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PDF LT3479 Data sheet ( Hoja de datos )
| Número de pieza | LT3479 | |
| Descripción | 3A. Full Featured DC/DC Converter | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT3479 (archivo pdf) en la parte inferior de esta página. Total 20 Páginas | ||
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LT3479www.DataSheet4U.com
3A, Full Featured DC/DC
Converter with Soft-Start
and Inrush Current Protection
FEATURES
n Wide Input Voltage Range: 2.5V to 24V
n 3A, 42V Internal Switch
n High Efficiency Power Conversion: Up to 89%
n Soft-Start
n Frequency Set by External Resistor: 200kHz to
3.5MHz
n Protection Against Input Short Circuits and
Hot Plugging
n Low VCESAT Switch: 0.3V at 2.5A (Typical)
n Capable of Positive and Negative Outputs
n Available in Thermally Enhanced 14-Lead
(4mm × 3mm) DFN and 16-Lead TSSOP Packages
APPLICATIONS
n High Power LED Driver
n DSL Modems
n Distributed Power
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 and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
5V to 12V Boost Converter
VIN
5V
2.2μF
4.7μH
VIN VS L
SW
FBN
SHDN
LT3479 VREF
SS
10nF
FBP
RT GND VC
200k
VOUT
12V
0.8A
10μF
23.2k
17.8k
10k
2.2nF
3479 TA01
5V to 12V Efficiency
90
85
80
75
70
65
60
0
0.2 0.4 0.6 0.8
IOUT (A)
3479 TA02
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LT3479www.DataSheet4U.com
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.
VS (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 VIN and VS together.
VIN (Pin 5/Pin 5): Input Supply. Must be locally bypassed.
Powers the internal control circuitry.
RT (Pin 6/Pin 6): Timing Resistor Pin. Adjusts the switch-
ing frequency. Do not leave this pin open. See Table 4 for
RT 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.
VREF (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.
VC (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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LT3479www.DataSheet4U.com
APPLICATIONS INFORMATION
Increasing switching frequency reduces output voltage
ripple but also reduces efficiency. The user should set
the frequency for the maximum tolerable output voltage
ripple. Figure 8 shows a reduction in efficiency of about
4% between 1MHz and 2MHz operation in a typical
application.
Inrush Current Protection
The LT3479 features a novel inductor current sensing
circuit that protects the LT3479 during hot plugging and
short circuits. An internal resistor in series with the ex-
ternal inductor senses the inductor current at all times.
When it exceeds 5A, a soft-start cycle is initiated. Figure 9
shows an output overload with inrush current protection
disabled. Notice that soft-start remains high, and that
the inductor current does not return to zero. Figure 10
illustrates the benefits of inrush current protection. The
output short initiates a new soft start cycle reducing the
inductor current. After the fault has passed, the inductor
current slowly returns to its equilibrium value. To ensure
bond wire integrity, the inductor current should not exceed
8A for more than 10ms.
Bypassing the 8.5mΩ inductor current sense resistor
disables inrush current protection. Connect the inductor
supply trace and bypass capacitor to the L pin and leave
the VS pin open to disable this feature.
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0.1
10
RT (kΩ)
100
3479 F07
Figure 7. Switching Frequency
90
1MHz
85
80 2MHz
75
70
65
60
55
50
0
0.2 0.4 0.6 0.8
IOUT (A)
3479 F08
Figure 8. Efficiency vs Switching Frequency
VSW
10V/DIV
IL
4V/DIV
VSS
2V/DIV
VOUT
20V/DIV
20μs/DIV
3479 F09
Figure 9. Output Overload with
Inrush Current Protection Enabled
VSW
10V/DIV
IL
4V/DIV
VSS
2V/DIV
VOUT
20V/DIV
20μs/DIV
3479 F10
Figure 10. Output Overload with
Inrush Current Protection Disabled
VIN VOUT
LT3479
BOOST
REGULATOR
0.5Ω
3479 F11
Figure 11. Circuit for Output Overload
3479fb
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11 Page | ||
| Páginas | Total 20 Páginas | |
| PDF Descargar | [ Datasheet LT3479.PDF ] | |
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