LT1054

SWITCHEDCAPACITOR VOLTAGE CONVERTERS

WITH REGULATORS

SLVS033F − FEBRUARY 1990 − REVISED NOVEMBER 2004

8

POST OFFICE BOX 655303

PRINCIPLES OF OPERATION

A review of a basic switched-capacitor building block is helpful in understanding the operation of the LT1054. When

the switch shown in Figure 12 is in the left position, capacitor C1 charges to the voltage at V1. The total charge on

C1 is q1 = C1V1. When the switch is moved to the right, C1 is discharged to the voltage at V2. After this discharge

time, the charge on C1 is q2 = C1V2. The charge has been transferred from the source V1 to the output V2. The

amount of charge transferred is shown in equation 1.

Dq + q1 * q2 + C1(V1 * V2)

If the switch is cycled f times per second, the charge transfer per unit time (i.e., current) is as shown in equation 2.

I

+ f

Dq + f

C1(1

* V2)

To obtain an equivalent resistance for a switched-capacitor network, this equation can be rewritten in terms of voltage

and impedance equivalence as shown in equation 3.

I

+ V1 * V2

1 fC1

+ V1 * V2

R

EQUIV

C2

C1

V2

f

V1

RL

Figure 12. Switched-Capacitor Building Block

shown in Figure 13. The LT1054 has the same switching action as the basic switched-capacitor building block. Even

though this simplification does not include finite switch-on resistance and output-voltage ripple, it provides an insight

into how the device operates.

C2

V2

V1

RL

REQUIV

R

1

fC1

Figure 13. Switched-Capacitor Equivalent Circuit

These simplified circuits explain voltage loss as a function of oscillator frequency (see Figure 7). As oscillator

frequency is decreased, the output impedance eventually is dominated by the 1/fC1 term, and voltage losses rise.

Voltage losses also rise as oscillator frequency increases. This is caused by internal switching losses that 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 voltage losses

again rise.

The oscillator of the LT1054 is designed to operate in the frequency band where voltage losses are at a minimum.

(1)

(2)

(3)