LTC3440_15 Datasheet, PDF (12/20 Pages) Linear Technology – Micropower Synchronous Buck-Boost DC/DC Converter

LTC3440
APPLICATIONS INFORMATION
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 frequen-
cy 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 sup-
port 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:
www.murata.com
[phone] [phone]
[phone]
Sumida
USA:
[phone]
Japan:
81(3) 3607-5111
[phone]
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 transient
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 ceramic
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
WEB SITE
AVX
[phone] [phone] www.avxcorp.com
Sanyo
[phone] [phone] www.sanyovideo.com
Taiyo Yuden [phone] [phone] 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.
3440fc
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