LMC7660IM datasheet(7/12 Pages) NSC

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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 1⁄2 cycle S2 charges C

through D1; D2 is reverse biased. In the next 1⁄2 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 1⁄2 cycle that D1 is charging C

p1, Cp2 is connected

from ground to −V

out via S2 and S4, and Cr2 is storing Cp2’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

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 IL = 10 mA, and Vin = 6V. For Vin

> 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.

DS009136-12

FIGURE 8. Split V

+ in Half

DS009136-13

FIGURE 9. Positive Voltage Multiplier

DS009136-14

FIGURE 10. Combined Negative Converter and Positive Multiplier

LMC7660IM Datasheet(PDF) 7 Page - National Semiconductor (TI)

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