Datasheet

LTC3425
17
3425f
COMPONENT SELECTION
Inductor Selection
The high frequency, multiphase operation of the LTC3425
allows the use of small surface mount inductors. The
minimum inductance value is proportional to the operat-
ing frequency and is limited by the following constraints:
L
f
and L
VV V
f Ripple V
IN MIN OUT MAX IN MIN
OUT MAX
>>
()
2
() ( ) ()
()
•–
••
where:
f = Operating frequency in MHz (of each phase)
Ripple = Allowable inductor current ripple (amps
peak-peak)
V
IN(MIN)
= Minimum input voltage
V
OUT(MAX)
= Maximum output voltage
The inductor current ripple is typically set to 20% to 40%
of the maximum inductor current.
For high efficiency, choose an inductor with high fre-
quency core material, such as ferrite to reduce core loses.
The inductor should have low ESR (equivalent series
resistance) to reduce the I
2
R losses, and must be able to
handle the peak inductor current without saturating. To
minimize radiated noise, use a shielded inductor. (Note
that the inductance of shielded types will drop more as
current increases, and will saturate more easily). See
Table 2 for a list of inductor manufacturers.
Table 2. Inductor Vendor Information
SUPPLIER PHONE FAX WEB SITE
Coilcraft [phone] [phone] www.coilcraft.com
Murata USA: USA: www.murata.com
[phone] [phone]
Sumida USA: USA: www.japanlink.com/
[phone] [phone] sumida
Japan: Japan:
81-3-3607-5111 81-3-3607-5144
TDK [phone] [phone] www.component.
tdk.com
Some example inductor part types are:
Coilcraft DO-1608, DS-1608 and DT-1608 series
Murata LQH3C, LQH4C, LQH32C and LQN6C series
Sumida CDRH3D16, CDRH4D18, CDRH4D28, CR32,
CR43 series
TDK RLF5018T and NLFC453232T series
Output Capacitor Selection
The output voltage ripple has three components to it. The
bulk value of the capacitor is set to reduce the ripple due
to charge into the capacitor each cycle. The max ripple due
to charge is given by:
V
IV
CVf
RBULK
PIN
OUT OUT
=
•
•••4
where:
I
P
= peak inductor current
f = switching frequency of one phase
APPLICATIO S I FOR ATIO
WUUU
LTC3425
L1
L2 L3
L4
3425 F05
C
IN
C
OUT
C
OUT
C
OUT
C
OUT
C
SS
R
T
Figure 5. Typical Board Layout
LTC3425
3425 F06
Figure 6. Example Board Layout for a 10W, 4-Phase Boost
Converter. Total Area = 0.50in
2
(with All Components Mounted
on the Topside of Board)