Detailed Description

The MAX1044/ICL7660 are charge-pump voltage

converters. 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 negative terminal

to VOUT. This connects C1 in parallel with 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 resistance (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 the right position, C1 is discharged to VOUT. The

charge transferred per cycle is: ΔQ = C1(V+ - VOUT).

If the switch is cycled at frequency f, then the resulting

current is: I = f x ΔQ = f x C1(V+ - VOUT). Rewriting

this equation in Ohm’s law form defines an equivalent

resistance synthesized by the switched-capacitor circuit

where:

(

)

OUT

EQUIV

VV

I

1/ ( f x C1)

and

1

R

f x C1

+−

=

=

Figure 1. Maxim MAX1044/ICL7660 Test Circuit

Pin Description

PIN

NAME

FUNCTION

1

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.

N.C.

(ICL7660)

No Connection

2

CAP+

Connection to positive terminal of Charge-Pump Capacitor

3

GND

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

4

CAP-

Connection to negative terminal of Charge-Pump Capacitor

5

VOUT

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

connected to this pin.

6

LV

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

ICL7660: Leave open for supply voltages above 5V.

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.

MAX1044

ICL7660

BOOST

CAP+

GND

CBYPASS

= 0.1µF

V+

RL

CAP-

V+

OSC

C1

10µF

LV

VOUT

C2

10µF

COSC

EXTERNAL

OSCILLATOR

VOUT

MAX1044/ICL7660

Switched-Capacitor Voltage Converters

www.maximintegrated.com

Maxim Integrated │ 6