LTC3555-X_15 Datasheet, PDF (25/32 Pages) Linear Technology – High Efficiency USB Power Manager Triple Step-Down DC/DC

LTC3555/LTC3555-X
APPLICATIONS INFORMATION
by application. Some inductors that may be suitable for
this application are listed in Table 6.
Table 6. Recommended Inductors
MAX MAX
INDUCTOR L I_DC DCR
TYPE
(µH) (A) (Ω)
SIZE in mm
(L × W × H)
LPS4018 3.3 2.2 0.08 _{3.9 × 3.9 × 1.7}
D53LC
DB318C
WE-TPC
Type M1
CDRH6D12
CDRH6D38
3.3 2.26 0.034 5 × 5 × 3
3.3 1.55 0.070 3.8 × 3.8 × 1.8
3.3 1.95 0.065 4.8 × 4.8 × 1.8
3.3 2.2 0.0625 6.7 × 6.7 × 1.5
3.3 3.5 0.020 7 × 7 × 4
MANUFACTURER
Coilcraft
www.coilcraft.com
Toko
www.toko.com
Wurth Elektronik
www.we-online.com
Sumida
www.sumida.com
V_BUS and V_OUT Bypass Capacitors
The style and value of capacitors used with the LTC3555
family determine several important parameters such as
regulator control-loop stability and input voltage ripple.
Because the LTC3555 family uses a step-down switching
power supply from V_BUS to V_OUT, its input current wave-
form 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 current. 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 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 versus voltage with
a 1V_RMS AC test signal and as a result, overstate the ca-
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.
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 regula-
tor 3. 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 1.2V output, efficiency 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 specified for the step-down converters.
Different core materials and shapes will change the size/
current and price/current relationship of an inductor. 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
3555fd
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