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LT3479EFE#TRPBF

P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20
LT3479
7
3479fc
OPERATION
The LT3479 uses a fi xed frequency, current mode control
scheme to provide excellent line and load regulation. Op-
eration can be best understood by referring to the Block
Diagram. The start of each oscillator cycle sets the SR latch
and turns on power switch Q1. The signal at the inverting
input of the PWM comparator (SLOPE) is proportional to
the sum of the switch current and oscillator ramp. When
SLOPE exceeds V
C
(the output of the feedback amplifi er),
the PWM comparator resets the latch and turns off the
power switch. In this manner, the feedback amplifi er and
PWM comparators set the correct peak current level to
keep the output in regulation.
The LT3479 also features a soft-start function. During
start-up, 10μA of current charges the external soft-start
capacitor. The SS pin directly limits the rate of voltage rise
on the V
C
pin, which in turn limits the peak switch cur-
rent. The switch current is constantly monitored and not
allowed to exceed the nominal value of 3A. If the switch
current reaches 3A, the SR latch is reset regardless of the
output of the PWM comparator. Current limit protects the
power switch and external components.
Soft-start plays an important role in applications where the
switch will reach levels of 30V or higher. During startup,
an overshoot in the switch current together with the pres-
ence of high switch voltage can overstress the switch. A
properly used soft-start feature will greatly improve the
robustness of such designs.
In addition to soft-start, inrush current protection protects
the LT3479 against shorts and line transients. During such
faults, the inductor current can momentarily exceed 3A and
damage the switch. Through an internal 8.5mΩ resistor
placed in series with the inductor, the inrush current protec-
tion comparator measures the inductor current. If it exceeds
5A, a soft-start cycle is initiated. The LT3479 will remain in
the soft-start condition until the fault has passed.
LT3479
8
3479fc
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 better than other dielectrics. A 4.7μF
to 10μF output capacitor is suffi cient for most high output
current designs. Converters with lower output currents
may need only a 1μF or 2.2μF output capacitor.
Table 1. Ceramic Capacitor Manufacturers
MANUFACTURER PHONE WEB
Taiyo Yuden (408) 573-4150 www.t-yuden.com
AVX (803) 448-9411 www.avxcorp.com
Murata (714) 852-2001 www.murata.com
Inductor Selection
Several inductors that work well with the LT3479 are listed
in Table 2. However, there are many other manufacturers
and devices that can be used. Consult each manufacturer
for more detailed information and their entire range of
parts. Ferrite core inductors should be used to obtain the
best effi ciency. Choose an inductor that can handle the
necessary peak current without saturating, and ensure
that the inductor has a low DCR (copper-wire resistance)
to minimize I
2
R power losses. A 4.7μH or 10μH inductor
will suffi ce for most LT3479 applications.
Inductor manufacturers specify the maximum current
rating as the current where the inductance falls to some
percentage of its nominal value—typically 65%. An inductor
APPLICATIONS INFORMATION
Table 2. Suggested Inductors
MANUFACTURER
PART NUMBER
I
DC
(A)
INDUCTANCE
(μH)
MAX DCR
(mΩ)
L × W × H
(mm)
MANUFACTURER
CDRH6D283R0
CDRH6D28100
CDRH4D284R7
3
1.7
1.32
3
10
4.7
24
65
72
6.7 × 6.7 × 3.0
6.7 × 6.7 × 3.0
5.0 × 5.0 × 3.0
Sumida
www.sumida.com
LM N 05D B4R7M
LM N 05D B100K
2.2
1.6
4.7
10
49
10
5.9 × 6.1 × 2.8
5.9 × 6.1 × 2.8
Taiyo Yuden
www.t-yuden.com
LQH55DN4R7M01L
LQH55DN100M01K
2.7
1.7
4.7
10
57
130
5.7 × 5.0 × 4.7
5.7 × 5.0 × 4.7
Murata
www.murata.com
FDV0630-4R7M 4.2 4.7 49
7.0 × 7.7 × 3.0
Toko
www.toko.com
Figure 1. Effi ciency vs Inductor Size
I
OUT
(A)
0
EFFICIENCY (%)
70
80
0.8
3479 F01
60
50
0.2
0.4
0.6
90
65
75
55
85
SUMIDA CDRH4D28-4R7
TOKO FDV0630-4R7
can pass a current larger 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 board space. Consult each manufacturer
to determine how the maximum inductor current is
measured and how much more current the inductor can
reliably conduct.
Physically larger inductors provide better effi ciency than
smaller ones. Figure 1 shows a 3% to 4% effi ciency gain
in using a larger inductor in a 1MHz, 5V to 12V application.
The effi ciency of the TOKO FDV0630-4R7M, which mea-
sures 7mm × 7.7mm and 3 mm thick, peaks at 87%. The
smaller Sumida CDRH4D28-4R7 which is 5mm × 5mm and
3mm thick yields a peak effi ciency of 85% in an identical
application. Thus, if board space is abundant, then larger
inductors should be used to maximize effi ciency.
LT3479
9
3479fc
Setting Negative Output Voltages
To set a negative output voltage, select the values of R3 and
R4 (see Figure 3) according to the following equation:
VV
R
R
OUT
=
⎛
⎝
⎜
⎞
⎠
⎟
–.1 235
3
4
Figure 2. Positive Output Voltage Feedback Connections
R2
R1
3479 F02
FBN
V
REF
V
OUT
LT3479
FBP
Figure 3. Negative Output Voltage Feedback Connections
R4
R3
3479 F03
FBP
V
REF
–V
OUT
LT3479
FBN
APPLICATIONS INFORMATION
Diode Selection
Schottky diodes, with their low forward voltage drop and
fast switching speed, are ideal for LT3479 applications.
Table 3 lists several Schottky diodes that work well with the
LT3479. The diode’s average current rating must exceed
the average output current. The diode’s maximum reverse
voltage must exceed the output voltage. The diode conducts
current only when the power switch is turned off (typically
less than 50% duty cycle), so a 3A diode is suffi cient for
most designs. The companies below also offer Schottky
diodes with high voltage and current ratings.
Table 3. Suggested Diodes
MANUFACTURER
PART NUMBER
MAX
CURRENT (A)
MAX REVERSE
VOLTAGE (V) MANUFACTURER
UPS340
UPS315
3
3
40
15
Microsemi
www.microsemi.com
B220
B230
B240
B320
B330
B340
SBM340
2
2
2
3
3
3
3
20
30
40
20
30
40
40
Diodes, Inc
www.diodes.com
Setting Positive Output Voltages
To set a positive output voltage, select the values of R1 and
R2 (see Figure 2) according to the following equation:
VV
R
R
OUT
=+
⎛
⎝
⎜
⎞
⎠
⎟
1 235 1
1
2
.
Board Layout
As with all switching regulators, careful attention must
be paid to the PCB board layout and component place-
ment. To maximize effi ciency, switch rise and fall times
are made as short as possible. To prevent radiation and
high frequency resonance problems, proper layout of the
high frequency switching path is essential. Minimize the
length and area of all traces connected to the SW pin and
always use a ground plane under the switching regulator
to minimize interplane coupling. The signal path including
the switch, output diode D1 and output capacitor C
OUT
,
contains nanosecond rise and fall times and should be
kept as short as possible. Recommended component
placement is shown in Figure 4.
Soft-Start
For many applications, it is necessary to minimize the
inrush current at start-up. The built-in soft-start circuit
signifi cantly reduces the start-up current spike and output
voltage overshoot. A typical value is 10nF for 1.65ms.
Figure 5 shows the start-up output voltage and induc-
tor current waveforms in a typical application without a
soft-start capacitor. Notice the output voltage overshoot
and the large initial current. The addition of a 22nF capaci-
tor eliminates the output overshoot and reduces the peak
inductor current (Figure 6).
P1-P3P4-P6P7-P9P10-P12P13-P15P16-P18P19-P20

LT3479EFE#TRPBF

Mfr. #:
Buy LT3479EFE#TRPBF
Manufacturer:
Analog Devices / Linear Technology
Description:
Switching Voltage Regulators 3A, 42V, 3MHz Step- Up Regulator
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