LMC7660 Datasheet, PDF (6/12 Pages) National Semiconductor (TI) – Switched Capacitor Voltage Converter

Typical Applications (Continued)
Increasing Output Voltage
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.
FIGURE 5. Lowering Output Resistance by Paralleling Devices
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FIGURE 6. Higher Voltage by Cascade
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FIGURE 7. Getting −15V from +5V
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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 ¹â„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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