LTC3440 Datasheet, PDF (12/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

LTC3440
APPLICATIO S I FOR ATIO
Ripple = allowable inductor current ripple
(e.g., 0.2 = 20%)
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 the peak inductor currents in the 1A to 2A
region. To minimize radiated noise, use a toroid, pot core
or shielded bobbin inductor. See Table 1 for suggested
components and Table 2 for a list of component suppliers.
Table 1. Inductor Vendor Information
SUPPLIER PHONE
FAX
WEB SITE
Coilcraft [phone] [phone] www.coilcraft.com
Coiltronics [phone] [phone] www.coiltronics.com
Murata
USA:
USA:
[phone] [phone]
[phone]
www.murata.com
Sumida
USA:
[phone] [phone]
Japan:
81(3) 3607-5111 81(3) 3607-5144
www.japanlink.com/
sumida
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 =} ^IOUT(MAX) ^•
V_IN(MAX) – V_OUT
• 100
%
C_OUT • V_IN(MAX) • V_OUT • f
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 ce-
ramic capacitors, AVX TPS series tantalum capacitors or
Sanyo POSCAP are recommended.
Input Capacitor Selection
Since the V_IN pin is the supply voltage for the IC it is
recommended to place at least a 4.7μF, low ESR bypass
capacitor.
Table 2. Capacitor Vendor Information
SUPPLIER PHONE
FAX
AVX
[phone] [phone]
Sanyo
[phone] [phone]
Taiyo Yuden [phone] [phone]
WEB SITE
www.avxcorp.com
www.sanyovideo.com
www.t-yuden.com
Optional Schottky Diodes
To achieve a 1%-2% efficiency improvement above 50mW,
Schottky diodes can be added across synchronous switches
B (SW1 to GND) and D (SW2 to V_OUT). The Schottky
diodes will provide a lower voltage drop during the break-
before-make time (typically 15ns) of the NMOS to PMOS
transition. General purpose diodes such as a 1N914 are
not recommended due to the slow recovery times and will
compromise efficiency. If desired a large Schottky diode,
such as an MBRM120T3, can be used from SW2 to V_OUT.
A low capacitance Schottky diode is recommended
from GND to SW1 such as a Phillips PMEG2010EA or
equivalent.
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