LTC3555 Datasheet, PDF (25/32 Pages) Linear Technology – High Effi ciency 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
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
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 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.
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 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 pos-
sible 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
3555fd
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