MC7660

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6

Efficient Positive Voltage Multiplication/Conversion

Since the switches that allow the charge pumping

operation are bidirectional, the charge transfer can be

performed backwards as easily as forwards. Figure 10 shows

a MC7660 transforming –5V to +5V (or +5V to +10V, etc.).

The only problem here is that the internal clock and

switch–drive section will not operate until some positive

voltage has been generated. An initial inefficient pump, as

shown in Figure 9, could be used to start this circuit up, after

never turn on), or else the diode and resistor shown dotted

in Figure 10 can be used to ”force” the internal regulator on.

Figure 10. Positive Voltage Conversion

MC7660

1

2

3

4

8

7

6

5

VOUT = –V–

C1

+

V– INPUT

+

10

mF

1 M

W

10

mF

Voltage Splitting

The same bidirectional characteristics used in Figure 10

can also be used to split a higher supply in half, as shown in

Figure 11. The combined load will be evenly shared between

the two sides. Once again, a high value resistor to the LV pin

ensures start–up. Because the switches share the load in

parallel, the output impedance is much lower than in the

standard circuits, and higher currents can be drawn from the

device. By using this circuit, and then the circuit of Figure

5, +15V can be converted (via +7.5V and –7.5V) to a

nominal –15V, though with rather high series resistance

(~250

W).

Figure 11. Splitting a Supply in Half

MC7660

1

2

3

4

8

7

6

5

1 M

W

100 k

W

100 k

W

50

mF

50

mF

+

50

mF

+

+

–

RL1

RL2

V

OUT +

2

–

V–

V+