
LED
Step1: Choose CTRL1 = 700mV for I = 700mA
LED
slope of CTRL2 versus temperature it is easier to keep
the exact same NTC resistor value and B-constant (there
are limited choices) and simply adjust R4 and the type
of resistor network used for the CTRL2 pin. By changing
the resistor network to option C it is possible to place a
CTRL1 = V /(1 + R2/R1)
REF
R2 = R1 • [(V /CTRL1) – 1]
REF
For V = 1.24V and choosing R1 = 22.1k,
REF
temperature independent resistor in series with R
reduce the effects of R
temperature.
to
NTC
R2 = 22.1k [(1.24/0.7) – 1]
R2 = 17k (choose 16.9k)
on the CTRL2 pin voltage over
NTC
CTRL1 = 1.24/(1 + (16.9/22.1))
Step 4: Calculate the resistor value required for R in
Y
resistor network option (c) (Figure 7) to provide an I
LED
CTRL1 = 703mV (I
= 703mA)
LED
slope of –200mA/25°C between 25°C and 50°C ambient
temperature.
Step 2: Choose resistor network option A (Figure 7) and
CTRL2 = CTRL1 for 25°C breakpoint
CTRL2 (25°C) = 0.7V = 1.24/(1 + (R4/(R (25°C)+
NTC
start with R4 = R2 = 16.9k, R
available)
= 22k (closest value
NTC
R ))
Y
R4 = 0.77 (R (25°C) + R )
(a)
NTC
Y
CTRL2 = 701mV (I
701mA)
= Min(CTRL1, CTRL2) • 1A =
LED
for –200mA/25°C slope ≥ CTRL2(50°C) = 0.7 – 0.2 =
0.5
Step 3: Calculate CTRL2 slope between 25°C and 50°C
CTRL2 (T) = 1.24/(1 + R4/R (T))
CTRL2(50°C) = 0.5V = 1.24/(1 + (R4/(R
+ R ))
Y
NTC
NTC
R4 = 1.48 (R (50°C) + R )
(b)
NTC
Y
at T = T = 25°C, CTRL2 = 701mV
O
Equating (a) = (b) and knowing R (25°C) = 22k and
NTC
x
at T = 50°C, R
– 1/298)]
(T) = R
(T ).e , x = B [(1/(T + 273)
NTC
NTC O
R
(50°C) = 7.9k gives,
NTC
0.77 (22k + R ) = 1.48 (7.9k + R )
Y
Y
(B=B-constant;linearoverthe25°Cto50°Ctemperature
range)
17k + 0.77 R = 11.7 k + 1.48 R
Y
Y
R = (17k – 11.7k)/(1.48 – 0.77)
Y
For R
B-constant = 3950 and T = 50°C
NTC
R = 7.5k
Y
x = 3950 [(1/323) – 1/298] = –1.026
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The value for R4 can now be solved using equation (a)
where,
for the output LED(s) is programmed for a given bright-
ness/color and “chopped” over a PWM duty cycle range
(Figure 10) from 100% to as low as 0.033%.
R4 = 0.77 (R (25°C) + R ) = 0.77 (22k + 7.5k)
NTC
Y
D2
R4 = 22.7k (choose 22.6k)
C
OUT
I
I
slope can now be calculated from,
V
L
SW
LED
LED
S
V
IN
V
OUT
slope = [CTRL2(50°C) – CTRL2(25°C)]/25°C
SHDN
where CTRL2 (50°C) = 1.24/(1 + 22.6/(7.9 + 7.5)) =
503mV
(LT3478)
V
REF
LT3478/
R
SENSE
LT3478-1
CTRL2
CTRL1
and CTRL2 (25°C) = 1.24/(1 + 39.2/(22 + 28.7)) =
699mV
OVPSET
LED
D1
R
V
PWM
T
C
giving I
slope (from 25°C to 50°C)
LED
= 503mV – 699mV/25°C
= –196mV/25°C => I slope = –196mA/25°C
PWM DIMMING
CONTROL
3478 F09
LED
Figure 9. PWM Dimming Control Using the LT3478/LT3478-1
Using the Murata simulation tool for the resistor network
and values in the above example shows a CTRL2 volt-
age curve that flattens out as temperatures approach
100°C ambient. The final resistor network chosen for the
derating curve in Figure 6 used option C network with
T
PWM
PWM
(= 1/f
)
PWM
TON
PWM
R4 = 19.3k, R
= 22k (NCP15XW223J0SRC) and R
NTC
Y
= 3.01k. Although the CTRL2 downward slope is greater
than –200mA/25°C initially, the slope is required to avoid
exceedingmaximumallowedLEDcurrentsathighambient
temperatures (see Figure 6).
INDUCTOR
CURRENT
MAX I
LED
CURRENT
LED
3478 F10
PWM Dimming
Figure 10. PWM Dimming Waveforms Using the
LT3478/LT3478-1
Many LED applications require an accurate control of the
brightness of the LED(s). In addition, being able to main-
tain a constant color over the entire dimming range can
be just as critical. For constant color LED dimming, the
LT3478/LT3478-1 provide a PWM pin and special internal
circuitry to allow up to a 3000:1 wide PWM dimming
range. With an N-channel MOSFET connected between
the LED(s) and ground and a PWM signal connected to
the gate of the MOSFET and the PWM pin (Figure 9), it
is possible to control the brightness of the LED(s) based
on PWM signal duty cycle only. This form of dimming is
superior to dimming control using an analog input voltage
(reducingCTRL1voltage)becauseitallowsconstantcolor
to be maintained during dimming. The maximum current
Some general guidelines for LED Current Dimming using
the PWM pin (see Figure 10):
(1) PWM Dimming Ratio (PDR) = 1/(PWM duty cycle) =
1/(TON
• f
)
PWM PWM
(2) Lower f
allows higher PWM Dimming Ratios
= 100Hz to avoid visible flicker and
PWM
(use minimum f
PWM
to maximize PDR)
(3)Higherf valueimprovesPDR(allowslowerTON
)
PWM
OSC
but will reduce efficiency and increase internal heating. In
general, minimum operational TON = 3 • (1/f ).
PWM
OSC
(4) Lower inductor value improves PDR
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To limit inductor current overshoot to <0.5A when SS
charges past the V level required for loop control, the C
(5) Higher output capacitor value improves PDR
C
SS
(6) Choose the schottky diode (D2, Figure 9) for minimum
reverse leakage
capacitor should be chosen using the following formula:
C
= C (7.35 – 0.6(I • V /V ))
SS(MIN)
C
LED
OUT
S
See Typical Performance Characteristics graph “LED Cur-
rent vs PWM Duty Cycle”.
Example: V = 8V, V
= 16V, I
= 1.05A, C = 0.1µF,
S
OUT
LED C
C
= 0.1µF (7.35 – 0.6(1.05 • 16/8))
SS(MIN)
Soft-Start
= 0.612µF (choose 0.68µF).
To limit inrush current and output voltage overshoot dur-
ing startup/recovery from a fault condition, the LT3478/
LT3478-1 provide a soft-start pin SS. The SS pin is used
to program switch current ramp up timing using a ca-
pacitor to ground. The LT3478/LT3478-1 monitor system
High Inductor Current “Inrush” Protection
The LT3478/LT3478-1 provide an integrated resistor
between the V and L pins to monitor inductor current
S
(Figure 1). During startup or “hotplugging” of the induc-
tor supply, it is possible for inductor currents to exceed
the maximum switch current limit. When inductor current
exceeds 6A, the LT3478/LT3478-1 protect the internal
power switch by turning it off and triggering a soft-start
latch. Thisprotectionpreventstheswitchfromrepetitively
turning on during excessive inductor currents by delay-
ing switching until the fault has been removed. To defeat
inductor current sensing the inductor supply should be
parametersforthefollowingfaults:V <2.8V, SHDN<1.4,
IN
inductor current >6A and boosted output voltage >OVP.
On detection of any of these faults, the LT3478/LT3478-1
stop switching immediately and a soft-start latch is set
causing the SS pin to be discharged (see Timing Diagram
for the SS pin in Figure 11). When all faults no longer ex-
ist and the SS pin has been discharged to at least 0.25V,
the soft-start latch is reset and an internal 12µA supply
charges the SS pin. A gradual ramp up of SS pin voltage
is equivalent to a ramp up of switch current limit until SS
connected to the L pin and the V pin left open. See details
S
in the Applications Information section “Soft-Start”.
exceeds V .
C
LED Open Circuit Protection and Maximum PWM
Dimming Ratios
The ramp rate of the SS pin is given by:
ΔV /Δt = 12µA/C
SS
SS
The LT3478/LT3478-1 LED drivers provide optimum pro-
tection from open LED faults by clamping the converter
output to a programmable overvoltage protection level
(OVP). In addition, the programmable OVP feature draws
zero current from the output during PWM = 0 to allow
higher PWM dimming ratios. This provides an advantage
overotherLEDdriverapplicationswhichconnectaresistor
SW
SS
FAULTS TRIGGERING
SOFT-START LATCH
WITH SW TURNED OFF
IMMEDIATELY:
0.65V (ACTIVE THRESHOLD)
0.25V (RESET THRESHOLD)
0.15V
divider directly from V
.
OUT
V
< 2.8V OR
SHDN < 1.4V OR
IN
An open LED fault occurs when the connection to the
LED(s) becomes broken or the LED(s) fails open. For an
LED driver using a step-up switching regulator, an open
circuit LED fault can cause the converter output to exceed
the voltage capabilities of the regulator’s power switch,
SOFT-START LATCH RESET:
SS < 0.25V AND
> 2.8V AND
V
I
> OVP OR
OUT
> 6A
SOFT-START
LATCH SET:
(INDUCTOR)
V
IN
SHDN > 1.4V AND
V
I
< OVP AND
< 6A
OUT
3478 F11
(INDUCTOR)
Figure 11. LT3478 Fault Detection and SS Pin Timing Diagram
causingpermanentdamage. WhenV
exceedsOVP, the
OUT
34781f
16
LT3478/LT3478-1
U
W U U
APPLICATIO S I FOR ATIO
LT3478/LT3478-1immediatelystopswitching,asoft-start
V = inductor supply input
S
latch is set and the SS pin is discharged. The SS latch can
D = switch duty cycle = (V + V – V )/(V + V – V )
SAT
OUT
F
S
OUT
F
only be reset when V
falls below OVP and the SS pin
OUT
V = forward voltage drop of external Schottky diode
F
hasbeendischargedbelow0.25V(Figure11).IftheLED(s)
simply go open circuit and are reconnected, however, the
OVP used to protect the switch might be too high for the
reconnectedLED(s).TheLT3478/LT3478-1thereforeallow
OVP to be programmable to protect both the LED driver
switch and the LED(s). (The minimum allowable OVP for
normal operation for a given LED string depends on the
number of LEDs and their maximum forward voltage rat-
ings.) OVP is programmed using the OVPSET pin (front
page), given by,
V
= I
• R
L(AVE) SW
SAT
(2) Switch AC loss = P
SW(AC)
= t (1/2)I
(V
+ V )(F
)
EFF
L(AVE) OUT
F
OSC
t
= effective switch current and switch V voltage
CE
EFF
overlap time during turn on and turn off = 2 • (t
+
ISW
t
t
t
f
)
VSW
ISW
VSW
OSC
= I
rise/fall time = I
• 2ns
SWITCH
L(AVE)
OVP = (OVPSET • 41)V
= SW fall/rise time = (V
+ V ) • 0.7ns
OUT F
wheretheprogrammablerangefortheOVPSETpinis0.3V
to 1V resulting in an OVP range of 12.3V to 41V.
= switching frequency
(3) Current sensing loss = P
=
SENSE
TheOVPSETpincanbeprogrammedwithasingleresistor
P
+ P
SENSE(IL)
SENSE(ILED)
2
by tapping off of the resistor divider from V
used to
REF
P
P
= I
• 9.5mΩ
SENSE(IL)
L(AVE)
program CTRL1. If both CTRL1 and CTRL2 are connected
2
= I
• 0.1Ω
directlytoV (maximumLEDcurrentsetting)thenOVP-
SENSE(ILED)
LED
REF
SET requires a simple 2 resistor divider from V
.
REF
(4) Input quiescent loss = P = V • I where
Q
IN
Q
I = (6.2mA + (100mA • D))
Q
Thermal Calculations
Example (Using LT3478-1):
To maximize output power capability in an application
withoutexceedingtheLT3478/LT3478-1125°Cmaximum
operational junction temperature, it is useful to be able
to calculate power dissipation within the IC. The power
dissipation within the IC comes from four main sources:
switch DC loss, switch AC loss, Inductor and LED cur-
rent sensing and input quiescent current. These formulas
assume a boost converter architecture, continuous mode
operation and no PWM dimming.
For V = V = 8V, I
= 700mA, V
= 24.5V (7 LEDs),
IN
S
LED
= 0.2Mhz,
OUT
V = 0.5V and f
F
OSC
η = 0.89 (initial assumption)
= (24.5 • 0.7)/(0.89 • 8) = 2.41A
I
L(AVE)
D = (24.5 + 0.5 – 8)/(24.5 + 0.5 – 0.17) = 0.684
= 2 • ((2.41 • 2)ns + (24.5 + 0.5) • 0.7)ns = 45ns
T
EFF
(1) Switch DC loss = P
Total Power Dissipation:
SW(DC)
2
= (R • I
• D)
P = P
+ P
+ P
+ P
SW L(AVE)
IC
SW(DC)
SW(AC)
2
SENSE Q
R
= switch resistance = 0.07Ω (at T = 125°C)
P
P
P
= 0.07 • (2.41) • 0.684 = 0.278W
SW
J
SW(DC)
SW(AC)
I
= P /(η • V )
= 45ns • 0.5 • 2.41 • 25 • 0.2MHz = 0.271W
L(AVE)
OUT
S
2
2
P
= V
• I
= ((2.41) • 0.0095) + ((0.7) • 0.1) = 0.104W
SENSE
OUT
OUT LED
η = converter efficiency = P /(P
+ P
)
P = 8 • (6.2mA + (100mA • 0.684)) = 0.597W
Q
OUT OUT
LOSS
P = 0.278 + 0.271 + 0.104 + 0.597 = 1.25W
IC
34781f
17
LT3478/LT3478-1
U
W U U
APPLICATIO S I FOR ATIO
LocalheatingfromthenearbyinductorandSchottkydiode
will also add to the final junction temperature of the IC.
Based on empirical measurements, the effect of diode and
inductor heating on the LT3478-1 junction temperature
can be approximated as:
If an application is built, the inductor current can be mea-
suredandanewvalueforjunctiontemperatureestimated.
Ideally a thermal measurement should be made to achieve
the greatest accuracy for T .
J
Note: The junction temperature of the IC can be reduced
ΔT (LT3478-1) = 5°C/W • (P
+ P
)
if a lower V supply is available – separate from the
J
DIODE
= (1 – D) • V • I
F L(AVE)
INDUCTOR
IN
inductor supply V . In the above example, driving V
S
IN
P
DIODE
from an available 3V source (instead of V = 8V) reduces
S
1 – D = 0.316
V = 0.5V
input quiescent losses in item(4) from 0.597W to 0.224W,
resulting in a reduction of T from 118°C to 105°C.
J
F
I
= 2.41
Layout Considerations
L(AVE)
P
P
= 0.316 • 0.5 • 2.41 = 0.381W
As with all switching regulators, careful attention must be
given to PCB layout and component placement to achieve
optimal thermal,electrical and noise performance (Figure
12). The exposed pad of the LT3478/LT3478-1 (Pin 17)
is the only GND connection for the IC. The exposed pad
should be soldered to a continuous copper ground plane
underneath the device to reduce die temperature and
maximize the power capability of the IC. The ground path
DIODE
2
= I
• DCR
INDUCTOR
L(AVE)
DCR = inductor DC resistance (assume 0.05Ω)
2
P
= (2.41) • 0.05 = 0.29W
INDUCTOR
The LT3478/LT3478-1 use a thermally enhanced FE pack-
age. With proper soldering to the Exposed Pad on the
undersideofthepackagecombinedwithafullcopperplane
for the R resistor and V capacitor should be taken from
T
C
underneath the device, thermal resistance (θ ) will be
JA
A
nearby the analog ground connection to the exposed pad
(near Pin 9) separate from the power ground connection
to the exposed pad (near Pin 16). The bypass capacitor
about 35°C/W. For an ambient temperature of T = 70°C,
the junction temperature of the LT3478-1 for the example
application described above, can be calculated as:
for V should be placed as close as possible to the V
IN
IN
T (LT3478-1)
pinandtheanaloggroundconnection. SWpinvoltagerise
and fall times are designed to be as short as possible for
maximumefficiency.Toreducetheeffectsof bothradiated
and conducted noise, the area of the SW trace should be
kept as small as possible. Use a ground plane under the
switching regulator to minimize interplane coupling. The
schottky diode and output capacitor should be placed as
close as possible to the SW node to minimize this high
frequencyswitchingpath.TominimizeLEDcurrentsensing
errors for the LT3478, the terminals of the external sense
J
= T + θ (P ) + 5(P
+ P
)
A
JA TOT
DIODE
INDUCTOR
= 70 + 35(1.25) + 5(0.671)
= 70 + 44 + 4
= 118°C
In the above example, efficiency was initially assumed to
be η = 0.89. A lower efficiency (η) for the converter will
increase I
for T . η can be calculated as:
and hence increase the calculated value
L(AVE)
J
resistor R
should be tracked to the V
and LED
SENSE
OUT
pins separate from any high current paths.
η = P /(P
+ P
)
OUT OUT
LOSS
P
P
= V
• I
= 17.15W
OUT
OUT LED
(estimated) = P + P
+ P
= 1.92W
LOSS
IC
DIODE
INDUCTOR
η = 17.15/(17.15 + 1.92) = 0.9
34781f
18
LT3478/LT3478-1
U
W U U
APPLICATIO S I FOR ATIO
(CONNECT MULTIPLE GROUND PLANES
THROUGH VIAS UNDERNEATH THE IC)
V
S
C
VS
V
OUT
GND
OUTPUT CAPACITOR
V
IN
C
SCHOTTKY
DIODE
VIN
SOLDER THE EXPOSED PAD (PIN 17)
TO THE ENTIRE COPPER GROUND PLANE
UNDERNEATH THE DEVICE
LT3478/LT3478-1
POWER GND
C
SW
SW
1
2
3
4
5
6
7
8
16
15
14
13
12
11
10
9
SS
SS
SW
INDUCTOR
R
R
T
T
V
PWM
R
IN
V
S
CTRL2
CTRL1
SHDN
R
R
R
L
L
V
OUT
R
EXPOSED PAD
PIN 17
SENSE
LED
V
REF
(LT3478 ONLY)
C
R
OVPSET
V
C
F
ANALOG GND
C
C
C
C
V
BYPASS CAP
IN
R
R
3252 F08
Figure 12. Recommended Layout for LT3478/LT3478-1 (Boost Configuration)
U
TYPICAL APPLICATIO S
15W, 6 LEDs at 700mA, Boost LED Driver
L1
10µH
D1
V
IN
LT3478-1 PWM Dimming
Waveforms
8V TO 16V
C1
4.7µF
25V
C2
10µF
V
V
L
SW
IN
S
25V
SHDN
OUT
PWM
5V/DIV
V
REF
R1
45.3k
f
= 100Hz
PWM
700mA
CTRL2
LT3478-1
LED
INDUCTOR
CURRENT
1A/DIV
OVPSET
R4
54.9k
I
LED
CTRL1
PWM
0.5A/DIV
3478 TA02b
R2
130k
SS
V
R
T
C
2µs/DIV
PWM DIMMING RATIO = 1000:1
(SEE EFFICIENCY ON PAGE 1)
R
C
C
C
0.1µF
T
SS
L1: CDRH104R-100NC
D1: PDS560
69.8k
1µF
Q1: Si2318DS
f
= 500kHz
OSC
LEDs: LUXEON III (WHITE)
3.3V
0V
Q1
PWM
DIMMING RATIO = 1000:1
100Hz
R3
10k
3478 TA02a
34781f
19
LT3478/LT3478-1
U
TYPICAL APPLICATIO S
17W, 15 LEDs at 350mA, Boost LED Driver plus LT3003
V
S
8V TO 14V
C1
4.7µF
16V
L1
5.2µH
D1
V
OUT
Efficiency vs Input V
V
S
IN
3.3V
C3
3.3µF
10V
C2
3.3µF
25V
90
85
80
75
70
V
V
L
SW
IN
S
V
= 3.3V
= 350mA
= 1MHz
IN
I
f
LED
OSC
SHDN
OUT
PWM DUTY CYCLE = 100%
V
REF
1.05A
CTRL2
CTRL1
R1
24k
LT3478-1
LED
OVPSET
PWM
R2
100k
SS
V
R
T
C
C
V
SS
1µF
C
R
T
15 LEDs
(5 SERIES x 3 CHANNELS)
LUXEON I (WHITE)
C
C
31.6k
L1: CDRH104R-5R2
D1: PDS560
LEDs: LUXEON I (WHITE)
0.1µF
f
= 1MHz
OSC
8
10
12
14
V
(V)
S
3478 TA03b
3.3V
LED1
MAX
LED2
LED3
OT1
OT2
V
V
V
OUT
0V
LT3003
100Hz
PWM
DIMMING RATIO = 3000:1
V
IN
IN
V
C
PWM
GND
V
EE
3478 TA03a
16W, 12 LEDs at 350mA, Buck-Boost Mode LED Driver plus LT3003
V
S
12V TO 16V
C1
4.7µF
25V
L1
8.2µH
D1
V
OUT
Efficiency vs Input V
V
S
IN
5V
C3
3.3µF
10V
C2
10µF
50V
90
85
80
75
70
65
60
55
50
V
V
L
SW
IN
S
V
= 5V
= 350mA
= 500kHz
IN
I
f
LED
OSC
SHDN
OUT
PWM DUTY CYCLE = 100%
V
REF
1.05A
CTRL2
CTRL1
R1
24k
LT3478-1
LED
OVPSET
PWM
R2
100k
SS
V
R
T
C
C
V
SS
1µF
C
R
T
12 LEDs
(4 SERIES x 3 CHANNELS)
LUXEON I (WHITE)
C
C
69.8k
L1: CDRH105R-8R2
D1: PDS560
0.1µF
D2: 7.5V ZENER
f
= 500kHz
OSC
D2
12
13
14
(V)
15
16
LEDs: LUXEON I (WHITE)
C4
1µF
V
V
S
OUT
3478 TA04b
3.3V
0V
LED1
MAX
LED2
LED3
OT1
OT2
V
V
LT3003
100Hz
PWM
DIMMING RATIO = 200:1
IN
V
C
PWM
GND
V
EE
3478 TA04a
34781f
20
LT3478/LT3478-1
U
TYPICAL APPLICATIO S
4W, 1 LED at 1A, Buck-Boost Mode LED Driver
V
IN
3.8V TO 6.5V
C1
L1
6.8µH
D1
NiMH 4× 10µF
Efficiency vs V
IN
10V
80
75
70
65
60
55
50
C2
I
f
= 1A
LED
OSC
V
V
L
SW
IN
S
4.7µF
= 500kHz
16V
ON OFF
PWM DUTY CYCLE = 100%
SHDN
OUT
V
REF
Q2
1A
CTRL2
CTRL1
R1
LT3478-1
LED
100k
R4
510Ω
OVPSET
PWM
R2
34k
L1: CDRH105R-6R8
D1: B320
Q1: Si2302ADS
Q2: Si2315BDS
LED: LUXEON III (WHITE)
SS
V
R
T
C
C
SS
1µF
R
T
R5
510Ω
SINGLE LED
C
C
69.8k
LUXEON III (WHITE)
0.1µF
3
4
5
6
7
3.3V
0V
f
= 500kHz
OSC
V
(V)
IN
3478 TA06b
Q1
PWM
DIMMING RATIO = 200:1
1kHz
R3
10k
3478 TA06a
34781f
21
LT3478/LT3478-1
U
TYPICAL APPLICATIO S
24W, 4 LEDs at 1.5A, Buck Mode LED Driver
PV
IN
32V
C1
R
3.3µF
SENSE
0.068Ω
50V
1.5A
4 LEDs
R4
365Ω
TYPICAL EFFICIENCY = 90%
FOR CONDITIONS/COMPONENTS SHOWN
(PWM DUTY CYCLE = 100%, T =25°C)
C3
A
10µF
25V
Q2
L1
10µH
R5
510Ω
V
IN
3.3V
C2
4.7µF
10V
D1
V
V
L
OUT LED SW
PWM
IN
S
Q1
SHDN
R3
10k
V
REF
LT3478
CTRL2
CTRL1
R1
24k
PWM
DIMMING RATIO = 3000:1
OVPSET
L1: CDRH105R-100
D1: PDS560
3.3V
R2
100k
Q1: 2N7002
SS
V
C
R
T
0V
100Hz
Q2: Si2319DS
LEDs: LXK2 (WHITE)
C
SS
R
T
1µF
C
C
69.8k
0.1µF
f
= 500kHz
OSC
3478 TA07a
34781f
22
LT3478/LT3478-1
U
PACKAGE DESCRIPTIO
FE Package
16-Lead Plastic TSSOP (4.4mm)
(Reference LTC DWG # 05-08-1663)
Exposed Pad Variation BC
4.90 – 5.10*
(.193 – .201)
3.58
(.141)
3.58
(.141)
16 1514 13 12 1110
9
6.60 0.10
4.50 0.10
2.94
(.116)
6.40
2.94
SEE NOTE 4
(.252)
(.116)
BSC
0.45 0.05
1.05 0.10
0.65 BSC
5
7
8
1
2
3
4
6
RECOMMENDED SOLDER PAD LAYOUT
1.10
(.0433)
MAX
4.30 – 4.50*
(.169 – .177)
0.25
REF
0° – 8°
0.65
(.0256)
BSC
0.09 – 0.20
(.0035 – .0079)
0.50 – 0.75
(.020 – .030)
0.05 – 0.15
(.002 – .006)
FE16 (BC) TSSOP 0204
0.195 – 0.30
(.0077 – .0118)
TYP
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS 4. RECOMMENDED MINIMUM PCB METAL SIZE
FOR EXPOSED PAD ATTACHMENT
*DIMENSIONS DO NOT INCLUDE MOLD FLASH. MOLD FLASH
SHALL NOT EXCEED 0.150mm (.006") PER SIDE
MILLIMETERS
(INCHES)
2. DIMENSIONS ARE IN
3. DRAWING NOT TO SCALE
34781f
Information furnished by Linear Technology Corporation is believed to be accurate and reliable.
However, no responsibility is assumed for its use. Linear Technology Corporation makes no representa-
tion that the interconnection of its circuits as described herein will not infringe on existing patent rights.
23
LT3478/LT3478-1
U
TYPICAL APPLICATIO
6W, 6 LEDs at 250mA, Boost LED Driver
V
S
8V TO 16V
C1
4.7µF
25V
L1
10µH
D1
Efficiency vs Input V
V
S
IN
3.3V
C3
3.3µF
10V
C2
3.3µF
25V
100
95
90
85
80
75
70
65
60
V
V
L
SW
IN
S
V
= 3.3V
= 250mA
= 2MHz
IN
I
f
LED
OSC
SHDN
OUT
R
SENSE
PWM DUTY CYCLE = 100%
V
REF
0.42Ω
CTRL2
CTRL1
R1
8.25k
LT3478
250mA
LED
OVPSET
PWM
R2
10k
SS
V
R
T
C
C
SS
R
T
1µF
C
C
10k
L1: CDRH6D28
D1: ZLLS1000
Q1: Si2318DS
0.1µF
6 LEDs = LUXEON I (WHITE)
12 14 16
(V)
f
= 2MHz
OSC
8
10
LEDs: LUXEON I (WHITE)
V
S
3478 TA05b
3.3V
0V
Q1
PWM
DIMMING RATIO = 1000:1
100Hz
R3
10k
3478 TA05a
RELATED PARTS
PART NUMBER DESCRIPTION
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IN
OUT(MAX)
DFN, TSSOP16E Packages
34781f
LT 0107 • PRINTED IN USA
LinearTechnology Corporation
1630 McCarthy Blvd., Milpitas, CA 95035-7417
24
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© LINEAR TECHNOLOGY CORPORATION 2007
(408) 432-1900 FAX: (408) 434-0507 www.linear.com
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