3-40

Detailed Description

The ICL7660S contains all the necessary circuitry to

complete a negative voltage converter, with the exception of

2 external capacitors which may be inexpensive 10

µF

polarized electrolytic types. The mode of operation of the

device may be best understood by considering Figure 13,

which shows an idealized negative voltage converter.

situation more closely than existing non-mechanical circuits.

In the ICL7660S, the 4 switches of Figure 13 are MOS power

N-Channel devices. The main difficulty with this approach is

must always remain reverse biased with respect to their

sources, but not so much as to degrade their “ON”

resistances. In addition, at circuit start up, and under output

be sensed and the substrate bias adjusted accordingly.

Failure to accomplish this would result in high power losses

and probable device latchup.

This problem is eliminated in the ICL7660S by a logic network

the correct level to maintain necessary reverse bias.

The voltage regulator portion of the ICL7660S is an integral

part of the anti-latchup circuitry, however its inherent voltage

drop can degrade operation at low voltages. Therefore, to

improve low voltage operation “LV” pin should be connected

to GND, disabling the regulator. For supply voltages greater

than 3.5V the LV terminal must be left open to insure latchup

proof operation, and prevent device damage.

Theoretical Power Efficiency

Considerations

In theory a voltage converter can approach 100% efficiency

if certain conditions are met:

1. The drive circuitry consumes minimal power.

2. The output switches have extremely low ON resistance

and virtually no offset.

3. The impedance of the pump and reservoir capacitors are

negligible at the pump frequency.

FIGURE 11. OUTPUT SOURCE RESISTANCE AS A FUNCTION OF OSCILLATOR FREQUENCY

NOTE:

supply current goes directly to the positive side of the load, and the other half, through the ICL7660S, to the negative side of the load. Ideally,

400

300

200

100

0

100

1k

10k

100k

OSCILLATOR FREQUENCY (Hz)

V+ = 5V

I = 10mA

10

µF

1

µF

100

µF

1

2

3

4

8

7

6

5

+

-

10

µF

(+5V)

10

µF

ICL7660S

V+

+

-

should be increased to 100

µF.

FIGURE 12. ICL7660S TEST CIRCUIT

ICL7660S