LTC3567 datasheet(21/28 Pages) LINER

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LTC3567

3567f

APPLICATIONS INFORMATION

VBUS and VOUT 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 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 ripple for a given load cur-

rent. Increasing the size of this capacitor will reduce the

input 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 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 1VRMS 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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