LTC3204-3.3/LTC3204-5/

LTC3204B-3.3/LTC3204B-5

8

3204fa

Power Efficiency

The power efficiency (η) of the LTC3204-3.3/LTC3204-5/

LTC3204B-3.3/LTC3204B-5 is similar to that of a linear

regulator with an effective input voltage of twice the actual

input voltage. This occurs because the input current for a

voltage doubling charge pump is approximately twice the

output current. In an ideal regulating voltage doubler the

power efficiency would be given by:

η =

=

=

P

P

V

I

V

I

V

V

OUT

IN

OUT OUT

IN

OUT

OUT

IN

•

• 2

2

At moderate to high output power, the switching losses

and the quiescent current of the LTC3204-3.3/LTC3204-5/

LTC3204B-3.3/LTC3204B-5 are negligible and the expres-

is 81.8% which is in close agreement with the theoretical

83.3% calculation.

Maximum Available Output Current

For the LTC3204-3.3/LTC3204-5/LTC3204B-3.3/

LTC3204B-5,the maximum available output current and

voltage can be calculated from the effective open-loop

the external capacitors. A first order approximation for

R

R

f

C

OL

S

OSC

FLY

≅ ∑

+

2

1

•

in Figure 2.

Figure 1. Equivalent Open-Loop Circuit

From Fig. 1, the available current is given by:

I

V

V

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

depends on many factors such as the oscillator frequency

ThestyleandvalueofcapacitorsusedwiththeLTC3204-3.3/

LTC3204-5/LTC3204B-3.3/LTC3204B-5determineseveral

important parameters such as regulator control loop sta-

bility, output ripple, charge pump strength and minimum

start-up time.

To reduce noise and ripple, it is recommended that low

and aluminum capacitors are not recommended because

of their high ESR.

will reduce the output ripple at the expense of higher

minimum turn-on time. The peak-to-peak output ripple

is approximately given by the expression:

V

I

f

C

RIPPLE P P

OUT

OSC

OUT

(

)

•

−

≅

2

+–

3204 F01

+

–

S=1 TO 4

APPLICATIO S I FOR ATIO

3204 F02

TEMPERATURE (

°C)

100

0

50

–50

8

7

6

5

4