© 2006 Microchip Technology Inc.

DS21453C-page 5

TC682

4.0

TYPICAL APPLICATIONS

4.1

Negative Doubling Converter

The most common application of the TC682 is as a

charge pump voltage converter which provides a

negative output of two times a positive input voltage

(Figure 4-1).

FIGURE 4-1:

Inverting Voltage Doubler

4.2

Capacitor Selection

The output resistance of the TC682 is determined, in

part, by the ESR of the capacitors used. An expression

Assuming all switch resistances are approximately

equal:

about 80-90

Ω. It can be seen easily that increasing or

decreasing values of C1 and C2 will affect efficiency by

term can quickly become dominant with large electro-

values of C1 and C2 (assume 0.5

Ω ESR). C1 must be

rated at 6VDC or greater while C2 and C3 must be

rated at 12VDC or greater.

Output voltage ripple is affected by C3. Typically the

larger the value of C3 the less the ripple for a given load

For a 10

at the output will be less then 60 mV. In most

1-5

μF pump capacitors will suffice. Table 4-2 shows

TABLE 4-1:

OUTPUT RESISTANCE

VS. C1, C2

TABLE 4-2:

GND

GND

TC682

22

μF

22

μF

22

μF

7

6

5

4

3

2

1

+

+

+

C1, C2 (

μF)

0.05

4085

0.10

2084

0.47

510

1.00

285

3.30

145

5.00

125

10.00

105

22.00

94

100.00

87

C3 (

μF)

0.50

1020

1.00

520

3.30

172

5.00

120

10.00

70

22.00

43

100.00

25