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LT3478IFE-1#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P21P22-P24
LT3478/LT3478-1
10
34781f
APPLICATIO S I FOR ATIO
WUU
U
Inductor Selection
Several inductors that work well with the LT3478/LT3478-1
are listed in Table 1. However, there are many other manu-
facturers 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 effi ciency. 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 I
2
R power losses. Values between 4.7µH and
22µH will suffi ce for most applications.
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.
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 suffi cient for most high output
current designs. Some suggested manufacturers are
listed in Table 2.
Diode Selection
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’s average output current. The diode’s maximum
reverse voltage must exceed the application’s output volt-
age. A 4.5A diode is suffi cient 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.
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
4
4.5
5.5
10
4.7
10
5.2
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
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 PHONE NUMBER WEB
Taiyo Yuden (408) 573-4150 www.t-yuden.com
AVX (803) 448-9411 www.avxcorp.com
Murata (714) 852-2001 www.murata.com
Table 3. Suggested Diodes
MANUFACTURER PART NUMBER MAX CURRENT (A) MAX REVERSE VOLTAGE WEB
UPS340 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
LT3478/LT3478-1
11
34781f
Shutdown and Programming Undervoltage Lockout
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 defi ne 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 fl ows
into the
⎯
S
⎯
H
⎯
D
⎯
N pin. After part turn on, 0µA fl ows from the
⎯
S
⎯
H
⎯
D
⎯
N pin. Calculation of the on/off thresholds for a system
input supply using the LT3478/LT3478-1
⎯
S
⎯
H
⎯
D
⎯
N pin can
be made as follows:
V
SUPPLY
OFF = 1.4 [1 + R1/R2)]
V
SUPPLY
ON = V
SUPPLY
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.
Programming Switching Frequency
The switching frequency is programmed using an external
resistor (R
T
) connected between the R
T
pin and ground. The
internal free-running oscillator is programmable between
200kHz and 2.25MHz. Table 4 shows the typical R
T
values
required for a range of switching frequencies.
Selecting the optimum switching frequency depends
on several factors. Inductor size is reduced with higher
frequency but effi ciency 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.
APPLICATIO S I FOR ATIO
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With the
⎯
S
⎯
H
⎯
D
⎯
N pin connected directly to the V
IN
pin, an
internal undervoltage lockout threshold exists for the V
IN
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
recovering from such faults as
⎯
S
⎯
H
⎯
D
⎯
N <1.4V and/or V
IN
<2.8V. See details in the Applications Information section
“Soft-Start”.
Figure 2. Programming Undervoltage Lockout (UVLO)
with Hysteresis
Table 4. Switching Frequencies vs R
T
Values
SWITCHING FREQUENCY (MHz) R
T
(kΩ)
2.25 9.09
1 31.6
0.2 200
Figure 3. Switching Frequency vs R
T
Resistor Value
R
T
(kΩ)
1
100
SWITCHING FREQUENCY (kHz)
1000
10000
10 100 1000
3478 F03
T
A
= 25°C
SHDN
1.4V
10µA
V
SUPPLY
R1
11
R2
3478 F02
ONOFF
+
–
LT3478/LT3478-1
12
34781f
CTRL1 (V)
0
LED CURRENT (mA)
1.40
3478 F04
0.35 0.70 1.05
1400
1050
700
350
0
T
A
= 25°C
CTRL2 = V
REF
(FOR LT3478 SCALE
BY 0.1Ω/R
SENSE
)
LT3478-1
V
REF
Programming Maximum LED current
Maximum LED current can be programmed using the CTRL1
pin with CTRL2 tied to the V
REF
pin (see Figures 4 and 5).
The maximum allowed LED current is defi ned as:
(LT3478-1) Max LED Current = Min(CTRL1, 1.05) Amps
()
(,.)•
.
LT Max LED Current
Min CTRL
R
S
3478
1105
01
=
EENSE
Amps
LED current vs CTRL1 is linear for approximately
0.1V < CTRL1 < 0.95V
For maximum possible LED current, connect CTRL1 and
CTRL2 to the V
REF
pin.
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 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 coeffi cient 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 defi ned 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 defi ne the LED current curve breakpoint
and slope versus temperature (Figure 8).
A variety of resistor networks and NTC resistors with differ-
ent temperature coeffi cients can be used for programming
APPLICATIO S I FOR ATIO
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Figure 6. LED Current Derating Curve vs Ambient Temperature
maximum allowed LED current versus temperature to
warn against exceeding this current limit and damaging
the LED (Figure 6).
Figure 5. Programming LED Current
Figure 4. LED Current vs CTRL1 Voltage
LT3478/LT3478-1
(LT3478)
V
REF
CTRL2
CTRL1
V
OUT
LED
10
13
12
R2 R
SENSE
R1
3478 F05
T
A
AMBIENT TEMPERATURE (°C)
0
I
f
FORWARD CURRENT (mA)
900
LUXEON V EMITTER
(GREEN, CYAN, BLUE, ROYAL BLUE)
θ
JA
= 20°C/W
700
800
0
500
400
300
200
100
600
25
3478 F06
50 75 100
EXAMPLE
LT3478-1
PROGRAMMED LED
CURRENT DERATING CURVE
LUXEON V EMITTER
CURRENT DERATING
CURVE
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
Luxeon V (Maximum) and LT3478-1
(Programmed) Current Derating
Curves vs Temperature
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LT3478IFE-1#TRPBF

Mfr. #:
Buy LT3478IFE-1#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
LED Lighting Drivers 4.5A 42V, 2.25MHz Boost LED Driver w/ True Color PWM Dimming
Lifecycle:
New from this manufacturer.
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