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Cascading Devices

1

2

3

4

8

7

6

5

TL7660

"1"

1

2

3

4

8

7

6

5

10 µF

+

+

-

+

+

-

V+

+

-

10 µF

V

= –nV+

OUT

10 µF

TL7660

"n"

+

-

10 µF

Changing the TL7660 Oscillator Frequency

1

2

3

4

8

7

6

5

TL7660

10 µF

+

-

CMOS

Gate

+

+

–

10 µF

V

OUT

V+

V+

–

TL7660

CMOS VOLTAGE CONVERTER

SCAS794 – JUNE 2006

APPLICATION INFORMATION (continued)

The TL7660 may be cascaded as shown to produced larger negative multiplication of the initial supply voltage

(see Figure 6). However, due to the finite efficiency of each device, the practical limit is 10 devices for light

loads. The output voltage is defined by:

VOUT = –n (VIN)

Where n is an integer representing the number of devices cascaded. The resulting output resistance would be

approximately the weighted sum of the individual TL7660 ROUT values.

Figure 6. Cascading Devices for Increased Output Voltage

It may be desirable in some applications, due to noise or other considerations, to increase the oscillator

frequency. This is achieved by overdriving the oscillator from an external clock, as shown in Figure 7. To prevent

possible device latchup, a 1-k

Ω resistor must be used in series with the clock output. When the external clock

frequency is generated using TTL logic, the addition of a 10-k

Ω pullup resistor to V

CC supply is required. Note

that the pump frequency with external clocking, as with internal clocking, will be 1/2 of the clock frequency.

Output transitions occur on the positive-going edge of the clock.

Figure 7. External Clocking

11

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