LTC3775 View Datasheet(PDF) - Linear Technology
Part Name
Description
Manufacturer
LTC3775 Datasheet PDF : 34 Pages
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LTC3775
APPLICATIONS INFORMATION
ΔIL may be calculated using the equation:
IL
=
VOUT
L • fSW
1–
VOUT
VIN
Since ΔIL increases with input voltage, the output ripple
voltage is highest at maximum input voltage. Typically,
once the ESR requirement is satisfied, the capacitance is
adequate for filtering and has the necessary RMS current
rating.
Manufacturers such as Sanyo, Panasonic and Cornell
Dublilier should be considered for high performance
through-hole capacitors. The OS-CON semiconductor
electrolyte capacitor available from Sanyo has a good
(ESR)(size) product. An additional ceramic capacitor in
parallel with OS-CON capacitors is recommended to offset
the effect of lead inductance.
In surface mount applications, multiple capacitors may
have to be connected in parallel to meet the ESR or tran-
sient current handling requirements of the application.
Aluminum electrolytic and dry tantalum capacitors are
both available in surface mount configurations. New special
polymer surface mount capacitors offer very low ESR also
but have much lower capacitive density per unit volume.
In the case of tantalum, it is critical that the capacitors are
surge tested for use in switching power supplies. Several
excellent output capacitor choices are the Sanyo POSCAP
TPD, POSCAP TPB, AVX TPS, AVX TPSV, the Kemet T510
series of surface mount tantalums, Kemet AO-CAPs or the
Panasonic SP series of surface mount special polymer
capacitors available in case heights ranging from 2mm
to 4mm. Other capacitor types include Nichicon PL series
and Sprague 595D series. Consult the manufacturer for
other specific recommendations.
Inductor Selection
The inductor in a typical LTC3775 application circuit is
chosen based on the required ripple current, its size and
its saturation current rating. The inductor should not be al-
lowed to saturate below the hard current limit threshold.
The inductor value sets the ripple current, which is com-
monly chosen at around 40% of the anticipated full load
current. Lower ripple current reduces core losses in the
inductor, ESR losses in the output capacitors and out-
put voltage ripple. Highest efficiency is obtained at low
frequency with small ripple current. However, achieving
high efficiency requires a large inductor and generates
higher output voltage excursion during load transients.
There is a trade-off between component size, efficiency
and operating frequency. Given a specified limit for ripple
current, the inductor value can be obtained using the fol-
lowing equation:
L=
fSW
VOUT
• IL(MAX)
• 1–
VOUT
VIN(MAX)
Once the value for L is known, the type of inductor must
be selected. High efficiency converters generally cannot
afford the core loss found in low cost powdered iron cores,
forcing the use of more expensive ferrite, molypermalloy or
Kool Mμ® cores. A variety of inductors designed for high
current, low voltage applications are available from manu-
facturers such as Sumida, Panasonic, Coiltronics, Coilcraft
and Toko. See the Current Limit Programming section for
calculation of the inductor saturation current.
Current Limit Programming
If current sensing is implemented with a sense resistor,
the topside current limit can be programmed by setting
RILIMT as follows:
RILIMT
=
CF
• RSENSE
•
IO(MAX) + 0.5 •
ILIMIT(MIN)
IL
where:
RSENSE = Sense resistor value
IO(MAX) = Maximum output current
ΔIL = Inductor ripple current (refer to the Output Capaci-
tor Selection section).
ILIMT(MIN) = ILIMT pin minimum pull-down current of
90μA
CF = Correction factor to provide safety margin and
account for RSENSE tolerance; use a value of CF = 1.2
is reasonable.
3775fa
22
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