cat660 Catalyst Semiconductor, cat660 Datasheet - Page 6

• Manufacturer Part Number: cat660
• Description: 100ma Cmos Charge Pump Inverter/doubler
• Manufacturer: Catalyst Semiconductor

Figure 2. Switched-Capacitor Building Block
Doc. No. MD-5000, Rev. W
APPLICATION INFORMATION
Circuit Description and Operating Theory
The CAT660 switches capacitors to invert or double an
input voltage.
Figure 2 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:
If the switch is cycled "F" times per second, the current
(charge transfer per unit time) is:
Rearranging in terms of impedance:
I = F
I=
Q = Q1 - Q2 = C1
V1
(V1-V2)
(1/FC1)
Q = F
C1 (V1 - V2)
=
C1
(V1 - V2)
V1-V2
REQ
C2
R L
V2
6
Figure 3. Switched-Capacitor Equivalent Circuit
The 1/FC1 term can be modeled as an equivalent
impedance REQ. A simple equivalent circuit is shown in
figure 3. 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 F 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 4. The
CAT660 is a replacement for the MAX660 and the
LTC660.
V1
REQ = 1
FC1
REQ
C2
R L
V2