Datasheet

LTC3454
9
3454fa
APPLICATIONS INFORMATION
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3454 allows the
use of small surface mount inductors. The inductor cur-
rent ripple is typically set to 20% to 40% of the maximum
average inductor current. For a given ripple the inductance
term in boost mode is:
L
VVV
fI
IN MIN OUT IN MIN
OUT MAX
>
()
() ()
()
•– •%
•
2
100
••% •Ripple V
OUT
2
and in buck mode is:
L
VVV
fV Ripp
IN MAX OUT OUT
IN MAX
>
()
()
()
–••%
••%
100
ll e I
OUT
•
where f = operating frequency, Hz
%Ripple = allowable inductor current ripple, %
V
IN(MIN)
= minimum input voltage, V
V
IN(MAX)
= maximum input voltage, V
V
OUT
= output voltage, V
I
OUT(MAX)
= maximum output load current
For high effi ciency, choose an inductor with a 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.
Molded chokes or chip inductors usually do not have
enough core to support peak inductor currents >1A. To
minimize radiated noise, use a toroid, pot core or shielded
bobbin inductor. For white LED application, a 4.7μH/5μH
inductor value is recommended. See Table 1 for a list of
component suppliers.
Table 1. Inductor Vendor Information
SUPPLIER WEB SITE
Coilcraft www.coilcraft.com
Cooper/Coiltronics www.cooperet.com
Murata www.murata.com
Sumida www.japanlink.com/sumida
Toko www.toko.com
Vishay-Dale www.vishay.com
Input Capacitor Selection
Since the V
IN
pin is the supply voltage for the IC it is recom-
mended to place at least a 2.2μF, low ESR bypass capacitor
to ground. See Table 2 for a list of component suppliers.
Table 2. Capacitor Vendor Information
SUPPLIER WEB SITE
AVX www.avxcorp.com
Sanyo www.sanyovideo.com
Taiyo Yuden www.t-yuden.com
TDK www.component.tdk.com
Output Capacitor Selection
The bulk value of the capacitor is set to reduce the ripple
due to charge into the capacitor each cycle. The steady-
state ripple due to charge is given by:
%_
•– •
() ()
Ripple Boost
IVV
OUT MAX OUT IN MIN
=
()
100%%
••
%_
–•
()
CV f
Ripple Buck
VV
OUT OUT
IN MAX OUT
2
=
()
1100
8
2
%
••••
()
VfLC
IN MAX OUT
where C
OUT
= output fi lter capacitor, F
The output capacitance is usually many times larger in
order to handle the transient response of the converter. For
a rule of thumb, the ratio of operating frequency to unity-
gain bandwidth of the converter is the amount the output
capacitance will have to increase from the above calcula-
tions in order to maintain desired transient response.
The other component of ripple is due to ESR (equivalent
series resistance) of the output capacitor. Low ESR ca-
pacitors should be used to minimize output voltage ripple.
For surface mount applications, Taiyo Yuden, TDK, AVX
ceramic capacitors, AVX TPS series tantalum capacitors
or Sanyo POSCAP are recommended. For the white LED
application, a 10μF capacitor value is recommended. See
Table 2 for a list of component suppliers.
Optional Schottky Diodes
Schottky diodes across the synchronous switches B and
D are not required, but provide a lower drop during the
break-before-make time (typically 20ns) of the NMOS to
PMOS transition, improving effi ciency. Use a Schottky