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LTC1044AC

12V CMOS Voltage Converter

Linear Technology 

LTC1044A

Test Circuit

1

8

2

7

+ C1 3 LTC1044A 6

10µF 4

5

COSC

V+ (5V)

IS

EXTERNAL

OSCILLATOR RL

IL

VOUT

C2

10µF

1044a TC

Applications Information

Theory of Operation

To understand the theory of operation of the LTC1044A,

a review of a basic switched-capacitor building block is

helpful.

In Figure 1, when the switch is in the left position, capaci-

tor C1 will charge to voltage V1. The total charge on C1

will be q1 = C1V1. The switch then moves to the right,

discharging C1 to voltage V2. After this discharge time,

the charge on C1 is q2 = C1V2. Note that charge has been

transferred from the source, V1, to the output, V2. The

amount of charge transferred is:

∆q = q1 – q2 = C1(V1 – V2)

If the switch is cycled f times per second, the charge

transfer per unit time (i.e., current) is:

I = f • ∆q = f • C1(V1 – V2)

V1

V2

f

RL

C1

C2

1044a F01

Figure 1. Switched-Capacitor Building Block

Rewriting in terms of voltage and impedance equivalence,

I

=

V1–

1

V2

=

V1– V2

REQUIV

(f •C1)

A new variable, REQUIV, has been defined such that REQUIV

= 1/(f • C1). Thus, the equivalent circuit for the switched-

capacitor network is as shown in Figure 2.

REQUIV

V1

V2

C2

RL

REQUIV

=

f

1

× C1

1044a F02

Figure 2. Switched-Capacitor Equivalent Circuit

Examination of Figure 3 shows that the LTC1044A has the

same switching action as the basic switched-capacitor

building block. With the addition of finite switch-on

resistance and output voltage ripple, the simple theory

although not exact, provides an intuitive feel for how the

device works.

For example, if you examine power conversion efficiency

as a function of frequency (see typical curve), this simple

theory will explain how the LTC1044A behaves. The loss,

and hence the efficiency, is set by the output impedance.

As frequency is decreased, the output impedance will

eventually be dominated by the 1/(f • C1) term, and power

efficiency will drop. The typical curves for Power Efficiency

vs Frequency show this effect for various capacitor values.

Note also that power efficiency decreases as frequency

goes up. This is caused by internal switching losses which

occur due to some finite charge being lost on each switching

cycle. This charge loss per unit cycle, when multiplied by

the switching frequency, becomes a current loss. At high

frequency this loss becomes significant and the power

efficiency starts to decrease.

1044afa

6

For more information www.linear.com/LTC1044A

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