5

LTC1044A

1

2

3

4

8

7

6

5

LTC1044A

+

C1

10

µF

C2

10

µF

EXTERNAL

OSCILLATOR

LTC1044A • TC

S

APPLICATI

I FOR ATIO

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, capacitor

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

q1 = C1V1. The switch then moves to the right, discharg-

ing 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

LTC1044A • F01

V2

C1

f

C2

Figure 1. Switched-Capacitor Building Block

Rewriting in terms of voltage and impedance equivalence,

I =

=

V1 – V2

1/(f

× C1)

V1 – V2

= 1/(f

× C1). Thus, the equivalent circuit for the switched-

capacitor network is as shown in Figure 2.

V1

LTC1044A • F02

V2

C2

1

f

× C1

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 resis-

tance and output voltage ripple, the simple theory al-

though 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 Effi-

ciency 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.

TEST CIRCUIT