CAT661 Datasheet, PDF (7/16 Pages) Catalyst Semiconductor – High Frequency 100mA CMOS Charge Pump, Inverter/Doubler

CAT661
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:
(V1-V2)
V1-V2
I=
=
(1/FC1)
REQ
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 150µF 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
V1
V2
C1 C2 RL
Figure 4. Switched-Capacitor Equivalent Circuit
REQ
V1
V2
REQ = ¹
FC1
C2 RL
7
Doc. No. 5003, Rev. J