Datasheet

LTC3556
27
3556f
APPLICATIONS INFORMATION
400mA Step-Down Switching Regulator Input/Output
Capacitor Selection
Low ESR (equivalent series resistance) MLCC capacitors
should be used at both buck regulator outputs as well as at
each buck regulator input supply (V
IN1
and V
IN2
). Only X5R
or X7R ceramic capacitors should be used because they
retain their capacitance over wider voltage and temperature
ranges than other ceramic types. A 10F output capaci-
tor is sufficient for most applications. For good transient
response and stability the output capacitor should retain
at least 4F of capacitance over operating temperature
and bias voltage. Each buck regulator input supply should
be bypassed with a 1F capacitor. Consult with capacitor
manufacturers for detailed information on their selection
and specifications of ceramic capacitors. Many manufac-
turers now offer very thin (<1mm tall) ceramic capacitors
ideal for use in height-restricted designs. Table 8 shows a
list of several ceramic capacitor manufacturers.
Table 8. Recommended Ceramic Capacitor Manufacturers
AVX www.avxcorp.com
Murata www.murata.com
Taiyo Yuden www.t-yuden.com
Vishay Siliconix www.vishay.com
TDK www.tdk.com
Buck-Boost Regulator Inductor Selection
Inductor selection criteria for the buck-boost are similar to
those given for the 400mA step-down switching regulators.
The buck-boost converter is designed to work with induc-
tors in the range of 1µH to 5µH. For most applications a
2.2µH inductor will suffi ce. Choose an inductor with a DC
current rating at least 2 times larger than the maximum
load current to ensure that the inductor does not saturate
during normal operation. If output short circuit is a pos-
sible condition, the inductor should be rated to handle
the maximum peak current specifi ed for the buck-boost
converter.
Table 9 shows several inductors that work well with the
LTC3556’s buck-boost regulator. These inductors offer a
good compromise in current rating, DCR and physical
size. Consult each manufacturer for detailed information
on their entire selection of inductors.
Table 9. Recommended Inductors for Buck-Boost Regulator
INDUCTOR
TYPE
L
(μH)
MAX
I
DC
(A)
MAX
DCR
(Ω)
SIZE IN mm
(L × W × H) MANUFACTURER
LPS4018 3.3
2.2
2.2
2.5
0.08
0.07
3.9 × 3.9 × 1.7
3.9 × 3.9 × 1.7
Coilcraft
www.coilcraft.com
D53LC 2.0 3.25 0.02
5.0 × 5.0 × 3.0
Toko
www.toko.com
7440430022 2.2 2.5 0.028
4.8 × 4.8 × 2.8
Würth-Elektronik
www.we-online.com
CDRH4D22/
HP
2.2 2.4 0.044
4.7 × 4.7 × 2.4
Sumida
www.sumida.com
SD14 2.0 2.56 0.045
5.2 × 5.2 ×
1.45
Cooper
www.cooperet.com
Buck-Boost Regulator Input/Output Capacitor
Selection
Low ESR MLCC capacitors should also be used at both the
buck-boost regulator output (V
OUT3
) and the buck-boost
regulator input supply (V
IN3
). Again, only X5R or X7R ce-
ramic capacitors should be used because they retain their
capacitance over wider voltage and temperature ranges
than other ceramic types. A 22µF output capacitor is suf-
fi cient for most applications. The buck-boost regulator
input supply should be bypassed with a 2.2µF capacitor.
Refer to Table 8 for recommended ceramic capacitor
manufacturers.
Buck-Boost Regulator Output Voltage Programming
The buck-boost regulator can be programmed for output
voltages greater than 2.75V and less than 5.5V. The full-
scale output voltage is programmed using a resistor divider
from the V
OUT3
pin connected to the FB3 pin such that:
VV
R
R
OUT FB33
1
2
1=+
⎛
⎝
⎜
⎞
⎠
⎟
where V
FB3
ranges from 0.425V to 0.8V.