LTC3556 Datasheet, PDF (27/36 Pages) Linear Technology – High Effi ciency USB Power Manager with Dual Buck and Buck-Boost DC/DCs

LTC3556
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 10μF output capaci-
tor is sufficient for most applications. For good transient
response and stability the output capacitor should retain
at least 4μF of capacitance over operating temperature
and bias voltage. Each buck regulator input supply should
be bypassed with a 1μF 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 suffice. 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 specified 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
MAX MAX
INDUCTOR L I_DC DCR
TYPE
(μH) (A) (Ω)
SIZE IN mm
(L × W × H) MANUFACTURER
LPS4018 3.3 2.2 0.08 3.9 × 3.9 × 1.7 Coilcraft
2.2 2.5 0.07 3.9 × 3.9 × 1.7 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/ 2.2 2.4 0.044 4.7 × 4.7 × 2.4 Sumida
HP
www.sumida.com
SD14
2.0 2.56 0.045 5.2 × 5.2 × Cooper
1.45
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-
ficient 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:
V_OUT3
=
V_FB3
⎛
⎝⎜
R1
R2
+
1^{⎞⎠⎟}
where V_FB3 ranges from 0.425V to 0.8V.
3556f
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