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PDF LT3481 Data sheet ( Hoja de datos )
| Número de pieza | LT3481 | |
| Descripción | 2.2.8MHz Step-Down Switching Regulator | |
| Fabricantes | Linear Technology | |
| Logotipo |  |
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Hay una vista previa y un enlace de descarga de LT3481 (archivo pdf) en la parte inferior de esta página. Total 24 Páginas | ||
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LT3481www.DataSheet4U.com
36V, 2A, 2.8MHz Step-Down
Switching Regulator with
50µA Quiescent Current
FEATURES
■ Wide Input Range: 3.6V to 34V Operating,
36V Maximum
■ 2A Maximum Output Current
■ Low Ripple Burst Mode® Operation
50μA IQ at 12VIN to 3.3VOUT
Output Ripple < 15mV
■ Adjustable Switching Frequency: 300kHz to 2.8MHz
■ Low Shutdown Current: IQ < 1μA
■ Integrated Boost Diode
■ Power Good Flag
■ Saturating Switch Design: 0.18Ω On-Resistance
■ 1.265V Feedback Reference Voltage
■ Output Voltage: 1.265V to 20V
■ Soft-Start Capability
■ Synchronizable Between 275kHz to 475kHz
■ Small 10-Pin Thermally Enhanced MSOP and
(3mm x 3mm) DFN Packages
APPLICATIONS
■ Automotive Battery Regulation
■ Power for Portable Products
■ Distributed Supply Regulation
■ Industrial Supplies
■ Wall Transformer Regulation
DESCRIPTION
The LT®3481 is an adjustable frequency (300kHz to 2.8MHz)
monolithic buck switching regulator that accepts input
voltages up to 34V (36V maximum). A high efficiency
0.18Ω switch is included on the die along with a boost
Schottky diode and the necessary oscillator, control, and
logic circuitry. Current mode topology is used for fast
transient response and good loop stability. Low ripple
Burst Mode operation maintains high efficiency at low
output currents while keeping output ripple below 15mV
in a typical application. In addition, the LT3481 can fur-
ther enhance low output current efficiency by drawing
bias current from the output when VOUT is above 3V.
Shutdown reduces input supply current to less than 1μA
while a resistor and capacitor on the RUN/SS pin provide a
controlled output voltage ramp (soft-start). A power good
flag signals when VOUT reaches 90% of the programmed
output voltage. The LT3481 is available in 10-Pin MSOP
and 3mm x 3mm DFN packages with exposed pads for
low thermal resistance.
, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
TYPICAL APPLICATION
VIN
4.5V TO
34V
4.7μF
330pF
3.3V Step-Down Converter
OFF ON
16.2k
60.4k
VIN
RUN/SS
BD
BOOST
VC LT3481 SW
RT
PG BIAS
GND FB
0.47μF
4.7μH
324k
200k
VOUT
3.3V
2A
22μF
3481 TA01
Efficiency
90
10000.0
80 1000.0
70 100.0
60 10.0
50
40
30
0.0001
1.0
VIN = 12V
VOUT = 3.3V
L = 4.7μ
F = 800 kHz
0.1
0.01
0.001 0.01 0.1
1
10
LOAD CURRENT (A)
3481 TA01b
3481fb
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TYPICAL PERFORMANCE CHARACTERISTICS
LT3481www.DataSheet4U.com
Switch Voltage Drop
700
600
500
400
300
200
100
0
0 500 1000 1500 2000 2500 3000 3500
SWITCH CURRENT (mA)
3481 G10
Boost Pin Current
90
80
70
60
50
40
30
20
10
0
0 500 1000 1500 2000 2500 3000 3500
SWITCH CURRENT (mA)
3481 G11
Feedback Voltage
1.290
1.285
1.280
1.275
1.270
1.265
1.260
1.255
1.250
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
4381 G12
Switching Frequency
1.20
RT = 45.3k
1.15
1.10
1.05
1.00
0.95
0.90
0.85
0.80
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
4381 G13
Frequency Foldback
1200
RT = 45.3k
1000
800
600
400
200
0
0 200 400 600 800 1000 1200 1400
FB PIN VOLTAGE (mV)
3481 G14
Minimum Switch On-Time
140
120
100
80
60
40
20
0
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (˚C)
3481 G15
Soft Start
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0
0 0.5 1 1.5 2 2.5 3 3.5
RUN/SS PIN VOLTAGE (V)
3481 G16
RUN/SS Pin Current
12
10
8
6
4
2
0
0 5 10 15 20 25 30 35
RUN/SS PIN VOLTAGE (V)
3481 G17
Boost Diode
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 0.5 1.0 1.5 2.0
BOOST DIODE CURRENT (A)
3481 G18
3481fb
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LT3481www.DataSheet4U.com
APPLICATIONS INFORMATION
Of course, such a simple design guide will not always
result in the optimum inductor for your application. A larger
value inductor provides a slightly higher maximum load
current and will reduce the output voltage ripple. If your
load is lower than 2A, then you can decrease the value of
the inductor and operate with higher ripple current. This
allows you to use a physically smaller inductor, or one
with a lower DCR resulting in higher efficiency. There are
several graphs in the Typical Performance Characteristics
section of this data sheet that show the maximum load
current as a function of input voltage and inductor value
for several popular output voltages. Low inductance may
result in discontinuous mode operation, which is okay
but further reduces maximum load current. For details
of maximum output current and discontinuous mode
operation, see Linear Technology Application Note 44.
Finally, for duty cycles greater than 50% (VOUT/VIN >
0.5), there is a minimum inductance required to avoid
subharmonic oscillations. See AN19.
Input Capacitor
Bypass the input of the LT3481 circuit with a ceramic capaci-
tor of X7R or X5R type. Y5V types have poor performance
over temperature and applied voltage, and should not be
used. A 4.7μF to 10μF ceramic capacitor is adequate to
bypass the LT3481 and will easily handle the ripple current.
Note that larger input capacitance is required when a lower
switching frequency is used. If the input power source has
high impedance, or there is significant inductance due to
long wires or cables, additional bulk capacitance may be
necessary. This can be provided with a low performance
electrolytic capacitor.
Step-down regulators draw current from the input supply
in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT3481 and to force this very high frequency
switching current into a tight local loop, minimizing EMI.
A 4.7μF capacitor is capable of this task, but only if it is
placed close to the LT3481 and the catch diode (see the
PCB Layout section). A second precaution regarding the
ceramic input capacitor concerns the maximum input
voltage rating of the LT3481. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT3481 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT3481’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safety section).
For space sensitive applications, a 2.2μF ceramic capaci-
tor can be used for local bypassing of the LT3481 input.
However, the lower input capacitance will result in in-
creased input current ripple and input voltage ripple, and
may couple noise into other circuitry. Also, the increased
voltage ripple will raise the minimum operating voltage
of the LT3481 to ~3.7V.
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by the
LT3481 to produce the DC output. In this role it determines
the output ripple, and low impedance at the switching
frequency is important. The second function is to store
energy in order to satisfy transient loads and stabilize the
LT3481’s control loop. Ceramic capacitors have very low
equivalent series resistance (ESR) and provide the best
ripple performance. A good starting value is:
COUT
=
100
VOUT fSW
where fSW is in MHz, and COUT is the recommended
output capacitance in μF. Use X5R or X7R types. This
choice will provide low output ripple and good transient
response. Transient performance can be improved with
a higher value capacitor if the compensation network is
also adjusted to maintain the loop bandwidth. A lower
value of output capacitor can be used to save space
and cost but transient performance will suffer. See the
Frequency Compensation section to choose an appropriate
compensation network.
3481fb
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11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT3481.PDF ] | |
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