LINER LT3478EFE 4.5a monolithic led drivers with true color pwm dimming Datasheet

LT3478/LT3478-1
4.5A Monolithic LED
Drivers with True Color
PWM Dimming
DESCRIPTIO
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FEATURES
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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)/RSENSE (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
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APPLICATIO S
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High Power LED Driver
Automotive Lighting
The LT®3478/LT3478-1 are 4.5A step-up DC/DC converters designed to drive LEDs with a constant current over
a wide programmable range. Series connection of the
LEDs provides identical LED currents for uniform brightness 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.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners. Patents Pending.
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TYPICAL APPLICATIO
Automotive TFT LCD Backlight
VIN 8V TO 16V
VIN
VS
L
10µF
SW
OUT
VREF
45.3k
CTRL2
LT3478-1
0.1Ω
RSENSE
(LT3478)
LED
OVPSET
54.9k
90
85
CTRL1
130k
ILED = 700mA
fOSC = 500kHz
PWM DUTY CYCLE = 100%
95
SHDN
EFFICIENCY (%)
4.7µF
Efficiency vs VIN
100
10µH
PWM
1µF
SS
700mA
15W
6 LEDs
(WHITE)
RT
VC
0.1µF
69.8k
PWM DIMMING
CONTROL
6 LEDs LUXEON III (WHITE)
80
8
10
12
VIN (V)
14
16
3478 TA01b
3478 TA01
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LT3478/LT3478-1
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ABSOLUTE
AXI U RATI GS
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PACKAGE/ORDER I FOR ATIO
(Note 1)
SW ............................................................................42V
VOUT, LED ..................................................................42V
VIN, VS, VL, ⎯S⎯H⎯D⎯N (Note 5) .......................................36V
PWM .........................................................................15V
CTRL1, 2 .....................................................................6V
SS, RT, VC, VREF, 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
TOP VIEW
SW
1
16 SS
SW
2
15 RT
VIN
3
14 PWM
VS
4
L
5
12 CTRL1
VOUT
6
11 SHDN
LED
7
10 VREF
OVPSET
8
9
17
13 CTRL2
VC
FE PACKAGE
16-LEAD PLASTIC TSSOP
TJMAX = 125°C, θJA = 35°C/W
EXPOSED PAD (PIN 17) IS PGND, MUST BE SOLDERED TO PCB.
ORDER PART NUMBER
FE PART MARKING
LT3478EFE
LT3478EFE-1
LT3478IFE
LT3478IFE-1
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.
ELECTRICAL CHARACTERISTICS
The ● denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at TA = 25°C. SW = open, VIN = VS = L = VOUT = ⎯S⎯H⎯D⎯N = 2.7V, LED = open, SS = open,
PWM = CTRL1, CTRL2 = 1.25V, VREF = open, VC = open, RT = 31.6k.
PARAMETER
CONDITIONS
Minimum Operating Voltage
(Rising)
Operational Input Voltage
VS
VIN (Note 5)
VIN Quiescent Current
VC = 0V (No Switching)
VIN Shutdown Current
⎯S⎯H⎯D⎯N = 0V
⎯S⎯H⎯D⎯N Pin Threshold (VSD_µp)
(Micropower)
⎯S⎯H⎯D⎯N Pin Threshold (VSD_UVLO)
(Switching)
⎯S⎯H⎯D⎯N Pin Current
⎯S⎯H⎯D⎯N = VSD_UVLO – 50mV
⎯S⎯H⎯D⎯N = VSD_UVLO + 50mV
VREF Voltage
I(VREF) = 0µA, VC = 0V
VREF Line Regulation
I(VREF) = 0µA, 2.7V < VIN < 36V
VREF Load Regulation
0 < I(VREF) < 100µA (Max)
Frequency: fOSC 200kHz
RT = 200k
Frequency: fOSC 1MHz
RT = 31.6k
MIN
●
TYP
MAX
2.4
2.8
V
36
36
V
V
2.8
2.8
6.1
●
●
mA
3
6
µA
0.1
0.4
0.7
V
1.3
1.4
1.5
V
8
10
0
12
µA
µA
1.213
0.18
●
UNITS
0.88
1.240
1.263
V
0.005
0.015
%/V
8
12
mV
0.2
0.22
MHz
1.12
MHz
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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 TA = 25°C. SW = open, VIN = VS = L = VOUT = ⎯S⎯H⎯D⎯N = 2.7V, LED = open, SS = open,
PWM = CTRL1, CTRL2 = 1.25V, VREF = open, VC = open, RT = 31.6k.
PARAMETER
CONDITIONS
Frequency: fOSC 2.25MHz
RT = 9.09k
Line Regulation fOSC
RT = 31.6k, 2.7V < VIN < 36V
MIN
TYP
MAX
2
2.25
2.6
MHz
0.05
0.2
%/V
Nominal RT Pin Voltage
UNITS
0.64
V
88
97
73
%
%
%
(Note 6)
770
µA/A
(Note 6)
400
V/A
13
A/V
Maximum Duty Cycle
RT = 31.6k
RT = 200k
RT = 9.09k
LED Current to VC Current Gain
LED Current to VC Voltage Gain
●
80
VC to Switch Current Gain
VC Source Current (Out of Pin)
CTRL1 = 0.4V, VC = 1V
40
µA
VC Sink Current
CTRL1 = 0V, VC = 1V
40
µA
VC Switching Threshold
0.65
V
VC High Level (VOH)
CTRL1 = 0.4V
1.5
V
VC Low Level (VOL)
CTRL1 = 0V
0.2
V
Inductor Current Limit
2.7V < VS < 36V
Switch Current Limit
Switch VCE SAT
ISW = 4.5A
Switch Leakage Current
SW = 42V, VC = 0V
VOUT Overvoltage Protection (OVP)
(Rising)
OVPSET = 1V
OVPSET = 0.3V
Full Scale LED Current (LT3478-1)
CTRL1 = VREF, Current Out of LED Pin
700mA LED Current (LT3478-1)
CTRL1 = 700mV, Current Out of LED Pin
350mA LED Current (LT3478-1)
100mA LED Current (LT3478-1)
●
4.5
●
4.5
6
6.8
6.3
7.5
A
A
270
mV
1
µA
41
12.3
V
V
1010
1050
1090
mA
655
700
730
mA
CTRL1 = 350mV, Current Out of LED Pin
325
350
375
mA
CTRL1 = 100mV, Current Out of LED Pin
70
100
130
mA
101
105
109
mV
67
70.5
74
mV
●
Full Scale LED Current VSENSE (LT3478) CTRL1 = VREF, VSENSE = VVOUT – VLED
●
CTRL1 = 700mV, VSENSE (LT3478)
CTRL1 = 700mV, VSENSE = VVOUT – VLED
CTRL1 = 350mV, VSENSE (LT3478)
CTRL1 = 350mV, VSENSE = VVOUT – VLED
33
35.5
38
mV
CTRL1 = 100mV, VSENSE (LT3478)
CTRL1 = 100mV, VSENSE = VVOUT – VLED
7
10
13
mV
CTRL1, 2 Input Currents
CTRL1 = 100mV, CTRL2 = 1.25V or
CTRL2 = 100mV, CTRL1 = 1.25V (Current Out of Pin)
40
nA
OVPSET Input Current
OVPSET = 1V, VOUT = 41V (Current Out of Pin)
200
nA
PWM Switching Threshold
0.8
1
1.2
V
VC Pin Current in PWM Mode
VC = 1V, PWM = 0
1
50
nA
OUT Pin Current in PWM Mode
PWM = 0
1
100
nA
SS Low Level (VOL)
I(SS) = 20µA
0.15
V
SS Reset Threshold
VC = 0V
0.25
V
SS High Level (VOH)
VC = 0V
1.5
V
Soft-Start (SS) Pin Charge Current
SS = 1V, Current Out of Pin, VC = 0V
12
µA
Soft-Start (SS) Pin Discharge Current
SS = 0.5V, VC = 0V
350
µA
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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 VIN is limited by thermal and
efficiency considerations. Power switch base current is delivered from VIN
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 • (RSENSE/0.1Ω).
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TYPICAL PERFOR A CE CHARACTERISTICS
LED Current vs CTRL1
1400
LED Current vs Temperature
1400
TA = 25°C
CTRL2 = VREF
(FOR LT3478 SCALE BY 0.1Ω/RSENSE)
LT3478-1
700
LED CURRENT (mA)
1050
LED CURRENT (mA)
LED CURRENT (mA)
1000
(FOR LT3478 SCALE BY 0.1Ω/RSENSE)
ILED = 1050mA, CTRL1 = CTRL2 = VREF
1050
LT3478-1
700
ILED = 100mA, CTRL1 = 100mV,
CTRL2 = VREF
VREF
0
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
0
0
0.35
0.70
CTRL1 (V)
1.05
1.40
240
50
CTRL1 = 0.1V
CTRL1 = 0.7V
CTRL1 = 0.9V
0
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
7.0
TA = 25°C
6.5
CURRENT LIMIT (A)
SWITCH VCE (SAT) (mV)
CTRL1 = 0.35V
180
120
60
3478 G04
6.0
SWITCH
INDUCTOR
5.5
5.0
0
125
100
Switch and Inductor Peak Current
Limits vs Temperature
210
30
0.1
1
10
PWM DUTY CYCLE (%)
3478 G03
Switch VCE (SAT) vs Switch
Current
CTRL1 Pin Current vs
Temperature
20 CTRL2 = VREF
CTRL1 AND CTRL2 PINS
INTERCHANGEABLE
10
0
0.01
125
3478 G02
3478 G01
40
TA = 25°C
VIN = VS = 12V
6 LEDS AT 500mA
PWM FREQ = 100Hz
100
CTRL1 = 0.5V
CTRL2 = VREF
FOSC = 1.6MHz
L = 2.2µH
10
1
350
350
CTRL1 PIN CURRENT X (–1) (nA)
LED Current vs PWM Duty Cycle
Wide PWM Dimming Range
(3000:1)
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 G06
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LT3478/LT3478-1
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TYPICAL PERFOR A CE CHARACTERISTICS
1.60
1.50
1.24
SHDN (V)
VREF (V)
1.26
15
SHDN PIN CURRENT (µA)
1.28
1.22
1.40
1.30
1.20
1.18
–50 –25
0
25
50
75 100
JUNCTION TEMPERATURE (°C)
125
1.20
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
3478 G07
JUST BEFORE PART TURNS ON
10
5
AFTER PART TURNS ON
0
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
125
3478 G08
VIN Shutdown Current vs
Temperature
50
⎯S⎯H⎯D⎯N Pin (Hysteresis) Current vs
Temperature
⎯S⎯H⎯D⎯N Threshold vs Temperature
VREF vs Temperature
3478 G09
VIN Quiescent Current vs
Temperature
VIN Quiescent Current vs VIN
SHDN = 0V
125
14
14
12
12
10
10
20
VIN = 36V
VIN = 20V
VIN CURRENT (mA)
VIN CURRENT (mA)
30
8
6
4
8
6
4
10
2
VIN = 2.8V
0
–50 –25
0
25
50
75 100
JUNCTION TEMPERATURE (°C)
0
125
0
3
6
9 12 15 18 21 24 27 30 33 36
VIN (V)
VIN = 2.8V
V = 0V
0 C
–50 –25
0
25
50
75 100
JUNCTION TEMPERATURE (°C)
3478 G11
VS, L, SW Shutdown Currents vs
Temperature
125
3478 G12
Switch Peak Current Limit
vs Duty Cycle
7
SHDN = 0V
VS = L = SW = 36V
SWITCH PEAK CURRENT LIMIT (A)
4
2
TA= 25°C
VC = 0V
3478 G10
PIN CURRENT (µA)
VIN CURRENT (µA)
40
2
I(VS PIN) = I(L PIN)
I(SW PIN)
0
–50 –25
0
25
50
75 100
JUNCTION TEMPERATURE (°C)
6
5
4
3
2
1
0
125
3478 G18
TA= 25°C
0
20
40
60
DUTY CYCLE (%)
80
100
3478 G19
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LT3478/LT3478-1
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TYPICAL PERFOR A CE CHARACTERISTICS
Switching Frequency vs
Temperature
Switching Frequency vs RT
1.20
1000
100
1
10
100
1000
RT (kΩ)
43.0
RT = 31.6k
1.15
42.5
1.10
42.0
VOUT CLAMP (V)
TA = 25°C
SWITCHING FREQUENCY (MHz)
SWITCHING FREQUENCY (kHz)
10000
Open-Circuit Output Clamp
Voltage vs Temperature
1.05
1.00
0.95
41.0
40.5
40.0
0.85
39.5
3478 G13
125
39.0
0
25
–50 –25
50
75 100
JUNCTION TEMPERATURE (°C)
3478 G14
125
3478 G15
VC Pin Active and Clamp Voltages
vs Temperature
SS Pin Charge Current vs
Temperature
14
1.8
1.5
VC CLAMP
13
1.2
VC (V)
SS PIN CURRENT (µA) (OUT OF PIN)
41.5
0.90
0.80
0
25
–50 –25
50
75 100
JUNCTION TEMPERATURE (°C)
OVPSET = 1V
12
0.9
0.6
VC ACTIVE THRESHOLD
11
0.3
10
–50 –25
50
75 100
0
25
JUNCTION TEMPERATURE (°C)
125
3478 G16
0
0
25
–50 –25
50
75 100
JUNCTION TEMPERATURE (°C)
125
3478 G17
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LT3478/LT3478-1
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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.
VIN (Pin 3): Input Supply. Must be locally bypassed with
a capacitor to ground.
VS (Pin 4): Inductor Supply. Must be locally bypassed
with a capacitor to ground. Can be shorted to VIN if only
one supply is available (see L (Pin 5) function).
L (Pin 5): Inductor Pin. An internal resistor between VS
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 connecting the inductor power supply directly to the L pin and
leaving the VS pin open (requires local bypass capacitor
to GND on L pin; not VS pin).
VOUT (Pin 6): Output voltage of the converter. Connect a
capacitor from this pin to ground. Internal circuitry monitors VOUT for protection against open LED faults.
LED (Pin 7): Connect the LED string from this pin to
ground. An internal (LT3478-1)/external (LT3478) resistor
between the VOUT and LED pins senses LED current for
accurate control.
OVPSET (Pin 8): Programs VOUT overvoltage protection
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.
VC (Pin 9): Output of the transconductance error amplifier
and compensation pin for the converter regulation loop.
VREF (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.
⎯ ⎯H⎯D⎯N (Pin 11): The ⎯S⎯H⎯D⎯N pin has an accurate 1.4V
S
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
hysteresis allows programming of undervoltage lockout
(UVLO) hysteresis. ⎯S⎯H⎯D⎯N above 1.4V turns the part on
and removes a 10µA sink current from the pin. ⎯S⎯H⎯D⎯N = 0V
⎯ H
⎯ D
⎯ N
⎯ can be directly connected
reduces VIN current < 3µA. S
to VIN. If left open circuit the part will be turned off.
CTRL1 (Pin 12): CTRL1 pin voltage is used to program
maximum LED current (CTRL2 = VREF). CTRL1 voltage
can be set by a resistor divider from VREF or an external
voltage source. Maximum LED current is given by:
(LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps
(LT3478) Max LED Current =
0.1
Min(CTRL, 1.05) •
Amps
RSENSE
(linear for 0.1V < CTRL1< 0.95V ; CTRL2 = VREF) For maximum LED current, short CTRL1 and CTRL2 pins to VREF.
CTRL2 (Pin 13): The CTRL2 pin is available for programming a decrease in LED current versus temperature
(setting temperature breakpoint and slope). This feature
allows the output LED(s) to be programmed for maximum
allowable current without damage at higher temperatures.
This maximizes LED usage and increases reliability. A
CTRL2 voltage with negative temperature coefficient is
created using an external resistor divider from VREF with
temperature dependant resistance. If not used, CTRL2
should be tied to VREF.
PWM (Pin 14): Input pin for PWM dimming control. Above
1V allows converter switching and below 1V disables
switching with VC pin level maintained. With an external
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
be connected to VREF.
RT (Pin 15): A resistor to ground programs switching
frequency between 200kHz and 2.25MHz.
SS (Pin 16): Soft-Start Pin. Placing a capacitor here programs soft-start timing to limit inductor inrush current
during start-up due to the converter. When inductor current
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LT3478/LT3478-1
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PI FU CTIO S
exceeds 6A or VOUT exceeds OVP, an internal soft-start
latch is set, the power NPN is immediately turned off and
the SS pin is discharged. The soft-start latch is also set
if VIN and/or ⎯S⎯H⎯D⎯N do not meet their turn on thresholds.
The SS pin only recharges when all faults are removed
and the pin has been discharged below 0.25V.
Exposed Pad (Pin 17): The ground for the IC and the converter. The FE package has an Exposed Pad underneath the
IC which is the best path for heat out of the package. Pin 17
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.
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BLOCK DIAGRA
SHDN
L
VS
11
4
SS
5
10µA
9.5mΩ
+
–
+
1.4V
VIN
REF
1.24V
3
1, 2
VOUT
VC
–
UVLO
SW
16
6
OVERVOLTAGE
DETECT
–
57mV
OVPSET
INRUSH
CURRENT
PROTECTION
+
100Ω
RSENSE
0.1Ω
(INTERNAL FOR
LT3478-1)
SOFT-START
RSENSE
(EXTERNAL FOR
LT3478)
LED
7
PWM
DETECT
VREF
10
OSC
S
Q
Q1
R
LED
1.05V
+
+
+
–
13
GM
LED
–
+
CTRL2
LED
SLOPE
COMP
Q2
–
12
LED
+
–
PWM
CTRL1
Σ
1V
PWM
14
+
+
1000Ω
RS
–
TO OVERVOLTAGE
DETECT CIRCUIT
8
15
OVPSET
17
EXPOSED PAD
(GND)
RT
9
3478 F01
VC
Figure 1
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LT3478/LT3478-1
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OPERATIO
The LT3478/LT3478-1 are high powered LED drivers with
a 42V, 4.5A internal switch and the ability to drive LEDs
with up to 1050mA for LT3478-1 and up to 105mV/RSENSE
for LT3478.
the VC voltage controls the peak switch current limit and
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.
The LT3478/LT3478-1 work similarly to a conventional
current mode boost converter but use LED current (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
in LED current versus temperature for maximum reliability
and utilization of the LED(s). A CTRL2 voltage with negative
temperature coefficient can be created using an external
resistor divider from VREF with temperature dependant
resistance. Unused CTRL2 is tied to VREF.
For the part to turn on, the VIN pin must exceed 2.8V and
the ⎯S⎯H⎯D⎯N pin must exceed 1.4V. The ⎯S⎯H⎯D⎯N 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 ⎯S⎯H⎯D⎯N pin
before part turn on and is removed after part turn on. This
current hysteresis allows programming of hysteresis for
the UVLO threshold. See “Shutdown Pin and Programming
Undervoltage Lockout” in the Applications Information
Section. For micropower shutdown the ⎯S⎯H⎯D⎯N pin at 0V
reduces VIN supply current to approximately 3µA.
Each LED driver is a current mode step-up switching regulator. A regulation point is achieved when the
boosted output voltage VOUT across the output LED(s) is
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.
The basic loop uses a pulse from an internal oscillator to set
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 True Color PWM dimming, the LT3478/LT3478-1
provide up to a 3000:1 wide PWM dimming range by allowing 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.
For robust operation, the LT3478/LT3478-1 monitor system
performance for any of the following faults : VIN or ⎯S⎯H⎯D⎯N
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
12µA supply charges the SS pin with a timing programmed
using a single external capacitor. A gradual ramp up of SS
pin voltage limits switch current during startup.
For optimum component sizing, duty cycle range and efficiency the LT3478/LT3478-1 allow for a separate inductor
supply VS and for switching frequency to be programmed
from 200kHz up to 2.25MHz using a resistor from the RT
pin to ground. The advantages of these options are covered
in the Applications Informations section.
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Inductor Selection
Capacitor Selection
Several inductors that work well with the LT3478/LT3478-1
are listed in Table 1. However, there are many other manufacturers and inductors that can be used. Consult each
manufacturer for more detailed information and their entire
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)
to minimize I2R power losses. Values between 4.7µH and
22µH will suffice for most applications.
Low ESR (equivalent series resistance) ceramic capacitors 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.
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
exceed the maximum current rating of the inductor to save
space. Consult each manufacturer to determine how the
maximum inductor current is measured and how much
more current the inductor can reliably conduct.
Schottky diodes, with their low forward voltage drop and
fast switching speed, are ideal for LT3478/LT3478-1 applications. Table 3 lists several Schottky diodes that work
well. The diode’s average current rating must exceed the
application’s average output current. The diode’s maximum
reverse voltage must exceed the application’s output voltage. 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.
Diode Selection
Table 1. Suggested Inductors
MANUFACTURER PART NUMBER
CDRH104R-100NC
CDRH103RNP-4R7NC-B
CDRH124R-100MC
CDRH104R-5R2NC
FDV0630-4R7M
IDC (A)
3.8
4
4.5
5.5
4.2
INDUCTANCE (µH)
10
4.7
10
5.2
4.7
MAX DCR (mΩ)
35
30
28
22
49
L × W × H (mm)
10.5 × 10.3 × 4.0
10.5 × 10.3 × 3.1
12.3 × 12.3 × 4.5
10.5 × 10.3 × 4.0
7.0 × 7.7 × 3.0
UP4B-220
7.6
22
34
22 × 15 × 7.9
MANUFACTURER
Sumida
www.sumida.com
Toko
www.toko.com
Cooper
www.cooperet.com
Table 2. Ceramic Capacitor Manufacturers
MANUFACTURER
Taiyo Yuden
AVX
Murata
PHONE NUMBER
(408) 573-4150
(803) 448-9411
(714) 852-2001
WEB
www.t-yuden.com
www.avxcorp.com
www.murata.com
Table 3. Suggested Diodes
MANUFACTURER PART NUMBER
UPS340
MAX CURRENT (A)
3
MAX REVERSE VOLTAGE
40
B520C
B530C
B340A
B540C
PDS560
5
5
3
5
5
30
30
40
40
60
WEB
Microsemi
www.microsemi.com
Diodes, Inc.
www.diodes.com
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Shutdown and Programming Undervoltage Lockout
Programming Switching Frequency
The LT3478/LT3478-1 have an accurate 1.4V shutdown
threshold at the ⎯S⎯H⎯D⎯N 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). ⎯S⎯H⎯D⎯N pin current
hysteresis allows programming of hysteresis voltage for
this UVLO threshold. Just before part turn on, 10µA flows
into the ⎯S⎯H⎯D⎯N pin. After part turn on, 0µA flows from the
⎯ H
⎯ D
⎯ N
⎯ pin. Calculation of the on/off thresholds for a system
S
input supply using the LT3478/LT3478-1 ⎯S⎯H⎯D⎯N pin can
be made as follows:
The switching frequency is programmed using an external
resistor (RT) connected between the RT pin and ground. The
internal free-running oscillator is programmable between
200kHz and 2.25MHz. Table 4 shows the typical RT values
required for a range of switching frequencies.
VSUPPLY OFF = 1.4 [1 + R1/R2)]
VSUPPLY ON = VSUPPLY OFF + (10µA • R1)
An open drain transistor can be added to the resistor
divider network at the ⎯S⎯H⎯D⎯N pin to independently control
the turn off of the LT3478/LT3478-1.
Selecting the optimum switching frequency depends
on several factors. Inductor size is reduced with higher
frequency but efficiency drops due to higher switching
losses. In addition, some applications require very high duty
cycles to drive a large number of LEDs from a low supply.
Low switching frequency allows a greater operational duty
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.
VSUPPLY
10000
11
R2
SHDN
SWITCHING FREQUENCY (kHz)
R1
–
1.4V
OFF ON
+
10µA
TA = 25°C
1000
3478 F02
100
Figure 2. Programming Undervoltage Lockout (UVLO)
with Hysteresis
With the ⎯S⎯H⎯D⎯N pin connected directly to the VIN pin, an
internal undervoltage lockout threshold exists for the VIN
pin (2.8V max). This prevents the converter from operating in an erratic mode when supply voltage is too low.
The LT3478/LT3478-1 provide a soft-start function when
recovering from such faults as ⎯S⎯H⎯D⎯N <1.4V and/or VIN
<2.8V. See details in the Applications Information section
“Soft-Start”.
1
10
100
1000
RT (kΩ)
3478 F03
Figure 3. Switching Frequency vs RT Resistor Value
Table 4. Switching Frequencies vs RT Values
SWITCHING FREQUENCY (MHz)
RT (kΩ)
2.25
9.09
1
31.6
0.2
200
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Programming Maximum LED current
Maximum LED current can be programmed using the CTRL1
pin with CTRL2 tied to the VREF pin (see Figures 4 and 5).
The maximum allowed LED current is defined as:
maximum allowed LED current versus temperature to
warn against exceeding this current limit and damaging
the LED (Figure 6).
Luxeon V (Maximum) and LT3478-1
(Programmed) Current Derating
Curves vs Temperature
(LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps
(LT3478) Max LED Current =
0.1
Min(CTRL1, 1.05) •
Amps
RSENSE
900
LED current vs CTRL1 is linear for approximately
0.1V < CTRL1 < 0.95V
For maximum possible LED current, connect CTRL1 and
CTRL2 to the VREF pin.
If FORWARD CURRENT (mA)
800
700
LUXEON V EMITTER
CURRENT DERATING
CURVE
600
500
EXAMPLE
LT3478-1
PROGRAMMED LED
CURRENT DERATING CURVE
400
300
200
100
0
0
1400
TA = 25°C
CTRL2 = VREF
(FOR LT3478 SCALE
BY 0.1Ω/RSENSE)
LED CURRENT (mA)
1050
LUXEON V EMITTER
(GREEN, CYAN, BLUE, ROYAL BLUE)
θJA = 20°C/W
350
VREF
0
0.35
0.70
CTRL1 (V)
1.05
1.40
3478 F04
Figure 4. LED Current vs CTRL1 Voltage
LT3478/LT3478-1
10
R2
13
12
R1
VREF
(LT3478)
VOUT
RSENSE
CTRL2
CTRL1
100
3478 F06
Figure 6. LED Current Derating Curve vs Ambient Temperature
LT3478-1
700
0
50
75
25
TA AMBIENT TEMPERATURE (°C)
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
Without the ability to back off LED current as temperature
increases, many LED drivers are limited to driving the
LED(s) at only 50% or less of their maximum rated currents.
This limitation requires more LEDs to obtain the intended
brightness for the application. The LT3478/LT3478-1 allow 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 negative temperature coefficient using a resistor divider with
temperature dependent resistance (Figures 7 and 8).
CTRL2 voltage is programmed higher than CTRL1 voltage.
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).
A variety of resistor networks and NTC resistors with different temperature coefficients can be used for programming
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CTRL2 to achieve the desired CTRL2 curve vs temperature.
The current derating curve shown in Figure 6 uses the
resistor network shown in option C of Figure 7.
10
R2
R4
13
12
R1
VREF
LT3478/LT3478-1
CTRL2
CTRL1
Table 5. NTC Resistor Manufacturers/Distributors
OPTION A TO D
R3
MANUFACTURER
RY
RNTC
RNTC
A
RX RNTC
B
RY
RNTC
C
D
Figure 7. Programming LED Current Derating Curve
vs Temperature (RNTC Located on LEDs PCB)
CTRL1, CTRL2 PIN VOLTAGES (mV)
1100
1000
900
700
CTRL1
600
500
400
CTRL2
300
200
LED CURRENT = MINIMUM
100 OF CTRL1, CTRL2
R3 = OPTION C
0
0
25
50
75
TA AMBIENT TEMPERATURE (°C)
Murata Electronics North America
www.murata.com
TDK Corporation
www.tdk.com
Digi-key
www.digikey.com
RX
3478 F07
800
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
resulting CTRL2 curve plotted versus temperature. Several
iterations of resistor value calculations may be required
to achieve the desired breakpoint and slope of the LED
current derating curve.
100
3478 F08
Figure 8. CTRL1, 2 Programmed Voltages vs Temperature
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 connected 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 nonlinear over a wide range of temperatures it is important
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.
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
to select from different resistor networks and NTC resistor
values and then simulate the exact output voltage curve
(CTRL2 behavior) over temperature. Referred to as the
‘Murata Chip NTC Thermistor Output Voltage Simulator’,
users can log onto www.murata.com/designlib and download the software followed by instructions for creating an
output voltage VOUT (CTRL2) from a specified VCC supply
(VREF). At any time during selection of circuit parameters
the user can access data on the chosen NTC resistor by
clicking on a link to the Murata catalog.
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 flattening characteristic (reduced downward slope) at higher
temperatures. To avoid LED current exceeding a maximum
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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.
RNTC (50°C) = RNTC (25°C).e–1.026
RNTC (50°C) = 22k • 0.358
RNTC (50°C) = 7.9k
CTRL2(50°C) = 1.24/(1 + 16.9/7.9) = 395mV
Example: Calculate the resistor values required for generating CTRL1 and CTRL2 from VREF based on the following
requirements:
CTRL2 slope (25°C to 50°C) = [CTRL2(50°C)
– CTRL2(25°C)]/25°C
(a) ILED = 700mA at 25°C
= (395 – 701)/25
(b) ILED derating curve breakpoint occurs at 25°C
= –306mV/25°C
(c) ILED derating curve has a slope of –200mA/25°C between 25°C and 50°C ambient temperature
Step1: Choose CTRL1 = 700mV for ILED = 700mA
CTRL1 = VREF/(1 + R2/R1)
R2 = R1 • [(VREF/CTRL1) – 1]
For VREF = 1.24V and choosing R1 = 22.1k,
R2 = 22.1k [(1.24/0.7) – 1]
R2 = 17k (choose 16.9k)
CTRL1 = 1.24/(1 + (16.9/22.1))
CTRL1 = 703mV (ILED = 703mA)
Step 2: Choose resistor network option A (Figure 7) and
CTRL2 = CTRL1 for 25°C breakpoint
start with R4 = R2 = 16.9k, RNTC = 22k (closest value
available)
CTRL2 = 701mV (ILED = Min(CTRL1, CTRL2) • 1A =
701mA)
Step 3: Calculate CTRL2 slope between 25°C and 50°C
CTRL2 (T) = 1.24/(1 + R4/RNTC (T))
at T = TO = 25°C, CTRL2 = 701mV
at T = 50°C, RNTC (T) = RNTC (TO).ex, x = B [(1/(T + 273)
– 1/298)]
(B = B-constant; linear over the 25°C to 50°C temperature
range)
ILED slope = –306mA/25°C
The required ILED slope is –200mA/25°C. To reduce the
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
temperature independent resistor in series with RNTC to
reduce the effects of RNTC on the CTRL2 pin voltage over
temperature.
Step 4: Calculate the resistor value required for RY in
resistor network option (c) (Figure 7) to provide an ILED
slope of –200mA/25°C between 25°C and 50°C ambient
temperature.
CTRL2 (25°C) = 0.7V = 1.24/(1 + (R4/(RNTC(25°C)+
RY))
R4 = 0.77 (RNTC(25°C) + RY)
(a)
for –200mA/25°C slope ≥ CTRL2(50°C) = 0.7 – 0.2 =
0.5
CTRL2(50°C) = 0.5V = 1.24/(1 + (R4/(RNTC + RY))
R4 = 1.48 (RNTC(50°C) + RY)
(b)
Equating (a) = (b) and knowing RNTC(25°C) = 22k and
RNTC(50°C) = 7.9k gives,
0.77 (22k + RY) = 1.48 (7.9k + RY)
17k + 0.77 RY = 11.7 k + 1.48 RY
For RNTC B-constant = 3950 and T = 50°C
RY = (17k – 11.7k)/(1.48 – 0.77)
x = 3950 [(1/323) – 1/298] = –1.026
RY = 7.5k
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The value for R4 can now be solved using equation (a)
where,
R4 = 0.77 (RNTC(25°C) + RY) = 0.77 (22k + 7.5k)
for the output LED(s) is programmed for a given brightness/color and “chopped” over a PWM duty cycle range
(Figure 10) from 100% to as low as 0.033%.
D2
R4 = 22.7k (choose 22.6k)
ILED slope can now be calculated from,
VIN
ILED slope = [CTRL2(50°C) – CTRL2(25°C)]/25°C
SHDN
where CTRL2 (50°C) = 1.24/(1 + 22.6/(7.9 + 7.5)) =
503mV
VREF
CTRL2
RT
= 503mV – 699mV/25°C
VOUT
(LT3478)
LT3478/
LT3478-1
RSENSE
LED
VC
D1
PWM
PWM DIMMING
CONTROL
= –196mV/25°C => ILED slope = –196mA/25°C
Many LED applications require an accurate control of the
brightness of the LED(s). In addition, being able to maintain 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
(reducing CTRL1 voltage) because it allows constant color
to be maintained during dimming. The maximum current
COUT
SW
OVPSET
giving ILED slope (from 25°C to 50°C)
PWM Dimming
L
CTRL1
and CTRL2 (25°C) = 1.24/(1 + 39.2/(22 + 28.7)) =
699mV
Using the Murata simulation tool for the resistor network
and values in the above example shows a CTRL2 voltage 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
R4 = 19.3k, RNTC = 22k (NCP15XW223J0SRC) and RY
= 3.01k. Although the CTRL2 downward slope is greater
than –200mA/25°C initially, the slope is required to avoid
exceeding maximum allowed LED currents at high ambient
temperatures (see Figure 6).
VS
3478 F09
Figure 9. PWM Dimming Control Using the LT3478/LT3478-1
TPWM
TONPWM
(= 1/fPWM)
PWM
INDUCTOR
CURRENT
LED
CURRENT
MAX ILED
3478 F10
Figure 10. PWM Dimming Waveforms Using the
LT3478/LT3478-1
Some general guidelines for LED Current Dimming using
the PWM pin (see Figure 10):
(1) PWM Dimming Ratio (PDR) = 1/(PWM duty cycle) =
1/(TONPWM • fPWM)
(2) Lower fPWM allows higher PWM Dimming Ratios
(use minimum fPWM = 100Hz to avoid visible flicker and
to maximize PDR)
(3) Higher fOSC value improves PDR (allows lower TONPWM)
but will reduce efficiency and increase internal heating. In
general, minimum operational TONPWM = 3 • (1/fOSC).
(4) Lower inductor value improves PDR
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(5) Higher output capacitor value improves PDR
(6) Choose the schottky diode (D2, Figure 9) for minimum
reverse leakage
See Typical Performance Characteristics graph “LED Current vs PWM Duty Cycle”.
Soft-Start
To limit inrush current and output voltage overshoot during 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 capacitor to ground. The LT3478/LT3478-1 monitor system
parameters for the following faults: VIN <2.8V, ⎯S⎯H⎯D⎯N <1.4,
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 exist 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
exceeds VC.
The ramp rate of the SS pin is given by:
ΔVSS/Δt = 12µA/CSS
SW
SS
FAULTS TRIGGERING
SOFT-START LATCH
WITH SW TURNED OFF
IMMEDIATELY:
VIN < 2.8V OR
SHDN < 1.4V OR
VOUT > OVP OR
I(INDUCTOR) > 6A
0.65V (ACTIVE THRESHOLD)
0.25V (RESET THRESHOLD)
0.15V
SOFT-START LATCH RESET:
SOFT-START
LATCH SET:
SS < 0.25V AND
VIN > 2.8V AND
SHDN > 1.4V AND
VOUT < OVP AND
I(INDUCTOR) < 6A
3478 F11
Figure 11. LT3478 Fault Detection and SS Pin Timing Diagram
To limit inductor current overshoot to <0.5A when SS
charges past the VC level required for loop control, the CSS
capacitor should be chosen using the following formula:
CSS(MIN) = CC (7.35 – 0.6(ILED • VOUT/VS))
Example: VS = 8V, VOUT = 16V, ILED = 1.05A, CC = 0.1µF,
CSS(MIN) = 0.1µF (7.35 – 0.6(1.05 • 16/8))
= 0.612µF (choose 0.68µF).
High Inductor Current “Inrush” Protection
The LT3478/LT3478-1 provide an integrated resistor
between the VS and L pins to monitor inductor current
(Figure 1). During startup or “hotplugging” of the inductor 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. This protection prevents the switch from repetitively
turning on during excessive inductor currents by delaying switching until the fault has been removed. To defeat
inductor current sensing the inductor supply should be
connected to the L pin and the VS pin left open. See details
in the Applications Information section “Soft-Start”.
LED Open Circuit Protection and Maximum PWM
Dimming Ratios
The LT3478/LT3478-1 LED drivers provide optimum protection 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
over other LED driver applications which connect a resistor
divider directly from VOUT.
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,
causing permanent damage. When VOUT exceeds OVP, the
34781f
16
LT3478/LT3478-1
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APPLICATIO S I FOR ATIO
LT3478/LT3478-1 immediately stop switching, a soft-start
latch is set and the SS pin is discharged. The SS latch can
only be reset when VOUT falls below OVP and the SS pin
has been discharged below 0.25V (Figure 11). If the LED(s)
simply go open circuit and are reconnected, however, the
OVP used to protect the switch might be too high for the
reconnected LED(s). The LT3478/LT3478-1 therefore allow
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 ratings.) OVP is programmed using the OVPSET pin (front
page), given by,
OVP = (OVPSET • 41)V
where the programmable range for the OVPSET pin is 0.3V
to 1V resulting in an OVP range of 12.3V to 41V.
The OVPSET pin can be programmed with a single resistor
by tapping off of the resistor divider from VREF used to
program CTRL1. If both CTRL1 and CTRL2 are connected
directly to VREF (maximum LED current setting) then OVPSET requires a simple 2 resistor divider from VREF.
Thermal Calculations
VS = inductor supply input
D = switch duty cycle = (VOUT + VF – VS)/(VOUT + VF – VSAT)
VF = forward voltage drop of external Schottky diode
VSAT = IL(AVE) • RSW
(2) Switch AC loss = PSW(AC)
= tEFF(1/2)IL(AVE)(VOUT + VF)(FOSC)
tEFF = effective switch current and switch VCE voltage
overlap time during turn on and turn off = 2 • (tISW +
tVSW)
tISW = ISWITCH rise/fall time = IL(AVE) • 2ns
tVSW = SW fall/rise time = (VOUT + VF) • 0.7ns
fOSC = switching frequency
(3) Current sensing loss = PSENSE =
PSENSE(IL) + PSENSE(ILED)
PSENSE(IL) = IL(AVE)2 • 9.5mΩ
PSENSE(ILED) = ILED2 • 0.1Ω
(4) Input quiescent loss = PQ = VIN • IQ where
IQ = (6.2mA + (100mA • D))
To maximize output power capability in an application
without exceeding the LT3478/LT3478-1 125°C maximum
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 current sensing and input quiescent current. These formulas
assume a boost converter architecture, continuous mode
operation and no PWM dimming.
Example (Using LT3478-1):
(1) Switch DC loss = PSW(DC)
Total Power Dissipation:
= (RSW • IL(AVE)2 • D)
For VIN = VS = 8V, ILED = 700mA, VOUT = 24.5V (7 LEDs),
VF = 0.5V and fOSC = 0.2Mhz,
η = 0.89 (initial assumption)
IL(AVE) = (24.5 • 0.7)/(0.89 • 8) = 2.41A
D = (24.5 + 0.5 – 8)/(24.5 + 0.5 – 0.17) = 0.684
TEFF = 2 • ((2.41 • 2)ns + (24.5 + 0.5) • 0.7)ns = 45ns
PIC = PSW(DC) + PSW(AC) + PSENSE + PQ
RSW = switch resistance = 0.07Ω (at TJ = 125°C)
PSW(DC) = 0.07 • (2.41)2 • 0.684 = 0.278W
IL(AVE) = POUT/(η • VS)
PSW(AC) = 45ns • 0.5 • 2.41 • 25 • 0.2MHz = 0.271W
POUT = VOUT • ILED
PSENSE = ((2.41)2 • 0.0095) + ((0.7)2 • 0.1) = 0.104W
η = converter efficiency = POUT/(POUT + PLOSS)
PQ = 8 • (6.2mA + (100mA • 0.684)) = 0.597W
PIC = 0.278 + 0.271 + 0.104 + 0.597 = 1.25W
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17
LT3478/LT3478-1
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APPLICATIO S I FOR ATIO
Local heating from the nearby inductor and Schottky diode
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 measured and a new value for junction temperature estimated.
Ideally a thermal measurement should be made to achieve
the greatest accuracy for TJ.
VF = 0.5V
Note: The junction temperature of the IC can be reduced
if a lower VIN supply is available – separate from the
inductor supply VS. In the above example, driving VIN
from an available 3V source (instead of VS = 8V) reduces
input quiescent losses in item(4) from 0.597W to 0.224W,
resulting in a reduction of TJ from 118°C to 105°C.
IL(AVE) = 2.41
Layout Considerations
PDIODE = 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
for the RT resistor and VC capacitor should be taken from
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
for VIN should be placed as close as possible to the VIN
pin and the analog ground connection. SW pin voltage rise
and fall times are designed to be as short as possible for
maximum efficiency. To reduce the effects of both radiated
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
frequency switching path. To minimize LED current sensing
errors for the LT3478, the terminals of the external sense
resistor RSENSE should be tracked to the VOUT and LED
pins separate from any high current paths.
ΔTJ (LT3478-1) = 5°C/W • (PDIODE + PINDUCTOR)
PDIODE = (1 – D) • VF • IL(AVE)
1 – D = 0.316
PINDUCTOR = IL(AVE)2 • DCR
DCR = inductor DC resistance (assume 0.05Ω)
PINDUCTOR = (2.41)2 • 0.05 = 0.29W
The LT3478/LT3478-1 use a thermally enhanced FE package. With proper soldering to the Exposed Pad on the
underside of the package combined with a full copper plane
underneath the device, thermal resistance (θJA) will be
about 35°C/W. For an ambient temperature of TA = 70°C,
the junction temperature of the LT3478-1 for the example
application described above, can be calculated as:
TJ (LT3478-1)
= TA + θJA(PTOT) + 5(PDIODE + PINDUCTOR)
= 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 IL(AVE) and hence increase the calculated value
for TJ. η can be calculated as:
η = POUT/(POUT + PLOSS)
POUT = VOUT • ILED = 17.15W
PLOSS (estimated) = PIC + PDIODE + PINDUCTOR = 1.92W
η = 17.15/(17.15 + 1.92) = 0.9
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18
LT3478/LT3478-1
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APPLICATIO S I FOR ATIO
(CONNECT MULTIPLE GROUND PLANES
THROUGH VIAS UNDERNEATH THE IC)
VS
CVS
VOUT
VIN
CVIN
OUTPUT CAPACITOR
SCHOTTKY
DIODE
SOLDER THE EXPOSED PAD (PIN 17)
TO THE ENTIRE COPPER GROUND PLANE
UNDERNEATH THE DEVICE
LT3478/LT3478-1
SW
INDUCTOR
L
RSENSE
(LT3478 ONLY)
SW
1
16 SS
CSS
SW
2
15 RT
RT
VIN
3
14 PWM
R
VS
4
13 CTRL2
R
L
5
12 CTRL1
R
VOUT
6
11 SHDN
R
LED
OVPSET
R
POWER GND
GND
EXPOSED PAD
7
10 VREF
PIN 17
8 ANALOG GND 9 VC
C
R
CF
RC
VIN BYPASS CAP
CC
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
VIN
8V TO 16V
C1
4.7µF
25V
VIN
VS
L
D1
SHDN
OUT
PWM
5V/DIV
VREF
R1
45.3k
CTRL2
LT3478-1
700mA
LED
R4
54.9k
ILED
0.5A/DIV
CTRL1
PWM
SS
CSS
1µF
L1: CDRH104R-100NC
D1: PDS560
Q1: Si2318DS
LEDs: LUXEON III (WHITE)
fPWM = 100Hz
INDUCTOR
CURRENT
1A/DIV
OVPSET
R2
130k
LT3478-1 PWM Dimming
Waveforms
C2
10µF
25V
SW
VC
RT
CC
0.1µF
2µs/DIV
PWM DIMMING RATIO = 1000:1
(SEE EFFICIENCY ON PAGE 1)
RT
69.8k
3478 TA02b
fOSC = 500kHz
3.3V
0V
100Hz
Q1
PWM
DIMMING RATIO = 1000:1
R3
10k
3478 TA02a
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LT3478/LT3478-1
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TYPICAL APPLICATIO S
17W, 15 LEDs at 350mA, Boost LED Driver plus LT3003
VS
8V TO 14V
VIN
3.3V
C1
4.7µF
16V
L1
5.2µH
C3
3.3µF
10V
VIN
VS
D1
L
VOUT
SW
SHDN
Efficiency vs Input VS
C2
3.3µF
25V
90
OUT
VREF
85
LT3478-1
1.05A
LED
EFFICIENCY (%)
CTRL2
R1
24k
CTRL1
OVPSET
R2
100k
PWM
SS
VC
RT
CSS
1µF
L1: CDRH104R-5R2
D1: PDS560
LEDs: LUXEON I (WHITE)
VIN = 3.3V
ILED = 350mA
fOSC = 1MHz
PWM DUTY CYCLE = 100%
80
75
VC
RT
31.6k
CC
0.1µF
15 LEDs
(5 SERIES x 3 CHANNELS)
LUXEON I (WHITE)
70
fOSC = 1MHz
8
10
12
14
VS (V)
3.3V
0V
LED1
VMAX
VOUT
100Hz
LT3003
VIN
VIN
PWM
LED2
PWM
OT1
OT2
GND
3478 TA03b
LED3
VEE
VC
DIMMING RATIO = 3000:1
3478 TA03a
16W, 12 LEDs at 350mA, Buck-Boost Mode LED Driver plus LT3003
VS
12V TO 16V
VIN
5V
C1
4.7µF
25V
L1
8.2µH
C3
3.3µF
10V
VIN
VS
L
D1
VOUT
SW
SHDN
Efficiency vs Input VS
C2
10µF
50V
90
85
OUT
VREF
80
LT3478-1
1.05A
LED
EFFICIENCY (%)
CTRL2
R1
24k
CTRL1
OVPSET
R2
100k
PWM
RT
69.8k
CC
0.1µF
100Hz
PWM
70
65
12 LEDs
(4 SERIES x 3 CHANNELS)
LUXEON I (WHITE)
55
50
fOSC = 500kHz
D2
75
60
VC
C4
1µF
3.3V
RT
VC
CSS
1µF
L1: CDRH105R-8R2
D1: PDS560
D2: 7.5V ZENER
LEDs: LUXEON I (WHITE)
0V
SS
VIN = 5V
ILED = 350mA
fOSC = 500kHz
PWM DUTY CYCLE = 100%
12
VOUT
LED1
VMAX
VIN
PWM
LED2
LT3003
VEE
14
VS (V)
15
16
3478 TA04b
LED3
OT1
OT2
GND
13
VC
DIMMING RATIO = 200:1
3478 TA04a
34781f
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LT3478/LT3478-1
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TYPICAL APPLICATIO S
4W, 1 LED at 1A, Buck-Boost Mode LED Driver
VIN
3.8V TO 6.5V
NiMH 4×
C1
10µF
10V
L1
6.8µH
VIN
ON OFF
VS
L
D1
Efficiency vs VIN
C2
4.7µF
16V
SW
SHDN
80
OUT
ILED = 1A
fOSC = 500kHz
75 PWM DUTY CYCLE = 100%
CTRL2
R1
100k
LT3478-1
Q2
1A
LED
CTRL1
R4
510Ω
OVPSET
R2
L1: CDRH105R-6R8
34k
D1: B320
Q1: Si2302ADS
Q2: Si2315BDS
LED: LUXEON III (WHITE)
PWM
SS
CSS
1µF
3.3V
0V
VC
CC
0.1µF
65
60
55
RT
69.8k
R5
510Ω
SINGLE LED
LUXEON III (WHITE)
50
3
Q1
PWM
DIMMING RATIO = 200:1
70
RT
fOSC = 500kHz
1kHz
EFFICIENCY (%)
VREF
4
5
VIN (V)
6
7
3478 TA06b
R3
10k
3478 TA06a
34781f
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LT3478/LT3478-1
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TYPICAL APPLICATIO S
24W, 4 LEDs at 1.5A, Buck Mode LED Driver
PVIN
32V
C1
3.3µF
50V
RSENSE
0.068Ω
1.5A
4 LEDs
R4
365Ω
TYPICAL EFFICIENCY = 90%
FOR CONDITIONS/COMPONENTS SHOWN
(PWM DUTY CYCLE = 100%, TA =25°C)
C3
10µF
25V
Q2
L1
10µH
VIN
3.3V
C2
4.7µF
10V
D1
VIN
VS
L
OUT LED SW
SHDN
L1: CDRH105R-100
D1: PDS560
Q1: 2N7002
Q2: Si2319DS
LEDs: LXK2 (WHITE)
Q1
PWM
R3
10k
VREF
R1
24k
R5
510Ω
LT3478
CTRL2
PWM
CTRL1
DIMMING RATIO = 3000:1
OVPSET
3.3V
R2
100k
SS
CSS
1µF
RT
VC
CC
0.1µF
fOSC = 500kHz
0V
100Hz
RT
69.8k
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
6.60 ±0.10
9
2.94
(.116)
4.50 ±0.10
6.40
2.94
(.252)
(.116)
BSC
SEE NOTE 4
0.45 ±0.05
1.05 ±0.10
0.65 BSC
1 2 3 4 5 6 7 8
RECOMMENDED SOLDER PAD LAYOUT
4.30 – 4.50*
(.169 – .177)
0.09 – 0.20
(.0035 – .0079)
0.50 – 0.75
(.020 – .030)
NOTE:
1. CONTROLLING DIMENSION: MILLIMETERS
MILLIMETERS
2. DIMENSIONS ARE IN
(INCHES)
3. DRAWING NOT TO SCALE
0.25
REF
1.10
(.0433)
MAX
0° – 8°
0.65
(.0256)
BSC
0.195 – 0.30
(.0077 – .0118)
TYP
0.05 – 0.15
(.002 – .006)
FE16 (BC) TSSOP 0204
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
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 representation 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
VIN
3.3V
C1
4.7µF
25V
L1
10µH
C3
3.3µF
10V
VIN
VS
L
D1
SW
SHDN
OUT
100
VIN = 3.3V
ILED = 250mA
95
fOSC = 2MHz
PWM DUTY CYCLE = 100%
90
RSENSE
0.42Ω
VREF
CTRL2
R1
8.25k
Efficiency vs Input VS
C2
3.3µF
25V
LT3478
EFFICIENCY (%)
VS
8V TO 16V
250mA
LED
CTRL1
OVPSET
R2
10k
PWM
SS
VC
80
75
70
CSS
1µF
L1: CDRH6D28
D1: ZLLS1000
Q1: Si2318DS
LEDs: LUXEON I (WHITE)
RT
85
RT
10k
CC
0.1µF
65
6 LEDs = LUXEON I (WHITE)
60
fOSC = 2MHz
8
10
12
VS (V)
14
3.3V
0V
100Hz
16
3478 TA05b
Q1
PWM
DIMMING RATIO = 1000:1
R3
10k
3478 TA05a
RELATED PARTS
PART NUMBER DESCRIPTION
COMMENTS
LT1618
Constant Current, 1.4MHz, 1.5A Boost Converter with Analog/PWM
Dimming
VIN: 5V to 18V, VOUT(MAX) = 36V, ISD <1µA, MS10 Package
LT3003
Three Channel LED Ballaster with 3,000:1 True Color PWM Dimming
VIN: 3V to 48V, ISD <5µA, MSOP10 Package
LT3474
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VIN: 4V to 36V, VOUT(MAX) = 13.5V, ISD <1µA, TSSOP16E Package
LT3475
Dual 1.5A(ILED), 36V, 2MHz,Step-Down LED Driver 3,000:1 True
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VIN: 4V to 36V, VOUT(MAX) = 13.5V, ISD <1µA, TSSOP20E Package
LT3476
Quad Output 1.5A, 2MHz High Current LED Driver with 1,000:1 True
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VIN: 2.8V to 16V, VOUT(MAX) = 36V, ISD <10µA, 5mm × 7mm QFN
Package
LT3477
42V, 3A, 3.5MHz Boost, Buck-Boost, Buck LED Driver with Analog/
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Packages
LT3479
3A, 3.5MHz Full Featured DC/DC Converter with Soft-Start and
Inrush Current Protection and Analog/PWM Dimming
VIN: 2.5V to 24V, VOUT(MAX) = 40V, ISD <1µA, 4mm × 3mm DFN,
TSSOP16E Packages
LT3486
Dual 1.3A , 2MHz High Current LED Driver with 1,000:1 True Color
PWM Dimming
VIN: 2.5V to 24V, VOUT(MAX) = 36V, ISD <1µA, 5mm × 3mm DFN,
TSSOP16E Packages
LTC3783
High Current LED Controller with 3,000:1 True Color PWM Dimming
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DFN, TSSOP16E Packages
34781f
24 Linear Technology Corporation
LT 0107 • PRINTED IN USA
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