TC7662A

DS21468B-page 8

 2002 Microchip Technology Inc.

4.5

Changing the TC7662A Oscillator

Frequency

the TC7662A at low load levels by lowering the

oscillator frequency. This reduces the switching losses,

and is shown in Figure 4-5. However, lowering the

oscillator frequency will cause an undesirable increase

capacitors; this is overcome by increasing the values of

been reduced. For example, the addition of a 100pF

oscillator frequency to 2kHz from its nominal frequency

of 12kHz (multiple of 6), and thereby necessitate a

10

µF to 68µF).

FIGURE 4-5:

LOWERING OSCILLATOR

FREQUENCY

4.6

Positive Voltage Doubling

The TC7662A may be employed to achieve positive

voltage doubling using the circuit shown in Figure 4-6.

In this application, the pump inverter switches of the

the supply voltage minus the combined forward voltage

current of 10 mA, it will be approximately 60

Ω.

FIGURE 4-6:

POSITIVE VOLTAGE

MULTIPLIER

4.7

Combined Negative Voltage

Conversion and Positive Supply

Multiplication

Figure 4-7 combines the functions shown in Figure 4-1

and Figure 4-6 to provide negative voltage conversion

and positive voltage doubling simultaneously. This

approach would be, for example, suitable for generat-

ing +9V and -5V from an existing +5V supply. In this

reservoir functions, respectively, for the generation of

pump and reservoir, respectively, for the doubled

positive voltage. There is a penalty in this configuration

which combines both functions, however, in that the

source impedances of the generated supplies will be

somewhat higher due to the finite impedance of the

common charge pump driver at pin 2 of the device.

FIGURE 4-7:

COMBINED NEGATIVE

CONVERTER AND

POSITIVE DOUBLER

4.8

Voltage Splitting

The same bidirectional characteristics can be used to

split a higher supply in half, as shown in Figure 4-8.

The combined load will be evenly shared between the

two sides. Because the switches share the load in

parallel, the output impedance is much lower than in

the standard circuits, and higher currents can be drawn

from the device. By using this circuit, and then the

circuit of Figure 4-4, +15V can be converted (via +7.5V

and -7.5V) to a nominal -15V, though with rather high

series resistance (~250

Ω).

FIGURE 4-8:

SPLITTING A SUPPLY IN

HALF

1

2

3

4

8

7

6

5

+

+

TC7662A

10

µF

10

µF

1

2

3

4

8

7

6

5

+

+

TC7662A

1

2

3

4

8

7

6

5

+

+

+

TC7662A

+

+

–

2

50

µF

50

µF

V –

50

µF

+

1

2

8

7

TC7662A

3

4

6

5

+

–

–

–