LMC7660 Datasheet, PDF (7/12 Pages) National Semiconductor (TI) – Switched Capacitor Voltage Converter

Typical Applications (Continued)
Getting Up … and Down
The LMC7660 can also be used as a positive voltage multi-
plier. This application, shown in Figure 9, requires 2 addi-
tional diodes. During the first ¹â„2 cycle S2 charges C_p1
through D1; D2 is reverse biased. In the next ¹â„2 cycle S2 is
open and S1 is closed. Since C_p1 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 10.
In the ¹â„2 cycle that D1 is charging C_p1, C_p2 is connected
from ground to −V_out via S2 and S4, and C_r2 is storing C_p2’s
charge. In the interval that S1 and S3 are closed, C_p1 pumps
the junction of D1 and D2 above V⁺, while C_p2 is refreshed
from V⁺.
FIGURE 8. Split V⁺ in Half
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FIGURE 9. Positive Voltage Multiplier
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FIGURE 10. Combined Negative Converter and Positive Multiplier
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Thermometer Spans 180˚C
Using the combined negative and positive multiplier of Fig-
ure 11 with an LM35 it is possible to make a µPower ther-
mometer that spans a 180˚C temperature range. The LM35
temperature sensor has an output sensitivity of 10 mV/˚C,
while drawing only 50 µA of quiescent current. In order for
the LM35 to measure negative temperatures, a pull down to
a negative voltage is required. Figure 11 shows a thermom-
eter circuit for measuring temperatures from −55˚C to
+125˚C and requiring only two 1.5V cells. End of battery life
can be extended by replacing the up converter diodes with
Schottky’s.
Regulating −V_out
It is possible to regulate the output of the LMC7660 and still
maintain µPower performance. This is done by enclosing the
LMC7660 in a loop with a LP2951. The circuit of Figure 12
will regulate V_out to −5V for I_L = 10 mA, and V_in = 6V. For V_in
> 7V, the output stays in regulation up to I_L = 25 mA. The er-
ror flag on pin 5 of the LP2951 sets low when the regulated
output at pin 4 drops by about 5%. The LP2951 can be shut-
down by taking pin 3 high; the LMC7660 can be shutdown by
shorting pin 7 and pin 8.
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