LTC3444_15 Datasheet, PDF (14/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter for WCDMA Applications

LTC3444
U
OPERATIO
Inductor Selection
The high frequency operation of the LTC3444 allows the
use of small surface mount inductors. The internal loop
compensation is designed to work with a 2.2μH inductor
(1.5μH for V_IN < 3.1V). The 2.2μH inductor was selected to
optimize the transient response to the control input. The
use of a 2.2μH inductor pushes out the right half plane
(RHP) zero frequency and allows the loop crossover to
occur at frequencies higher than the output L-C double
pole.
For external compensation the inductor selection is based
on the desired inductor ripple current. The inductor ripple
current is typically set to 20% to 40% of the average
inductor current. Increased inductance results in lower
ripple current, however, higher inductance pulls in the
RHP zero frequency and limits the maximum crossover
frequency possible. Refer to Closing the Feedback Loop
for more information on the RHP zero. For a given ripple
the inductance terms are given as follows:
L_BOOST
>
V_IN(MIN) •(V_OUT – V_IN(MIN))H
f •I_OUT(MAX) • ΔI_L • V_OUT
L_BUCK
>
V_OUT • (V_IN(MAX) – V_OUT) _H
f •I_OUT(MAX) • ΔI_L • V_IN(MAX)
where f = operating frequency, Hz
ΔI_L = inductor ripple current, A
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
In most cases, the boost configuration will be used to
determine the minimum inductance allowed for a given
ripple current.
For high efficiency, choose a ferrite inductor with a high
frequency core material 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. To minimize radiated
noise, use a shielded inductor. See Table 2 for a suggested
list of inductor suppliers.
Table 2. Inductor Vendor Information
SUPPLIER
PHONE
Coilcraft
[phone]
CoEv Magnetics
[phone]
COOPER Bussmann
[phone]
Murata
[phone]
[phone]
Sumida
USA: [phone]
Japan: 81(3) 3607-5111
TDK
[phone]
TOKO
[phone]
FAX
[phone]
[phone]
1-800-544-2570
[phone]
USA: [phone]
Japan: 81(3) 3607-5144
[phone]
[phone]
WEB SITE
www.coilcraft.com
www.circuitprotection.com/magnetics.asp
www.coooperET.com
www.murata.com
www.sumida.com
www.component.tdk.com
www.tokoam.com
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