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LTC3566

3566fa

APPLICATIONS INFORMATION

VBUS and VOUT Bypass Capacitors

The style and value of capacitors used with the LTC3566

determine several important parameters such as regulator

control-loop stability and input voltage ripple. Because

the LTC3566 uses a step-down switching power supply

from VBUS to VOUT, 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 VBUS. Tantalum and

aluminum capacitors are not recommended because of

their high ESR. The value of the capacitor on VBUS 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 VOUT voltage steps during transient load

conditions, it is also recommended that a ceramic capaci-

tor be used to bypass VOUT. The output capacitor is used

in the compensation of the switching regulator. At least

4μF of actual capacitance with low ESR are required on

VOUT. 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 1VRMS 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 (VOUT1) and the buck-boost

regulator input supply (VIN1). 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 VBUS 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 VOUT is satisﬁed

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