LT3479
3A, Full Featured DC/DC
Converter with Soft-Start
and Inrush Current Protection
FEATURES
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DESCRIPTION
The LT
®
3479 is a current mode, fixed frequency step-up
DC/DC converter with an internal 3A, 42V switch. Efficien-
cies of up to 89% can be achieved in typical applications. It
features a programmable soft-start function to limit induc-
tor current during start-up and inrush current protection
to protect the LT3479 during shorts and line transients.
Both inputs of the error amplifier are available to the user
allowing positive and negative output voltages. Through
an external resistor, the user can program the switch-
ing frequency from 200kHz to 3.5MHz. The low profile
(0.75mm) 14-pin, 4mm
×
3mm DFN package provides
excellent thermal performance in a small footprint. The
LT3479 is also available in a thermally enhanced 16-pin
TSSOP package.
L,
LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear
Technology Corporation. All other trademarks are the property of their respective owners.
Wide Input Voltage Range: 2.5V to 24V
3A, 42V Internal Switch
High Efficiency Power Conversion: Up to 89%
Soft-Start
Frequency Set by External Resistor: 200kHz to
3.5MHz
Protection Against Input Short Circuits and
Hot Plugging
Low V
CESAT
Switch: 0.3V at 2.5A (Typical)
Capable of Positive and Negative Outputs
Available in Thermally Enhanced 14-Lead
(4mm
×
3mm) DFN and 16-Lead TSSOP Packages
APPLICATIONS
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High Power LED Driver
DSL Modems
Distributed Power
TYPICAL APPLICATION
5V to 12V Boost Converter
4.7μH
V
IN
5V
2.2μF
V
IN
V
S
L
SHDN
LT3479
SS
10nF
R
T
GND
17.8k
FBP
V
C
10k
2.2nF
3479 TA01
5V to 12V Efficiency
V
OUT
12V
0.8A
90
85
EFFICIENCY (%)
80
75
70
65
60
200k
SW
FBN
23.2k
V
REF
10μF
0
0.2
0.4
I
OUT
(A)
0.6
0.8
3479 TA02
3479fc
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LT3479
ABSOLUTE MAXIMUM RATINGS
(Note 1)
SW, L, V
S
Voltages ................................................... 42V
V
IN
,
SHDN
Voltages ................................................. 24V
FBP FBN, V
REF
, R
T
, V
C
Voltages ................................. 2V
,
Junction Temperature .......................................... 125°C
Operating Temperature Range (Note 2).... –40°C to 85°C
Storage Temperature Range................... –65°C to 125°C
Lead Temperature (Soldering, 10 sec)
TSSOP .............................................................. 300°C
PIN CONFIGURATION
TOP VIEW
TOP VIEW
SW
SW
L
V
S
V
IN
R
T
SHDN
1
2
3
4
5
6
7
15
14 GND
13 GND
12 SS
11 V
C
10 FBN
9
8
FBP
V
REF
SW
SW
L
V
S
V
IN
R
T
SHDN
GND
DE14 PACKAGE
14-LEAD (4mm 3mm) PLASTIC DFN
T
JMAX
= 125°C,
θ
JA
= 43°C/W
EXPOSED PAD (PIN 15) IS PGND (MUST BE SOLDERED TO PCB)
1
2
3
4
5
6
7
8
17
16 GND
15 GND
14 GND
13 SS
12 V
C
11 FBN
10 FBP
9
V
REF
FE PACKAGE
16-LEAD PLASTIC TSSOP
T
JMAX
= 125°C,
θ
JA
= 38°C/W
EXPOSED PAD (PIN 17) IS PGND (MUST BE SOLDERED TO PCB)
ORDER INFORMATION
LEAD FREE FINISH
LT3479EDE#PBF
LT3479EFE#PBF
LEAD BASED FINISH
LT3479EDE
LT3479EFE
TAPE AND REEL
LT3479EDE#TRPBF
LT3479EFE#TRPBF
TAPE AND REEL
LT3479EDE#TR
LT3479EFE#TR
PART MARKING
3479
3479EFE
PART MARKING
3479
3479EFE
PACKAGE DESCRIPTION
14-Lead (4mm
×
3mm) Plastic DFN
16-Lead Plastic TSSOP
PACKAGE DESCRIPTION
14-Lead (4mm
×
3mm) Plastic DFN
16-Lead Plastic TSSOP
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
TEMPERATURE RANGE
–40°C to 85°C
–40°C to 85°C
Consult LTC Marketing for parts specified with wider operating temperature ranges.
For more information on lead free part marking, go to:
http://www.linear.com/leadfree/
For more information on tape and reel specifications, go to:
http://www.linear.com/tapeandreel/
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LT3479
ELECTRICAL CHARACTERISTICS
PARAMETER
Minimum Input Voltage
Quiescent Current
Reference Voltage
Reference Voltage Line Regulation
Maximum V
REF
Pin Current
Soft-Start Pin Current
FBP Pin Bias Current
FBN Pin Bias Current
Feedback Amplifier Offset Voltage
Feedback Amplifier Voltage Gain
Feedback Amplifier Transconductance
Feedback Amplifier Sink Current
Feedback Amplifier Source Current
Switching Frequency
V
FBP
= 1.25V, V
FBN
= 1.5V, V
C
= 0.5V
V
FBP
= 1.25V, V
FBN
= 1V, V
C
= 0.5V
R
T
= 17.8k
R
T
= 113k
R
T
= 1.78k
R
T
= 17.8k
V
SHDN
= 5V
V
SHDN
= 0V
0.3
(Note 3)
(Note 3)
I
SW
= 1A (Note 3)
SW = 40V
3.5
3
l
The
l
denotes the specifications which apply over the full operating
temperature range, otherwise specifications are at T
A
= 25°C. V
IN
= 2.5V, V
SHDN
= 2.5V.
CONDITIONS
l
MIN
TYP
2.3
0.1
5
MAX
2.5
1
7.5
1.250
0.03
100
UNITS
V
μA
mA
V
%/V
μA
μA
nA
nA
mV
V/V
μS
μA
μA
V
IN
= 2.5V, V
SHDN
= 0V
V
IN
= 2.5V, V
SHDN
= 2.5V, V
C
= 0.3V (Not Switching)
Measured at V
REF
Pin
2.5V < V
IN
< 24V, V
C
= 0.3V
Out of Pin
SS = 0.5V, Out of Pin
l
1.216
1.235
0.01
9
25
25
100
100
6
FBP – FBN, V
C
= 1V
–2
2
250
150
10
10
0.9
160
2.7
84
1
200
3.5
93
30
0.1
1.5
5
4.5
120
0.2
1.15
240
4.1
60
1
2
6.5
6
200
5
MHz
kHz
MHz
%
μA
μA
V
A
A
mV
μA
Maximum Switch Duty Cycle
SHDN
Pin Current
SHDN
Pin Threshold
Inductor Current Limit
Switch Current Limit
Switch V
CESAT
Switch Leakage Current
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 LT3479 is guaranteed to meet performance specifications
from 0°C to 70°C. Specifications over the –40°C to 85°C operating
temperature range are assured by design, characterization and correlation
with statistical process controls.
Note 3:
Inductor Current Limit, Switch Current Limit and Switch V
CESAT
for
DE package guaranteed by design and/or correlation to static test.
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LT3479
TYPICAL PERFORMANCE CHARACTERISTICS
Switch V
CE(SAT)
0.5
6
INDUCTOR CURRENT LIMIT
0.4
125°C
0.3
25°C
–50°C
0.2
CURRENT (A)
V
CE(SAT)
(V)
5
4
3
2
0.1
1
0
–50 –25
SWITCH CURRENT LIMIT
V
REF
(V)
1.26
1.25
V
IN
= 24V
1.24
V
IN
= 2.5V
1.23
1.22
1.21
–50 –25
Inductor and Switch Current Limit
1.27
V
REF
0
0
0.5
1
1.5
2
SWITCH CURRENT (A)
2.5
3
3479 G01
50
25
75
0
TEMPERATURE (°C)
100
125
0
25 50 75 100 125 150
TEMPERATURE (°C)
3479 G03
3479 G02
SHDN
Pin Turn-On Threshold
1.750
50
SHDN
Pin Current
6
–50°C
V
IN
PIN CURRENT (mA)
V
IN
Pin Current
V
C
= 0.3V
SHDN THRESHOLD (V)
1.625
SHDN PIN CURRENT (μA)
40
5
30
25°C
1.500
4
20
125°C
1.375
10
3
1.250
–50
0
–25
75
0
25
50
TEMPERATURE (°C)
100
125
0
4
8
12
V
SHDN
(V)
16
20
24
3479 G05
2
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3479 G06
3479 G04
Soft-Start Pin Current
20
2.0
Oscillator Frequency
5
R
T
= 10k
OFFSET VOLTAGE (mV)
Feedback Amplifier Offset Voltage
15
FREQUENCY (MHz)
1.6
4
V
C
= 0.5V
3
I
SS
(μA)
1.2
R
T
= 15k
10
0.8
R
T
= 20k
2
V
C
= 1V
5
0.4
1
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3479 G07
0
–50 –25
0
25 50 75 100 125 150
TEMPERATURE (°C)
3479 G08
0
–50
–25
0
25
50
TEMPERATURE ( C)
75
100
3479 G09
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LT3479
PIN FUNCTIONS
(DFN/TSSOP)
SW (Pins 1, 2/Pins 1, 2):
Switch Pins. Collector of the
internal NPN power switch. Connect the inductor and diode
here and minimize the metal trace area connected to this
pin to minimize electromagnetic interference.
L (Pin 3/Pin 3):
Inductor Pin. Connect the inductor to
this pin.
V
S
(Pin 4/Pin 4):
Inductor Supply. Must be locally bypassed.
Powers the switch and the inductor. In case only one supply
voltage is available, tie V
IN
and V
S
together.
V
IN
(Pin 5/Pin 5):
Input Supply. Must be locally bypassed.
Powers the internal control circuitry.
R
T
(Pin 6/Pin 6):
Timing Resistor Pin. Adjusts the switch-
ing frequency. Do not leave this pin open. See Table 4 for
R
T
values and switching frequencies.
SHDN
(Pin 7/Pin 7):
Shutdown. Tie to 1.5V or greater to
enable the device. Tie below 0.3V to turn off the device.
V
REF
(Pin 8/Pin 9):
Bandgap Voltage Reference. Internally
set to 1.235V. Connect this pin to FBP if generating a posi-
tive output, or to an external resistor divider if generating a
negative voltage. This pin can provide up to 100μA of current
and can be locally bypassed with a 100pF capacitor.
FBP (Pin 9/Pin 10):
The Noninverting Input to the Error
Amplifier. Connect resistive divider tap here for negative
output voltage.
FBN (Pin 10/Pin 11):
The Inverting Input to the Error
Amplifier. Connect resistive divider tap here for positive
output voltage.
V
C
(Pin 11/Pin 12):
Compensation Pin for Error Amplifier.
Connect a series RC from this pin to GND. Typical values
are 10kΩ and 2.2nF.
SS (Pin 12/Pin 13):
Soft-Start. Place a soft-start capacitor
here. Leave floating if not in use.
GND (Pins 13, 14/Pins 8, 14, 15, 16):
Ground. Tie directly
to local ground plane.
Exposed Pad (Pin 15/Pin 17):
Power Ground. Must be
connected to electrical PCB ground.
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