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PDF LT1935 Data sheet ( Hoja de datos )
| Número de pieza | LT1935 | |
| Descripción | DC/DC Converter | |
| Fabricantes | Linear Technology | |
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LT1935
1.2MHz Boost DC/DC
Converter in ThinSOT
with 2A Switch
FEATURES
■ 1.2MHz Switching Frequency
■ High Output Voltage: Up to 38V
■ Wide Input Range: 2.3V to 16V
■ Low VCESAT Switch: 180mV at 2A
■ Soft-Start
■ Uses Small Surface Mount Components
■ 5V at 1A from 3.3V Input
■ 12V at 600mA from 5V Input
■ Low Shutdown Current: < 1µA
■ Pin-for-Pin Compatible with the LT1613 and LT1930
■ Low Profile (1mm) SOT-23 (ThinSOTTM) Package
U
APPLICATIO S
■ Digital Cameras
■ Battery Backup
■ LCD Bias
■ Local 5V or 12V Supply
■ PC Cards
■ xDSL Power Supply
■ TFT-LCD Bias Supply
DESCRIPTIO
The LT®1935 is the industry’s highest power SOT-23
switching regulator. Its unprecedented 2A, 40V internal
switch allows high output currents to be generated in a
small footprint. Intended for space-conscious applica-
tions, the LT1935 switches at 1.2MHz, allowing the use of
tiny, low profile inductors and capacitors 2mm or less in
height. The NPN switch achieves a VCESAT of just 180mV
at 2A independent of supply voltage, resulting in high
efficiency even at maximum power levels from a 3V input.
A constant frequency, internally compensated, current
mode PWM architecture results in low, predictable output
noise that is easy to filter. Low ESR ceramic capacitors can
be used on the output, further reducing noise to the
millivolt level. The high voltage switch on the LT1935 is
rated at 40V, making the device ideal for boost converters
up to 38V as well as for single-ended primary inductance
converter (SEPIC) and flyback designs. The device can
generate 5V at up to 1A from a 3.3V supply or 5V at 550mA
from four alkaline cells in a SEPIC design.
The LT1935 is available in a 5-lead SOT-23 package.
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation.
TYPICAL APPLICATIO
L1
VIN
4.2µH
D1
5V
4.7µF
ON OFF
VIN SW
LT1935
SHDN
FB
GND
84.5k
10k
VOUT
12V
600mA
10µF
D1: ON SEMI MBRM120
L1: SUMIDA CDRH5D28-4R2
1935 F01
Figure 1. 5V to 12V, 600mA Step-Up DC/DC Converter
www.DataSheet4U.com
Efficiency, VOUT = 12V
90
85 VIN = 5V
80 VIN = 3.3V
75
70
65
60
55
50
0 100 200 300 400 500 600 700
LOAD CURRENT (mA)
1935 F01b
1935f
1
1 page  
LT1935
APPLICATIONS INFORMATION
Inductor Selection
Use inductors that are intended for high frequency power
applications. The saturation current rating should be at
least 2A. The RMS current rating, which is usually based
on heating of the inductor, should be higher than the
average current in the inductor in your application. For
best efficiency, the DC resistance should be less than
100mΩ.
A good first choice for the inductor value results in a ripple
current that is 1/3 of the maximum switch current:
L = 3 (VIN/VOUT) (VOUT – VIN)/(IMAX • f)
IMAX is the maximum switch current of 2A and f is the
switching frequency. At lower duty cycles (less than 70%),
this value can be lowered somewhat in order to use a
physically smaller inductor.
Table 1 lists several inductor manufacturers, along with
part numbers for inductors that are a good match to the
LT1935.
Table 1. Inductor Suppliers
Supplier
Model Prefix
Sumida
CDRH4D18, CDRH4D28,
CDRH5D18, CDRH5D28, CR43
Coiltronics/Cooper
Wu..rth Elektronik
SD10, SD12, SD18, SD20
WE-PD2S, WE-PD3S, WE-PD4S
Coilcraft
MSS5131, MSS6132, DO1608
Diode Selection
Use a Schottky rectifier with a 1A or higher current rating,
such as the On Semiconductor MBRM120. Its 20V reverse
voltage rating is adequate for most applications. Higher
output voltages may require a 30V of 40V diode.
Use a 4.7µF ceramic capacitor to bypass the input of the
LT1935. Be aware that the switching regulators require a
low impedance input supply. Additional bulk capacitance
may be required if the LT1935 circuit is more than a few
inches away from the power source. If there are low ESR
capacitors nearby, the input bypass capacitor can be
reduced to 2.2µF.
The output capacitor supports the output under transient
loads and stabilizes the control loop of the LT1935. Look
at the typical application circuits as a starting point to
choose a value. Generally, a higher output capacitance is
required at higher load currents and lower input voltages.
Figure 3 shows transient response of the circuit in Fig-
ure 1. The load is stepped from 200mA to 400mA and back
to 200mA. The transient performance can be improved by
increasing the output capacitance, but may require a
phase lead capacitor between the output and the FB pin.
Figure 4 shows the transient response with the output
capacitor increased to 20µF. Figure 5 shows the additional
improvement resulting from the phase lead capacitor.
VOUT
100mV/DIV
ILOAD
200mA/DIV
0
50µs/DIV
1935 F03
Figure 3. Transient Response of the Circuit in Figure 1,
COUT = 10µF
Capacitor Selection
Use capacitors with low ESR (equivalent series resis-
tance). In most cases, multilayer ceramic capacitors are
the best choice. They offer high performance (very low
ESR) in a small package. Use only X5R or X7R types; they
maintain their capacitance over temperature and applied
voltage. Other suitable capacitor types include low-ESR
tantalum capacitors that are specified for power applica-
tions, and newer types of capacitors such as Sanyo’s
POSCAP and Panasonic’s SP CAP.
VOUT
100mV/DIV
ILOAD
200mA/DIV
50µs/DIV
1935 F04
Figure 4. Transient Response with COUT = 20µF
1935f
5
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| PDF Descargar | [ Datasheet LT1935.PDF ] | |
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