5

LTC1144

TEST CIRCUITS

Figure 1.

1

2

3

4

8

7

6

5

+

C1

10

µF

C2

10

µF

15V

EXTERNAL

OSCILLATOR

1144 F01

LTC1144

S

APPLICATI

I FOR ATIO

Theory of Operation

To understand the theory of operation of the LTC1144, a

review of a basic switched-capacitor building block is

helpful.

In Figure 2, 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)

V2

C2

C1

V1

f

1144 F02

Figure 2. Switched-Capacitor Building Block

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)

Rewriting in terms of voltage and impedance equivalence,

I

VV

fC

VV

=

−

×









=

−

12

1

1

12

= 1/(f

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

capacitor network is as shown in Figure 3.

Figure 3. Switched-Capacitor Equivalent Circuit

V2

C2

V1

1144 F03

1

f

× C1

Examination of Figure 4 shows that the LTC1144 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 Figure 5), this simple

theory will explain how the LTC1144 behaves. The loss,

Figure 4. LTC1144 Switched-Capacitor

Voltage Converter Block Diagram

SHDN

(6)

OSC

(7)

10X

(1)

BOOST

1144 F04

OSC

(8)

SW1

SW2

(2)

(4)

GND

(3)

(5)

C2

C1

+

+

φ

φ