MC7660 Datasheet, PDF (6/12 Pages) ON Semiconductor – Charge Pump DC-to-DC Voltage Converter

MC7660
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
which it will bypass the other (D₁ and D₂ in Figure 9 would
never turn on), or else the diode and resistor shown dotted
in Figure 10 can be used to ”force” the internal regulator on.
+
10C1mF
VOUT = –V–
1
8
2
7
MC7660
3
6
4
5
1 MW
+ _{10 mF}
V– INPUT
Figure 10. Positive Voltage Conversion
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
(~250W).
V+
RL1
50 mF ⁺
+ ^* VOUT
V⁾ V^–
2
RL2
50 mF +
–
100 kW 100 kW
1
8
2
7
MC7660
3
6
4
5
1 MW
50 mF
+
–
V–
Figure 11. Splitting a Supply in Half
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