current is: I = f x

∆Q = f x C1(V+ - V

equation in Ohm’s law form defines an equivalent resis-

tance synthesized by the switched-capacitor circuit

where:

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.

________________Design Information

The MAX1044/ICL7660 are designed to provide 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 external capacitors are needed. The type of

capacitor used is not critical.

Proper Use of the Low-Voltage (LV) Pin

Figure 4 shows an internal voltage regulator inside the

MAX1044/ICL7660. Use the LV pin to bypass this

regulator, in order to improve low-voltage performance

I

(V+ - V

)

1 / (f x C1)

R

1

f x C1

OUT

EQUIV

=

=

and

Switched-Capacitor Voltage Converters

6

_______________________________________________________________________________________

S1

V+

S2

S3

S4

C1

C2

Figure 2. Ideal Voltage Inverter

V+

C1

f

C2

Figure 3a. Switched Capacitor Model

1

V+

f

× C1

C2

Figure 3b. Equivalent Circuit

1M

BOOST

pin 1

OSC

pin 7

LV

pin 6

GND

pin 3

CAP-

pin 4

S2

S1

S4

S3

CAP+

pin 2

V+

pin 8

pin 5

÷ 2

Q

Q

Figure 4. MAX1044 and ICL7660 Functional Diagram