cycle, S5 and S7 are open, S6 and S8 are closed, and

the charge on C2 is transferred to C4, generating the

negative supply. The eight switches are CMOS power

MOSFETs. S1, S2, S4, and S5 are P-channel

switches, while S3, S6, S7, and S8 are N-channel

switches.

__________Efficiency Considerations

Theoretically, a charge-pump voltage multiplier can

approach 100% efficiency under the following con-

ditions:

• The charge-pump switches have virtually no offset

and extremely low on-resistance

• Minimal power is consumed by the drive circuitry

• The impedances of the reservoir and pump capaci-

tors are negligible

For the MAX680/MAX681, the energy loss per clock

cycle is the sum of the energy loss in the positive and

negative converters as below:

There will be a substantial voltage difference between

between V+ and V-, if the impedances of pump capaci-

tors C1 and C2 are high relative to their respective out-

put loads.

Larger C3 and C4 reservoir capacitor values reduce

output ripple. Larger values of both pump and reservoir

capacitors improve efficiency.

________Maximum Operating Limits

The MAX680/MAX681 have on-chip zener diodes that

V- to -12.4V. Never exceed the maximum supply volt-

age: excessive current may be shunted by these

diodes, potentially damaging the chip. The MAX680/

MAX681 operate over the entire operating temperature

range with an input voltage of +2V to +6V.

________________________Applications

Positive and Negative Converter

The most common application of the MAX680/MAX681

is as a dual charge-pump voltage converter that pro-

vides positive and negative outputs of two times a posi-

tive input voltage. For applications where PC board

space is at a premium, the MAX681, with its capacitors

internal to the package, offers the smallest footprint.

The simple circuit shown in Figure 3 performs the same

function using the MAX680 with external capacitors C1

and C3 for the positive pump and C2 and C4 for the

negative pump. In most applications, all four capacitors

are low-cost, 10µF or 22µF polarized electrolytics.

When using the MAX680 for low-current applications,

1µF can be used for C1 and C2 charge-pump capaci-

tors, and 4.7µF for C3 and C4 reservoir capacitors.

C1 and C3 must be rated at 6V or greater, and C2 and

C4 must be rated at 12V or greater.

+5V to ±10V Voltage Converters

_______________________________________________________________________________________

5

MAX680

C1

22

µF

C3

22

µF

V+ OUT

V- OUT

GND

C4

22

µF

C2

22

µF

C1-

8

7

6

5

C2+

V-

V+

1

2

3

4

C1+

GND

C2-

Figure 3. Positive and Negative Converter