Datasheet

LMC7660
www.ti.com SNOSBZ9C –APRIL 1997–REVISED APRIL 2013
SPLIT V
+
IN HALF
Figure 17 is one of the more interesting applications for the LMC7660. The circuit can be used as a precision
voltage divider (for very light loads), alternately it is used to generate a ½ supply point in battery applications. In
the ½ cycle when S1 and S3 are closed, the supply voltage divides across the capacitors in a conventional way
proportional to their value. In the ½ 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 voltage
can be raised to 20V giving exactly 10V at V
out
.
GETTING UP … AND DOWN
The LMC7660 can also be used as a positive voltage multiplier. This application, shown in Figure 18, requires 2
additional diodes. During the first ½ cycle S2 charges C
p
1 through D1; D2 is reverse biased. In the next ½ cycle
S2 is open and S1 is closed. Since C
p
1 is charged to V
+
− V
D1
and is referenced to V
+
through S1, the junction of
D1 and D2 is at V
+
+ (V
+
−V
D1
). D1 is reverse biased in this interval. This application uses only two of the four
switches in the 7660. The other two switches can be put to use in performing a negative conversion at the same
time as shown in Figure 19. In the ½ cycle that D1 is charging C
p
1, C
p
2 is connected from ground to −V
out
via S2
and S4, and C
r
2 is storing C
p
2's charge. In the interval that S1 and S3 are closed, C
p
1 pumps the junction of D1
and D2 above V
+
, while C
p
2 is refreshed from V
+
.
Figure 17. Split V
+
in Half
Figure 18. Positive Voltage Multiplier
Figure 19. Combined Negative Converter and Positive Multiplier
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