LTC3556 Datasheet, PDF (26/36 Pages) Linear Technology – High Effi ciency USB Power Manager with Dual Buck and Buck-Boost DC/DCs

LTC3556
APPLICATIONS INFORMATION
To prevent large V_OUT voltage steps during transient load
conditions, it is also recommended that a ceramic capaci-
tor be used to bypass V_OUT. The output capacitor is used
in the compensation of the switching regulator. At least
4μF of actual capacitance with low ESR are required on
V_OUT. Additional capacitance will improve load transient
performance and stability.
Multilayer ceramic chip capacitors typically have excep-
tional ESR performance. MLCCs combined with a tight
board layout and an unbroken ground plane will yield very
good performance and low EMI emissions.
There are several types of ceramic capacitors available,
each having considerably different characteristics. For
example, X7R ceramic capacitors have the best voltage
and temperature stability. X5R ceramic capacitors have
apparently higher packing density but poorer performance
over their rated voltage and temperature ranges. Y5V
ceramic capacitors have the highest packing density,
but must be used with caution, because of their extreme
nonlinear characteristic of capacitance vs voltage. The
actual in-circuit capacitance of a ceramic capacitor should
be measured with a small AC signal (ideally less than
200mV) as is expected in-circuit. Many vendors specify
the capacitance vs voltage with a 1V_RMS AC test signal and
as a result overstate the capacitance that the capacitor will
present in the application. Using similar operating condi-
tions as the application, the user must measure or request
from the vendor the actual capacitance to determine if the
selected capacitor meets the minimum capacitance that
the application requires.
400mA Step-Down Switching Regulator Inductor
Selection
Many different sizes and shapes of inductors are avail-
able from numerous manufacturers. Choosing the right
inductor from such a large selection of devices can be
overwhelming, but following a few basic guidelines will
make the selection process much simpler.
The buck converters are designed to work with inductors in
the range of 2.2μH to 10μH. For most applications a 4.7μH
inductor is suggested for both buck regulators. Larger
value inductors reduce ripple current which improves
output ripple voltage. Lower value inductors result in higher
ripple current and improved transient response time. To
26
maximize efficiency, choose an inductor with a low DC
resistance. For a 1.2V output, efficiency is reduced about
2% for 100mΩ series resistance at 400mA load current,
and about 2% for 300mΩ series resistance at 100mA load
current. Choose an inductor with a DC current rating at
least 1.5 times larger than the maximum load current to
ensure that the inductor does not saturate during normal
operation. If output short-circuit is a possible condition,
the inductor should be rated to handle the maximum peak
current specified for the buck converters.
Different core materials and shapes will change the
size/current and price/current relationship of an induc-
tor. Toroid or shielded pot cores in ferrite or Permalloy
materials are small and don’t radiate much energy, but
generally cost more than powdered iron core inductors
with similar electrical characteristics. Inductors that are
very thin or have a very small volume typically have much
higher core and DCR losses, and will not give the best ef-
ficiency. The choice of which style inductor to use often
depends more on the price vs size, performance and any
radiated EMI requirements than on what the LTC3556
requires to operate.
The inductor value also has an effect on forced burst
and Burst Mode operations. Lower inductor values will
cause the Burst Mode and forced Burst Mode switching
frequencies to increase.
Table 7 shows several inductors that work well with the
LTC3556’s buck regulators. 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 7. Recommended Inductors for 400mA Step-Down
Switching Regulators
NDUCTOR
TYPE
DE2818C
DE2812C
L MAX MAX SIZE IN mm
(μH) I_DC (A) DCR (Ω) (L × W × H) MANUFACTURER
4.7 1.25 0.072* 3.0 × 2.8 × 1.8 Toko
4.7 1.15 0.13* 3.0 × 2.8 × 1.2 ^www.toko.com
CDRH3D16 4.7 0.9
0.11 4.0 × 4.0 × 1.8 Sumida
www.sumida.com
SD3118
SD3112
4.7 1.3
4.7 0.8
0.162 3.1 × 3.1 × 1.8 Cooper
0.246
3.1 × 3.1 × 1.2
www.cooperet.
com
LPS3015 4.7 1.1
0.2 3.0 × 3.0 × 1.5 Coilcraft
www.coilcraft.com
*Typical DCR
3556f