Typical Applications
Increasing Output Voltage
(Continued)
Stacking the LMC7660s is an easy way to produce a greater
negative voltage. It should be noted that the input current re-
quired for each stage is twice the load current on that stage
as shown in
Figure 6.
The effective output resistance is ap-
proximately the sum of the individual R
out
values, and so
only a few levels of multiplication can be used.
It is possible to generate −15V from +5V by connecting the
second 7660’s pin 8 to +5V instead of ground as shown in
Figure 7.
Note that the second 7660 sees a full 20V and the
input supply should not be increased beyond +5V.
DS009136-9
FIGURE 5. Lowering Output Resistance by Paralleling Devices
DS009136-10
FIGURE 6. Higher Voltage by Cascade
DS009136-11
FIGURE 7. Getting −15V from +5V
Split V
+
In Half
Figure 8
is one of the more interesting applications for the
LMC7660. The circuit can be used as a precision voltage di-
vider (for very light loads), alternately it is used to generate a
1
⁄
2
supply point in battery applications. In the
1
⁄
2
cycle when
S1 and S3 are closed, the supply voltage divides across the
capacitors in a conventional way proportional to their value.
In the
1
⁄
2
cycle when S2 and S4 are closed, the capacitors
switch from a series connection to a parallel connection. This
forces the capacitors to have the same voltage; the charge
redistributes to maintain precisely V
+
/2, across C
p
and C
r
. In
this application all devices are only V
+
/2, and the supply volt-
age can be raised to 20V giving exactly 10V at V
out
.
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