_______________Detailed Description

The MAX1044/ICL7660 are charge-pump voltage con-

verters. They work by first accumulating charge in a

bucket capacitor and then transfer it into a reservoir

capacitor. The ideal voltage inverter circuit in Figure 2

illustrates this operation.

During the first half of each cycle, switches S1 & S3

close and switches S2 & S4 open, which connects the

bucket capacitor C1 across V+ and charges C1.

During the second half of each cycle, switches S2 & S4

close and switches S1 & S3 open, which connects the

positive terminal of C1 to ground and shifts the nega-

the reservoir capacitor C2. If the voltage across C2 is

smaller than the voltage across C1, then charge flows

from C1 to C2 until the voltages across them are equal.

During successive cycles, C1 will continue pouring

charge into C2 until the voltage across C2 reaches

- (V+). In an actual voltage inverter, the output is less

than - (V+) since the switches S1–S4 have resistance

and the load drains charge from C2.

Additional qualities of the MAX1044/ICL7660 can be

understood by using a switched-capacitor circuit

model. Switching the bucket capacitor, C1, between

the input and output of the circuit synthesizes a resis-

tance (Figures 3a and 3b.)

When the switch in Figure 3a is in the left position,

capacitor C1 charges to V+. When the switch moves to

charge transferred per cycle is:

∆Q = C1(V+ - V

the switch is cycled at frequency f, then the resulting

Switched-Capacitor Voltage Converters

_______________________________________________________________________________________

5

MAX1044

ICL7660

BOOST

CAP+

GND

= 0.1µF

V+

CAP-

V+

OSC

C1

10

µF

LV

C2

10

µF

EXTERNAL

OSCILLATOR

_____________________________________________________________ Pin Description

NAME

FUNCTION

BOOST

(MAX1044)

Frequency Boost. Connecting BOOST to V+ increases the oscillator frequency by a factor of six. When the

oscillator is driven externally, BOOST has no effect and should be left open.

PIN

1

N.C.

(ICL7660)

No Connection

3

GND

Ground. For most applications, the positive terminal of the reservoir capacitor is connected to this pin.

2

CAP+

Connection to positive terminal of Charge-Pump Capacitor

6

LV

Low-Voltage Operation. Connect to ground for supply voltages below 3.5V.

ICL7660: Leave open for supply voltages above 5V.

5

Negative Voltage Output. For most applications, the negative terminal of the reservoir capacitor is

connected to this pin.

4

CAP-

Connection to negative terminal of Charge-Pump Capacitor

7

OSC

Oscillator Control Input. Connecting an external capacitor reduces the oscillator frequency. Minimize stray

capacitance at this pin.

8

V+

Power-Supply Positive Voltage Input. (1.5V to 10V). V+ is also the substrate connection.

Figure 1. Maxim MAX1044/ICL7660 Test Circuit