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PDF LT1912 Data sheet ( Hoja de datos )
| Número de pieza | LT1912 | |
| Descripción | Step-Down Switching Regulator | |
| Fabricantes | Linear Technology Corporation | |
| Logotipo |  |
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LT1912
36V, 2A, 500kHz Step-Down
Switching Regulator
FEATURES
■ Wide Input Range:
Operation From 3.6V to 36V
■ 2A Maximum Output Current
■ Adjustable Switching Frequency: 200kHz to 500kHz
www.DataSh■eetL4Uo.wcomShutdown Current: IQ < 1μA
■ Integrated Boost Diode
■ Synchronizable Between 250kHz to 500kHz
■ Saturating Switch Design: 0.25Ω On-Resistance
■ 0.790V Feedback Reference Voltage
■ Output Voltage: 0.79V to 20V
■ Soft-Start Capability
■ Small 10-Pin Thermally Enhanced MSOP and
(3mm × 3mm) DFN Packages
APPLICATIONS
■ Automotive Battery Regulation
■ Set Top Box
■ Distributed Supply Regulation
■ Industrial Supplies
■ Wall Transformer Regulation
DESCRIPTION
The LT®1912 is an adjustable frequency (200kHz to
500kHz) monolithic step-down switching regulator that
accepts input voltages up to 36V. A high efficiency 0.25Ω
switch is included on the die along with a boost Schottky
diode and the necessary oscillator, control, and logic cir-
cuitry. Current mode topology is used for fast transient
response and good loop stability. The LT1912 allows the
use of ceramic capacitors resulting in low output ripple
while keeping total solution size to a minimum. The low
current shutdown mode 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). The LT1912 is available in 10-Pin MSOP and
3mm × 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 36V
3.3V Step-Down Converter
OFF ON
VIN
RUN/SS
BD
BOOST
4.7μF
470pF
20k
VC LT1912 SW
RT
68.1k
SYNC
GND
FB
VOUT
3.3V
2A
0.47μF 6.8μH
316k
100k
47μF
1912 TA01
Efficiency
100
VOUT = 5V
90
80 VOUT = 3.3V
70
60
VIN = 12V
L = 6.8μF
50 F = 500kHz
0 0.5 1.0 1.5
2
LOAD CURRENT (A)
1912 TA01b
1912f
1
1 page  
LT1912
TYPICAL PERFOR A CE CHARACTERISTICS TA = 25°C unless otherwise noted.
Feedback Voltage
840
820
800
www.DataSheet4U.com
780
760
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (°C)
1912 G09
Switching Frequency
1.20
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)
1912 G10
Frequency Foldback
1.2
1.0
0.8
0.6
0.4
0.2
0
0 100 200 300 400 500 600 700 800 900
FB PIN VOLTAGE (mV)
1912 G11
Minimum Switch On-Time
140
120
100
80
60
40
20
0
–50 –25
0 25 50 75 100 125 150
TEMPERATURE (˚C)
1912 G12
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
RUN/SS PIN VOLTAGE (V)
3.5
1912 G13
RUN/SS Pin Current
12
10
8
6
4
2
0
05
10 15 20 25 30 35
RUN/SS PIN VOLTAGE (V)
1912 G14
Boost Diode
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)
1912 G15
Error Amp Output Current
50
40
30
20
10
0
–10
–20
–30
–40
–50
–200
–100
0
100
FB PIN ERROR VOLTAGE (V)
200
1912 G16
1912f
5
5 Page 
LT1912
APPLICATIONS INFORMATION
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 opera-
www.DataShteioetn4,Us.ceoemLinear 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 LT1912 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 LT1912 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 lower performance
electrolytic capacitor.
Step-down regulators draw current from the input sup-
ply in pulses with very fast rise and fall times. The input
capacitor is required to reduce the resulting voltage
ripple at the LT1912 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 LT1912 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 LT1912. A ceramic input capacitor
combined with trace or cable inductance forms a high
quality (under damped) tank circuit. If the LT1912 circuit
is plugged into a live supply, the input voltage can ring to
twice its nominal value, possibly exceeding the LT1912’s
voltage rating. This situation is easily avoided (see the Hot
Plugging Safely section).
Output Capacitor and Output Ripple
The output capacitor has two essential functions. Along
with the inductor, it filters the square wave generated by the
LT1912 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
LT1912’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 compen-
sation network.
When choosing a capacitor, look carefully through the
data sheet to find out what the actual capacitance is under
operating conditions (applied voltage and temperature).
A physically larger capacitor, or one with a higher voltage
rating, may be required. High performance tantalum or
electrolytic capacitors can be used for the output capacitor.
Low ESR is important, so choose one that is intended for
use in switching regulators. The ESR should be specified
by the supplier, and should be 0.05Ω or less. Such a
capacitor will be larger than a ceramic capacitor and will
have a larger capacitance, because the capacitor must be
large to achieve low ESR. Table 2 lists several capacitor
vendors.
1912f
11
11 Page | ||
| Páginas | Total 24 Páginas | |
| PDF Descargar | [ Datasheet LT1912.PDF ] | |
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