LTC3221/

LTC3221-3.3/LTC3221-5

8

3221f

APPLICATIO S I FOR ATIO

depends on many factors such as the oscillator frequency

the external capacitors. A first order approximation for

RR

fC

OL

S

STO

OSC

FLY

≅

∑

+

=

2

1

14

•

in Figure 2.

ESR of the output capacitor. It is proportional to the input

voltage, the value of the flying capacitor and the ESR of

the output capacitor.

A smaller output capacitor and/ or larger output current

load will result in higher ripple due to higher output volt-

age slew rates.

There are several ways to reduce output voltage ripple.

For applications requiring lower peak-to-peak ripple, a

A larger capacitor will reduce both the low and high fre-

quency ripple due to the lower charging and discharging

slew rates, as well as the lower ESR typically found with

higher value (larger case size) capacitors. A low ESR ce-

ramic output capacitor will minimize the high frequency

ripple, but will not reduce the low frequency ripple unless

a high capacitance value is used.

The style and value of capacitors used with the LTC3221

family determine several important parameters such as

output ripple, charge pump strength and minimum start-

up time.

To reduce noise and ripple, it is recommended that low

These capacitors should be either ceramic or tantalum

and should be 2.2µF or greater. Aluminum capacitors are

not recommended because of their high ESR.

Flying Capacitor Selection

Warning: A polarized capacitor such as tantalum or alumi-

num should never be used for the flying capacitor since

its voltage can reverse upon start-up of the LTC3221.

Low ESR 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 to have at least 0.6µF of capacitance for the

flying capacitor. For very light load applications, the flying

capacitor may be reduced to save space or cost.From the

Open-Loop Output Resistance,” the theoretical minimum

output resistance of a voltage doubling charge pump can

6

15

14

13

12

11

7

5

9

10

8

TEMPERATURE (

°C)

–50

–25

100

3221 F02

050

25

75

Figure 2. Effective Open-Loop Output Resistance vs Temperature

Output Ripple

Low frequency regulation mode ripple exists due to the

hysteresis in the comparator CMP and propagation delay

in the charge pump control circuit. The amplitude and

frequency of this ripple are heavily dependent on the load

current, the input voltage and the output capacitor size.

current to the output. This helps to keep the output ripple

fairly constant over the full input voltage range. Typical

X5R case size 0603 output capacitor.

A high frequency ripple component may also be present

on the output capacitor due to the charge transfer action

of the charge pump. In this case the output can display

a voltage pulse during the charging phase. This pulse

results from the product of the charging current and the