LTC3426ES6 데이터 시트보기 (PDF) - Analog Devices
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LTC3426ES6 Datasheet PDF : 12 Pages
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LTC3426
OPERATION
The LTC3426 is a monolithic 1.2MHz boost converter
housed in a 6-lead SOT-23 package. The device features
fixed frequency, current mode PWM control for excellent
line and load regulation. The low RDS(ON) NMOS switch
enables the device to maintain high efficiency over a wide
range of load current. Operation of the feedback loop
which sets the peak inductor current to keep the output in
regulation can be best understood by referring to the Block
Diagram in Figure 1. At the start of each clock cycle a latch
in the PWM logic is set and the NMOS switch is turned on.
The sum of a voltage proportional to the switch current and
a slope compensating voltage ramp is fed to the positive
input to the PWM comparator. When this voltage exceeds
either a voltage proportional to the 2A current limit or the
PWM control voltage, the latch in the PWM logic is reset
and NMOS switch is turned off. The PWM control voltage
at the output of the error amplifier is the amplified and
compensated difference between the feedback voltage on
the FB pin and the internal reference voltage of 1.22V. If
the control voltage increases, more current is delivered
to the output. When the control voltage exceeds the ILIMIT
reference voltage, the peak current is limited to a mini-
mum of 2A. The current limit helps protect the LTC3426
internal switch and external components connected to it.
If the control voltage decreases, less current is delivered
to the output. During load transients control voltage may
decrease to the point where no switching occurs until the
feedback voltage drops below the reference. The LTC3426
has an integrated soft-start feature which slowly ramps
up the feedback control node from 0V. The soft-start is
initiated when SHDN is pulled high.
APPLICATIONS INFORMATION
Setting the Output Voltage
The output voltage, VOUT, is set by a resistive divider from
VOUT to ground. The divider tap is tied to the FB pin. VOUT
is set by the formula:
VOUT
=
1.22
•
1+
R1
R2
Inductor Selection
The LTC3426 can utilize small surface mount inductors
due to its 1.2MHz switching frequency. A 1.5µH or 2.2µH
inductor will be the best choice for most LTC3426 ap-
plications. Larger values of inductance will allow greater
output current capability by reducing the inductor ripple
current. Increasing the inductance above 3.3µH will in-
crease component size while providing little improvement
in output current capability. The inductor current ripple
is typically set for 20% to 40% of the maximum inductor
current (IP). High frequency ferrite core inductor materials
reduce frequency dependent power losses compared to
cheaper powdered iron types, improving efficiency. The
inductor should have low DCR (DC resistance) to reduce
the I2R power losses, and must be able to handle the peak
inductor current without saturating.
Several inductor manufacturers are listed in Table 1.
Table 1. Inductor Manufacturers
TDK
Sumida
Murata
www.tdk.com
www.sumida.com
www.murata.com
Output and Input Capacitor Selection
Low ESR (equivalent series resistance) capacitors should
be used to minimize the output voltage ripple. Multilayer
ceramic capacitors are an excellent choice as they have
extremely low ESR and are available in small footprints.
A 15µF to 30µF output capacitor is sufficient for most ap-
plications. X5R and X7R dielectric materials are preferred
for their ability to maintain capacitance over wide voltage
and temperature ranges.
Low ESR input capacitors reduce input switching noise
and reduce the peak current drawn from the input supply.
It follows that ceramic capacitors are also a good choice
for input decoupling and should be located as close as
For more information www.linear.com/LTC3426
3426fb
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