LTC3555 Linear Technology, LTC3555 Datasheet - Page 25

• Manufacturer Part Number: LTC3555
• Description: High Efficiency USB Power Manager + Triple Step-Down DC/DC
• Manufacturer: Linear Technology

www.DataSheet4U.com
APPLICATIONS INFORMATION
aluminum capacitors are not recommended because of
their high ESR. The value of the capacitor on V
controls the amount of input ripple for a given load cur-
rent. Increasing the size of this capacitor will reduce the
input ripple.
To prevent large V
conditions, it is also recommended that a ceramic capaci-
tor be used to bypass V
in the compensation of the switching regulator. At least
4μF of actual capacitance with low ESR are required on
V
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 appar-
ently 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 non-linear
characteristic of capacitance verse voltage. The actual
in-circuit capacitance of a ceramic capacitor should be
measured with a small AC signal as is expected in-circuit.
Many vendors specify the capacitance verse voltage with
a 1V
pacitance that the capacitor will present in the application.
Using similar operating conditions 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.
General Purpose 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.
OUT
RMS
. Additional capacitance will improve load transient
AC test signal and as a result, overstate the ca-
OUT
voltage steps during transient load
OUT
. The output capacitor is used
BUS
directly
The general purpose step-down 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 the
lower power switching regulators 1 and 2 and 2.2μH is
recommended for the more powerful switching regulator 3.
Larger value inductors reduce ripple current which im-
proves output ripple voltage. Lower value inductors result
in higher ripple current and improved transient response
time. To maximize effi ciency, choose an inductor with a
low DC resistance. For a 1.2V output, effi ciency is reduced
about 2% for 100mΩ series resistance at 400mA load cur-
rent, 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 specifi ed for the step-down 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-
fi ciency. 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 LTC3555
requires to operate.
The inductor value also has an effect on Forced Burst and
Burst Mode operations. Lower inductor values will cause
the Burst and Forced Burst Mode switching frequencies
to increase.
Table 7 shows several inductors that work well with the
LTC3555’s general purpose regulators. These inductors of-
fer a good compromise in current rating, DCR and physical
size. Consult each manufacturer for detailed information
on their entire selection of inductors.
LTC3555
25
3555f