LTC3567 Datasheet, PDF (21/28 Pages) Linear Technology – High Effi ciency USB Power Manager Plus 1A Buck-Boost Converter with I2C Control

LTC3567
APPLICATIONS INFORMATION
V_BUS and V_OUT Bypass Capacitors
The style and value of capacitors used with the LTC3567
determine several important parameters such as regulator
control-loop stability and input voltage ripple. Because
the LTC3567 uses a step-down switching power supply
from V_BUS to V_OUT, its input current waveform contains
high frequency components. It is strongly recommended
that a low equivalent series resistance (ESR) multilayer
ceramic capacitor be used to bypass V_BUS. Tantalum and
aluminum capacitors are not recommended because of
their high ESR. The value of the capacitor on V_BUS directly
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_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 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 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 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.
Buck-Boost 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-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. 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 maximize efficiency, choose
an inductor with a low DC resistance. For a 3.3V output,
efficiency is reduced about 3% for a 100mΩ series resis-
tance at 1A load current, and about 2% for 300mΩ series
resistance at 200mA load current. Choose an inductor
with a DC current rating at least two 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 2.5A maximum peak current specified for the
buck-boost converter.
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 do not 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 LTC3567
requires to operate.
The inductor value also has an effect on Burst Mode op-
eration. Lower inductor values will cause the Burst Mode
operation switching frequencies to increase.
Table 7 shows several inductors that work well with the
LTC3567’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.
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