CAT660

http://onsemi.com

6

Application Information

Circuit Description and Operating Theory

The CAT660 switches capacitors to invert or double an

input voltage.

Figure 13 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:

DQ + Q1 * Q2 + C1

(V1 * V2)

If the switch is cycled “F” times per second, the current

(charge transfer per unit time) is:

I + F

DQ + F

C1 (V1 * V2)

Rearranging in terms of impedance:

I +

(V1 * V2)

(1 FC1) +

V1 * V2

REQ

The 1/FC1 term can be modeled as an equivalent

impedance REQ. A simple equivalent circuit is shown in

Figure 14. 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

mF 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 CAT660 is shown in Figure 15.

The CAT660 is a replacement for the MAX660 and the

LTC660.

Figure 13. Switched−Capacitor Building Block

Figure 14. Switched−Capacitor Equivalent Circuit

V1

C1

V2

V1

V2

REQ

C2

C2

REQ + 1

FC1