9

LTC3202

3202fa

Flying Capacitor Selection

Warning: A polarized capacitor such as tantalum or alumi-

num should never be used for the flying capacitors since

their voltage can reverse upon start-up of the LTC3202.

Ceramic capacitors should always be used for the flying

capacitors.

The flying capacitor controls the strength of the charge

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

necessary to have at least 0.7

µF of capacitance for each of

the flying capacitors.

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. Z5U and Y5V capacitors may also

have a very poor voltage coefficient causing them to lose

60% or more of their capacitance when the rated voltage

is applied. Therefore, when comparing different capaci-

tors it is often more appropriate to compare the amount of

achievable capacitance for a given case size rather than

comparing the specified capacitance value. For example,

over rated voltage and temperature conditions, a 1

µF,10V,

Y5V ceramic capacitor in a 0603 case may not provide any

more capacitance than a 0.22

µF, 10V, X7R available in the

same 0603 case. The capacitor manufacturer’s data sheet

should be consulted to determine what value of capacitor

is needed to ensure minimum capacitances at all tempera-

tures and voltages.

Table 2 shows a list of ceramic capacitor manufacturers

and how to contact them:

Table 2 Recommended Capacitor Vendors

AVX

www.avxcorp.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

Vishay

www.vishay.com

For very light load applications the flying capacitor may be

reduced to save space or cost. The theoretical minimum

output resistance of a 2:3 fractional charge pump is given

by:

R

VV

If

C

OL MIN

IN

OUT

OUT

SC FLY

()

.–

≡=

15

1

charge pump will typically be weaker than the theoretical

limit due to additional switch resistance, however for very

light load applications the above expression can be used

as a guideline in determining a starting capacitor value.

Power Efficiency

The power efficiency (

η) of the LTC3202 is similar to that

of a linear regulator with an effective input voltage of 1.5

times the actual input voltage. This occurs because the

input current for a 2:3 fractional charge pump is approxi-

mately 1.5 times the load current. In an ideal regulating 2:3

charge pump the power efficiency would be given by:

η

IDEAL

OUT

IN

OUT

OUT

IN

OUT

OUT

IN

P

P

VI

VI

V

V

≡=

=

•

•

.

3

2

15

At moderate to high output power the switching losses

and quiescent current of the LTC3202 are negligible and

efficiency is 82% which is just under the theoretical 87.5%

calculation.

OPERATIO

Figure 5. 10nH Inductor Used for Input Noise Reduction

3202 F05

LTC3202

GND

5, 11

4

0.1

µF1µF

10nH