LTC3221/

LTC3221-3.3/LTC3221-5

9

3221f

be expressed by the following equation:

R

VV

If

C

OL MIN

IN

OUT

OUT

OSC

FLY

()

–

•

≡≅

21

is the value of the flying capacitor. The charge pump will

typically be weaker than the theoretical limit due to ad-

ditional switch resistance. However, for very light load ap-

plications, the above expression can be used as a guideline

in determining a starting capacitor value.

Ceramic 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°C to 85°C,

whereas, a Z5U or Y5V style capacitor will lose considerable

capacitance over that range. Z5U and Y5V capacitors may

also have a very strong voltage coefficient causing them

to lose 50% 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 capacitance for a given case size

rather than discussing the specified capacitance value.

For example, over rated voltage and temperature condi-

tions, a 1µF 10V Y5V ceramic capacitor in a 0603 case

may not provide any more capacitance than a 0.22µF 10V

X7R capacitor available in the same 0603 case. In fact,

for most LTC3221-3.3/LTC3221-5/LTC3221 applications,

these capacitors can be considered roughly equivalent. The

capacitor manufacturer’s data sheet should be consulted

to determine what value of capacitor is needed to ensure

0.6µF at all temperatures and voltages.

Table 1 shows a list of ceramic capacitor manufacturers

and how to contact them.

Table 1. Ceramic Capacitor Manufacturers

AVX

www.avxcorp.com

Kemet

www.kemet.com

Murata

www.murata.com

Taiyo Yuden

www.t-yuden.com

Vishay

www.vishay.com

APPLICATIO S I FOR ATIO

Programming the LTC3221 Output Voltage (FB Pin)

While the LTC3221-3.3/LTC3221-5 versions have internal

resistive dividers to program the output voltage, the pro-

grammable LTC3221 may be set to an arbitrary voltage via

an external resistive divider. Figure 3 shows the required

voltage divider connection.

Figure 3. Programming the Adjustable LTC3221

4

3

6

)

3221 F03

FB

R1

LTC3221

R2

GND

C1

R1

R2

The voltage divider ratio is given by the expression:

R

R

V

V

OUT

1

21 23

1

=

.

–

Since the LTC3221 employs a voltage doubling charge

pump, it is not possible to achieve output voltages greater

range required for regulation is given by the following

expression:

Minimum V

VI

R

or

V

IN

OUT

OUT

OL

=

+

()

•

.;

2

18

whichever is higher

operating voltage of 1.8V.

The sum of the voltage divider resistors can be made large

to keep the quiescent current to a minimum. Any standing

current in the output divider (given by 1.23/R2) will be

reflected by a factor of 2 in the input current. A reasonable

resistance value should be such that the standing current