LTC1625 View Datasheet(PDF) - Linear Technology
Part Name
Description
MFG CO.
'LTC1625' PDF : 24 Pages View PDF
LTC1625
APPLICATIONS INFORMATION
Ferrite designs have very low core loss and are preferred
at high switching frequencies, so design goals can con-
centrate on copper loss and preventing saturation. Ferrite
core material saturates “hard,” which means that induc-
tance collapses rapidly when the peak design current is
exceeded. This results in an abrupt increase in inductor
ripple current and consequent output voltage ripple. Do
not allow the core to saturate!
Molypermalloy (from Magnetics, Inc.) is a very good, low
loss core material for toroids, but it is more expensive than
ferrite. A reasonable compromise from the same manu-
facturer is Kool Mµ. Toroids are very space efficient,
especially when you can use several layers of wire.
Because they generally lack a bobbin, mounting is more
difficult. However, designs for surface mount are available
which do not increase the height significantly.
Schottky Diode Selection
The Schottky diode D1 shown in Figure 1 conducts during
the dead time between the conduction of the power
MOSFETs. This prevents the body diode of the bottom
MOSFET from turning on and storing charge during the
dead time, which could cost as much as 1% in efficiency.
A 1A Schottky diode is generally a good size for 3A to 5A
regulators. The diode may be omitted if the efficiency loss
can be tolerated.
CIN and COUT Selection
In continuous mode, the drain current of the top MOSFET
is approximately a square wave of duty cycle VOUT/ VIN. To
prevent large input voltage transients, a low ESR input
capacitor sized for the maximum RMS current must be
used. The maximum RMS current is given by:
IRMS
≅ IO(MAX)
VOUT
VIN
VIN
VOUT
1/ 2
− 1
This formula has a maximum at VIN = 2VOUT, where IRMS
= IO(MAX)/2. This simple worst-case condition is com-
monly used for design because even significant deviations
do not offer much relief. Note that ripple current ratings
from capacitor manufacturers are often based on only
2000 hours of life. This makes it advisable to further derate
the capacitor or to choose a capacitor rated at a higher
temperature than required. Several capacitors may also be
placed in parallel to meet size or height requirements in the
design.
The selection of COUT is primarily determined by the ESR
required to minimize voltage ripple. The output ripple
∆VOUT is approximately bounded by:
∆VOUT
≤
∆IL ESR
+
1
(8)(f)(COUT )
Since ∆IL increases with input voltage, the output ripple is
highest at maximum input voltage. Typically, once the ESR
requirement is satisfied the capacitance is adequate for
filtering and has the required RMS current rating.
Manufacturers such as Nichicon, United Chemicon and
Sanyo should be considered for high performance through-
hole capacitors. The OS-CON semiconductor dielectric
capacitor available from Sanyo has the lowest product of
ESR and size of any aluminum electrolytic at a somewhat
higher price.
In surface mount applications, multiple capacitors may
have to be placed in parallel to meet the ESR requirement.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount packages. In the case of
tantalum, it is critical that the capacitors have been surge
tested for use in switching power supplies. An excellent
choice is the AVX TPS series of surface mount tantalum,
available in case heights ranging from 2mm to 4mm. Other
capacitor types include Sanyo OS-CON, Nichicon PL se-
ries, and Sprague 593D and 595D series. Consult the
manufacturer for other specific recommendations.
INTVCC Regulator
An internal P-channel low dropout regulator produces the
5.2V supply which powers the drivers and internal cir-
cuitry within the LTC1625. The INTVCC pin can supply up
to 50mA and must be bypassed to ground with a minimum
of 4.7µF tantalum or low ESR electrolytic capacitance.
Good bypassing is necessary to supply the high transient
currents required by the MOSFET gate drivers.
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