LTC3566-2_15 Datasheet, PDF (21/28 Pages) Linear Technology – High Efficiency USB Power Manager Plus 1A Buck-Boost Converter

LTC3566/LTC3566-2
APPLICATIONS INFORMATION
V_BUS and V_OUT Bypass Capacitors
The style and value of capacitors used with the LTC3566
family determine several important parameters such as
regulator control-loop stability and input voltage ripple.
Because the LTC3566 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 voltage ripple for
a given load current. Increasing the size of this capacitor
will reduce the input voltage 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
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.
Buck-Boost Regulator Input/Output Capacitor
Selection
Low ESR MLCC capacitors should be used at both the
buck-boost regulator output (V_OUT1) and the buck-boost
regulator input supply (V_IN1). Only X5R or X7R ceramic
capacitors should be used because they retain their ca-
pacitance over wider voltage and temperature ranges than
other ceramic types. A 22μF output capacitor is sufficient
for most applications. The buck-boost regulator input sup-
ply should be bypassed with a 2.2μF capacitor. Consult
with capacitor manufacturers for detailed information on
their selection and specifications of ceramic capacitors.
Many manufacturers now offer very thin (<1mm tall)
ceramic capacitors ideal for use in height restricted de-
signs. Table 6 shows a list of several ceramic capacitor
manufacturers.
Table 6. Recommended Ceramic Capacitor Manufacturers
MANUFACTURER
WEBSITE
AVX
www.avxcorp.com
Murata
www.murata.com
Taiyo Yuden
www.t-yuden.com
Vishay Siliconix
www.vishay.com
TDK
www.tdk.com
Over-Programming the Battery Charger
The USB high power specification allows for up to 2.5W to
be drawn from the USB port (5V x 500mA). The PowerPath
switching regulator transforms the voltage at V_BUS to just
above the voltage at BAT with high efficiency, while limiting
power to less than the amount programmed at CLPROG.
In some cases the battery charger may be programmed
(with the PROG pin) to deliver the maximum safe charging
current without regard to the USB specifications. If there
is insufficient current available to charge the battery at the
programmed rate, the PowerPath regulator will reduce
charge current until the system load on V_OUT is satisfied
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