CAT661

7

Doc. No. MD-5003, Rev. K

The 1/FC1 term can be modeled as an equivalent

impedance REQ. A simple equivalent circuit is shown in

figure 4. This circuit does not include the switch resistance

nor does it include output voltage ripple. It does allow

one to understand the switch-capacitor topology and

make prudent engineering tradeoffs.

For example, power conversion efficiency is set by the

output impedance, which consists of REQ and switch

resistance. As switching frequency is decreased, REQ,

the 1/FC1 term, will dominate the output impedance,

causing higher voltage losses and decreased efficiency.

As the frequency is increased quiescent current

increases. At high frequency this current becomes

significant and the power efficiency degrades.

The oscillator is designed to operate where voltage

losses are a minimum. With external 150

µF capacitors,

the internal switch resistances and the Equivalent Series

Resistance (ESR) of the external capacitors determine

the effective output impedance.

A block diagram of the CAT661 is shown in figure 5.

Figure 3. Switched-Capacitor Building Block

Figure 4. Switched-Capacitor Equivalent Circuit

APPLICATION INFORMATION

Circuit Description and Operating Theory

The CAT661 switches capacitors to invert or double an

input voltage.

Figure 3 shows a simple switch capacitor circuit. In

position 1 capacitor C1 is charged to voltage V1. The

total charge on C1 is Q1 = C1V1. When the switch

moves to position 2, the input capacitor C1 is discharged

to voltage V2. After discharge, the charge on C1 is Q2 =

C1V2.

The charge transferred is:

∆Q = Q1 - Q2 = C1 × (V1 - V2)

If the switch is cycled "F" times per second, the current

(charge transfer per unit time) is:

I = F

× ∆Q = F × C1 (V1 - V2)

Rearranging in terms of impedance:

I=

(V1-V2)

V1-V2

(1/FC1)

REQ

=

V1

C1

C2

RL

V2

V1

C2

RL

V2

REQ

REQ =

1

FC1