LTC3290

10

Rev. 0

For more information www.analog.com

APPLICATIONS INFORMATION

with a ceramic capacitor to increase the total capacitance

but should not be used alone because of their high ESR.

at the expense of higher minimum turn-on time.

of bypass capacitance required at the input depends on

with at least 5µF of low ESR capacitance. A high ESR

capacitor such as tantalum or aluminum will have higher

input noise than a low ESR ceramic capacitor. Therefore,

a ceramic capacitor is recommended as the main bypass

capacitance with a tantalum or aluminum capacitor used

in parallel if desired.

Flying Capacitor Selection

charge pumps. A 1µF or greater ceramic capacitor is sug-

gested for the flying capacitor for applications requiring

the full rated output current of the charge pump. Polarized

capacitors such as aluminum or tantalum should not be

during startup.

For very light load applications, the flying capacitor may

be reduced to save space or cost. For example, a 0.2µF

capacitor might be sufficient for load currents up to 10mA.

A smaller flying capacitor leads to a larger effective open

current that can be delivered by the charge pump.

Ceramic Capacitors

Ceramic capacitors of different materials lose their capaci-

tancewithhighertemperatureandvoltageatdifferentrates.

For example, a capacitor made of X5R or X7R material

will retain most of its capacitance from –40°C to 85°C

whereasaZ5UorY5Vstylecapacitorwillloseconsiderable

capacitance over that range. Z5U and Y5V capacitors may

also have a poor voltage coefficient causing them to lose

60% or more of their capacitance when the rated voltage

isapplied.Thereforewhencomparingdifferentcapacitors,

it is often more appropriate to compare the amount of

achievable capacitance for a given case size rather than

discussing the specified capacitance value. The capacitor

manufacture’s data sheet should be consulted to ensure

the desired capacitance at all temperatures and voltages.

Below is a list of ceramic capacitor manufacturers and

their websites.

AVX

www.avxcorp.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

Vishay

www.vishay.com

TDK

www.component.tdk.com

Layout Considerations

Due to high switching frequency and high transient cur-

rents produced by the LTC3290, careful board layout is

necessary for optimum performance. A true ground plane

and short connections to all the external capacitors will

improve performance and ensure proper regulation under

all conditions. Figure 5 shows an example layout for the

LTC3290.

at a high frequency. These nodes should not be routed

Thermal Management

At high input voltages and maximum output current, there

can be substantial power dissipation in the LTC3290. If

the junction temperature increases above approximately

175°C, the thermal shutdown circuitry will automatically

deactivate the output. To reduce the maximum junction

temperature, a good thermal connection to the PC board

groundplaneisrecommended.Connectingtheexposedpad

of the package to a ground plane under the device on two

layers of the PC board can reduce the thermal resistance

of the package and PC board considerably.