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PDF LT3462 Data sheet ( Hoja de datos )
| Número de pieza | LT3462 | |
| Descripción | Inverting 1.2MHz/2.7MHz DC/DC Converters | |
| Fabricantes | Linear | |
| Logotipo |  |
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LT3462/LT3462A
Inverting 1.2MHz/2.7MHz
DC/DC Converters with
Integrated Schottky in ThinSOT
FEATURES
s Integrated Schottky Rectifier
s Fixed Frequency 1.2MHz/2.7MHz Operation
s Very Low Noise: 1mVP-P Output Ripple
s Low VCESAT Switch: 270mV at 250mA
s –5V at 100mA from 5V Input
s –12V at 30mA from 3.3V Input
s Low Input Bias Current GND Based FB Input
s Low Impedance (40Ω) 1.265V Reference Output
s High Output Voltage: Up to – 38V
s Wide Input Range: 2.5V to 16V
s Uses Tiny Surface Mount Components
s Low Shutdown Current: <10µA
s Low Profile (1mm) SOT-23 (ThinSOTTM) Package
U
APPLICATIO S
s CCD Bias
s LCD Bias
s GaAs FET Bias
s General Purpose Negative Voltage Supply
, LTC and LT are registered trademarks of Linear Technology Corporation.
ThinSOT is a trademark of Linear Technology Corporation
DESCRIPTIO
The LT®3462/LT3462A are general purpose fixed fre-
quency current mode inverting DC/DC converters. Both
devices feature an integrated Schottky and a low VCESAT
switch allowing a small converter footprint and lower parts
cost. The LT3462 switches at 1.2MHz while the LT3462A
switches at 2.7MHz. These high speeds enable the use of
tiny, low cost and low height capacitors and inductors.
The LT3462/LT3462A operate in a dual inductor inverting
topology that filters both the input and output currents.
Very low output voltage ripple approaching 1mVP-P can be
achieved when ceramic capacitors are used. Fixed fre-
quency switching ensures a clean output free from low
frequency noise typically present with charge pump solu-
tions. The 40V switch allows a VIN to VOUT differential of
up to 38V for dual inductor topologies.
Both devices provide a low impedance 1.265V reference
output to supply the feedback resistor network. A ground
referenced, high impedance FB input allows high feedback
resistor values without compromising output accuracy.
The LT3462/LT3462A are available in a 6-lead SOT-23
package.
TYPICAL APPLICATIO
5V to –5V, 100mA Inverting DC/DC Converter
VIN 22µH 1µF 22µH
5V
VOUT
–5V
SW D
267k
22pF
100mA
1µF VIN FB
LT3462A
68.1k
10µF
SDREF
GND
3462 TA01
75
TA = 25°C
70
65
Efficiency
VIN = 5V
VIN = 3.3V
60
55
0
20 40 60 80 100
LOAD CURRENT (mA)
3462 TA01b
3462af
1
1 page  
LT3462/LT3462A
APPLICATIO S I FOR ATIO
Inrush Current
The LT3462 has a built-in Schottky diode. When supply
voltage is applied to the VIN pin, the voltage difference
between VIN and VD generates inrush current flowing from
input through the inductor and the Schottky diode to
charge the flying capacitor to VIN. The maximum
nonrepetitive surge current the Schottky diode in the
LT3462 can sustain is 1.5A. The selection of inductor and
capacitor value should ensure the peak of the inrush
current to be below 1.5A. The peak inrush current can be
calculated as follows:
IP
=
VIN – O.6
L –1
C
exp
–
2
π
L
C
–
1
where L is the inductance between supply and SW, and C
is the capacitance between SW and D.
Table 3 gives inrush peak currents for some component
selections.
Table 3. Inrush Peak Current
VIN (V)
5
L (µH)
22
5 33
12 47
C (µF)
1
1
1
IP (A)
0.70
0.60
1.40
Inductor Selection
Each of the two inductors used with LT3462 should have
a saturation current rating (where inductance is approxi-
mately 70% of zero current inductance ) of approximately
0.25A or greater. If the device is used in the charge pump
mode, where there is only one inductor, then its rating
should be 0.35A or greater. DCR of the inductors should
be less than 1Ω. For LT3462, a value of 22µH is suitable
if using a coupled inductor such as Sumida CLS62-220. If
using two separate inductors, increasing the value to
47µH will result in the same ripple current. For LT3462A,
a value of 10µH for the coupled inductor and 22µH for two
inductors will be acceptable for most applications.
Capacitor Selection
Ceramic capacitors are recommended. An X7R or X5R
dielectric should be used to avoid capacitance decreasing
severely with applied voltage and at temperature limits.
The “flying” capacitor between the SW and D pins should
be a ceramic type of value 1µF or more. When used in the
dual inductor or coupled inductor topologies the flying
capacitor should have a voltage rating that is more than the
difference between the input and output voltages. For the
charge pump inverter topology, the voltage rating should
be more than the output voltage. The output capacitor
should be a ceramic type. Acceptable output capacitance
varies from 1µF for high VOUT (–36V), to 10µF for low VOUT
(–5V). The input capacitor should be a 1µF ceramic type
and be placed as close as possible to the LT3462/LT3462A.
Layout Hints
The high speed operation of the LT3462 demands careful
attention to board layout. You will not get advertised
performance with careless layout. Figure 2 shows the
recommended component placement. A ceramic capaci-
tor of 1µF or more must be placed close to the IC for input
supply bypassing.
C1
GND L1
C2
16
VIN
25
3 4 L2
C3 R2
R1 C4
3462 F03
VOUT
Figure 2. Suggested Layout
3462af
5
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet LT3462.PDF ] | |
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