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LTC3250-1.5/LTC3250-1.2

3250fa

Flying Capacitor Selection

Warning: A polarized capacitor such as tantalum or

aluminum should never be used for the flying capacitor

since its voltage can reverse upon start-up of the

LTC3250-1.5/LTC3250-1.2. Ceramic capacitors should

always be used for the flying capacitor.

The flying capacitor controls the strength of the charge

pump. In order to achieve the rated output current it is

necessary for the flying capacitor to have at least 0.4

µF of

capacitance over operating temperature with a 2V bias

(see “Ceramic Capacitor Selection Guidelines” section). If

only 100mA or less of output current is required for the

application the flying capacitor minimum can be reduced

to 0.15

µF.

Ceramic Capacitor Selection Guidelines

Capacitors of different materials lose their capacitance

with higher temperature and voltage at different rates. For

example, a ceramic capacitor made of X7R material will

retain most of its capacitance from –40

°Cto85°Cwhereas

a Z5U or Y5V style capacitor will lose considerable capaci-

tance over that range (60% to 80% loss typ.). Z5U and Y5V

capacitors may also have a very strong voltage coefficient

causing them to lose an additional 60% or more of their

capacitance when the rated voltage is applied. Therefore,

when comparing different capacitors it is often more

appropriate to compare the amount of achievable capaci-

tance for a given case size rather than discussing the

specified capacitance value. For example, over rated volt-

age and temperature conditions, a 4.7

µF, 10V, Y5V

ceramic capacitor in a 0805 case may not provide any

more capacitance than a 1

µF, 10V, X7R available in the

same 0805 case. In fact over bias and temperature range,

the 1

µF, 10V, X7R will provide more capacitance than the

4.7

µF, 10V, Y5V. The capacitor manufacturer’s data sheet

should be consulted to determine what value of capacitor

is needed to ensure minimum capacitance values are met

over operating temperature and bias voltage.

Below is a list of ceramic capacitor manufacturers and

how to contact them:

AVX

1-(803)-448-1943

www.avxcorp.com

Kemet

1-(864)-963-6300

www.kemet.com

Murata

1-(800)-831-9172

www.murata.com

Taiyo Yuden

1-(800)-348-2496

www.t-yuden.com

Vishay

1-(800)-487-9437

www.vishay.com

Layout Considerations

Due to the high switching frequency and transient currents

produced by the LTC3250-1.5/LTC3250-1.2 careful board

layout is necessary for optimal performance. A true ground

plane and short connections to all capacitors will improve

performance and ensure proper regulation under all con-

ditions. Figure 2 shows the recommended layout configu-

ration.

Figure 2. Recommended Layout

The flying capacitor pins, C+ and C– will have very high

edge rate wave forms. The large dv/dt on these pins can

couple energy capacitively to adjacent printed circuit board

runs. Magnetic fields can also be generated if the flying

capacitors are not close to the LTC3250-1.5/LTC3250-1.2

(i.e. the loop area is large). To decouple capacitive energy

transfer, a Faraday shield may be used. This is a grounded

PC trace between the sensitive node and the LTC3250-1.5/

LTC3250-1.2 pins. For a high quality AC ground it should

be returned to a solid ground plane that extends all the way

to the LTC3250-1.5/LTC3250-1.2.

OPERATIO

(Refer to Simplified Block Diagram)

GND

VOUT

VIN

SHDN

LTC3250-1.5/LTC3250-1.2

µF

4.7

µF

3250 F02

VIA TO GROUND PLANE

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