LTC3452 Datasheet, PDF (11/16 Pages) Linear Technology – Synchronous Buck-Boost MAIN/CAMERA White LED Driver

LTC3452
APPLICATIO S I FOR ATIO
COMPONENT SELECTION
Inductor Selection
The high frequency operation of the LTC3452 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
>
V_IN(MIN)2 • V_OUT – V_IN(MIN) • 100%
f •I_OUT(MAX) • %Ripple • V_OUT2
and in buck mode is:
( ) _L
>
V_OUT • V_IN(MAX) – V_OUT
f •I_OUT(MAX) • %Ripple •
• 100%
V_IN(MAX)
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 efficiency, 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 I²R 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 radi-
ated noise, use a toroid, pot core or shielded bobbin
inductor. For the white LED application, a 4.7µH inductor
value is recommended. See Table 2 for a list of component
suppliers.
Table 2. Inductor Vendor Information
SUPPLIER
WEB SITE
Coilcraft
www.coilcraft.com
Cooper/Coiltronics
www.cooperet.com
Murata
www.murata.com
Sumida
www.japanlink.com/sumida
Vishay-Dale
www.vishay.com
Input Capacitor Selection
Since the V_IN pin is the supply voltage for the IC it is
recommended to place at least a 2.2µF, low ESR bypass
capacitor to ground. See Table 3 for a list of component
suppliers.
Table 3. 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
=
I_OUT(MAX) • V_OUT – V_IN(MIN)
C_OUT • V_OUT2 • f
• 100
%
( ) _%Ripple
_ Buck
=
8
V_IN(MAX) – V_OUT
• V_IN(MAX) • f² •L
• 100
• C_OUT
%
where C_OUT = output filter 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 the operating frequency to
the unity-gain bandwidth of the converter is the amount
the output capacitance will have to increase from the
above calculations in order to maintain the desired tran-
sient response.
The other component of ripple is due to the ESR (equiva-
lent series resistance) of the output capacitor. Low ESR
capacitors should be used to minimize output voltage
ripple. For surface mount applications, Taiyo Yuden, TDK,
AVX ceramic capacitors, AVX TPS series tantalum capaci-
tors or Sanyo POSCAP are recommended. For the white
LED application, a 4.7µF capacitor value is recommended.
See Table 3 for a list of component suppliers.
3452f
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