LTC3221/

LTC3221-3.3/LTC3221-5

7

3221f

Power Efficiency

TheinputcurrentofadoublingchargepumpliketheLTC3221

family is always twice that of the output current. This is

true regardless of whether the output voltage is unregulated

or regulated or of the regulation method used. In an ideal

unregulated doubling charge pump, conservation of energy

implies that the input current has to be twice that of the

output current in order to obtain an output voltage twice

that of the input voltage. In a regulated charge pump like

(Input voltage plus the voltage across a fully charged flying

a pass transistor. So the input current of an ideal regulated

doubling charge pump is the same as an unregulated one,

which is equal to twice the output current. The efficiency

(n) of an ideal regulated doubler is therefore given by:

η=

=

=

P

P

VI

VI

V

V

OUT

IN

OUT

OUT

IN

OUT

OUT

IN

•

•2

2

At moderate to high output power, the switching losses and

quiescent current of the LTC3221 family are negligible and

sured efficiency of 82% which is in close agreement with

the theoretical 83.3% calculation. The LTC3221 product

family continues to maintain good efficiency even at fairly

light loads because of its inherently low power design.

Maximum Available Output Current

For the adjustable LTC3221, the maximum available output

current and voltage can be calculated from the effective

From Figure 1 the available current is given by:

I

VV

R

OUT

IN

OUT

OL

=

2–

pump is a very important parameter which determines the

strength of the charge pump. The value of this parameter

APPLICATIO S I FOR ATIO

OPERATIO

(Refer to Block Diagrams)

The LTC3221 has a FB pin in place of the ⎯S⎯H⎯D⎯N pin. This

allows the output voltage to be programmed using an

external resistive divider.

Burst Mode Operation

The LTC3221 family regulates the output voltage throughout

the full 60mA load range using Burst Mode control. This

keeps the quiescent current low at light load and improves

the efficiency at full load by reducing the switching losses.

All the internal circuitry except the comparator is kept off

if the output voltage is high and the flying capacitor has

drops below the comparator lower threshold. At light load,

time, this result in a very low average input current.

Soft-Start and Short-Circuit Protection

deliver current to the output. This helps to limit the input

and output current during start-up and output short-circuit

capacitor and output capacitor, this limits the input current

to approximately 240mA. During short-circuit condition,

the output current to approximately 120mA. This prevents

excessive self-heating that causes damage to the part.

Figure 1. Equivalent Open-Loop Circuit

+

–

+

3221 F01