S w it c h e d -Ca p a c it o r Vo lt a g e Co n ve rt e rs
current is: I = f x ∆Q = f x C1(V+ - V
). Rewriting this
OUT
equation in Ohm’s law form defines an equivalent resis-
tance synthesized by the switched-capacitor circuit
where:
S1
S3
S2
V+
(V+ - V
)
OUT
I =
C1
1 / (f x C1)
and
1
C2
R
=
S4
EQUIV
f x C1
V
OUT
= -(V+)
where f is one-half the oscillator frequency. This resis-
tance is a major component of the output impedance of
switched-capacitor circuits like the MAX1044/ICL7660.
As shown in Figure 4, the MAX1044/ICL7660 contain
MOSFET switches, the necessary transistor drive cir-
cuitry, and a timing oscillator.
MAX14/ICL760
Figure 2. Ideal Voltage Inverter
________________De s ig n In fo rm a t io n
The MAX1044/ICL7660 a re d e s ig ne d to p rovid e a
simple, compact, low-cost solution where negative or
doubled supply voltages are needed for a few low-
power components. Figure 5 shows the basic negative
voltage converter circuit. For many applications, only
two e xte rna l c a p a c itors a re ne e d e d . The typ e of
capacitor used is not critical.
f
V+
V
OUT
P ro p e r Us e o f t h e Lo w -Vo lt a g e (LV) P in
Figure 4 shows an internal voltage regulator inside the
MAX1044/ICL7660. Us e the LV p in to b yp a s s this
regulator, in order to improve low-voltage performance
C1
C2
R
LOAD
Figure 3a. Switched Capacitor Model
V+
pin 8
CAP+
pin 2
S2
S1
1M
BOOST
R
EQUIV
V+
V
OUT
Q
Q
pin 1
1
÷ 2
R
=
EQUIV
f × C1
OSC
pin 7
C2
R
LOAD
S3
S4
V
OUT
pin 5
GND
pin 3
CAP-
pin 4
LV
pin 6
Figure 4. MAX1044 and ICL7660 Functional Diagram
Figure 3b. Equivalent Circuit
6
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