4-57

TELCOM SEMICONDUCTOR, INC.

7

6

5

4

3

1

2

8

TC7660

CHARGE PUMP DC-TO-DC

VOLTAGE CONVERTER

Figure 5. Increased Output Voltage by Cascading Devices

Figure 4. Paralleling Devices Lowers Output Impedance

device latch-up, a 1k

Ω resistor must be used in series with

the clock output. In a situation where the designer has

generated the external clock frequency using TTL logic, the

addition of a 10k

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.

It is also possible to increase the conversion efficiency

of the TC7660 at low load levels by lowering the oscillator

frequency. This reduces the switching losses, and is achieved

Figure 7. Lowering the oscillator frequency will cause an

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

capacitor between pin 7 (OSC) and pin 8 (V

oscillator frequency to 1kHz from its nominal frequency of

10kHz (a multiple of 10), and necessitate a corresponding

Cascading Devices

The TC7660 may be cascaded as shown (Figure 6) to

produce larger negative multiplication of the initial supply

voltage. However, due to the finite efficiency of each device,

the practical limit is 10 devices for light loads. The output

voltage is defined by:

where n is an integer representing the number of devices

cascaded. The resulting output resistance would be ap-

proximately the weighted sum of the individual TC7660

Changing the TC7660 Oscillator Frequency

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 exter-

nal clock, as shown in Figure 6. In order to prevent possible

1

2

3

4

8

7

6

5

TC7660

V

+

1

2

3

4

8

7

6

5

TC7660

C1

RL

C2

C1

"n"

"1"

+

1

2

3

4

8

7

6

5

V

+

1

2

3

4

8

7

6

5

10

µF

10

µF

"n"

"1"

10

µF

VOUT

1. VOUT = –n V

+

for 1.5V

V

10V

+

NOTES:

*

*

≤≤

+

+

+

TC7660

TC7660