4-89

TELCOM SEMICONDUCTOR, INC.

7

6

5

4

3

1

2

8

TC7662B

CHARGE PUMP DC-TO-DC

VOLTAGE CONVERTER

1

2

3

4

8

7

6

5

TC7662B

1µF

+

+

1µF

–

+5 LOGIC SUPPLY

TTL DATA

INPUT

RS232

DATA

OUTPUT

11

3

12

16

4

15

13

14

1

IH5142

+5V

-5V

Figure 13. RS232 Levels from a Single 5V Supply

Regulated Negative Voltage Supply

In some cases, the output impedance of the TC7662B

can be a problem, particularly if the load current varies

substantially. The circuit of Figure 12 can be used to over-

come this by controlling the input voltage, via an ICL7611

low-power CMOS op amp, in such a way as to maintain a

nearly constant output voltage. Direct feedback is advisable,

since the TC7662B’s output does not respond instanta-

neously to change in input, but only after the switching delay.

The circuit shown supplies enough delay to accommodate

the TC7662B, while maintaining adequate feedback. An

increase in pump and storage capacitors is desirable, and

the values shown provide an output impedance of less than

5

Ω to a load of 10mA.

1

2

3

4

8

7

6

5

TC7662B

100µF

+

V+

100µF

-

50k

100k

+8V

50k

10µF

–

+

800k

250K

VOLTAGE

ADJUST

+8V

56k

–

+

Figure 12. Regulating the Output Voltage