4

TC7660H

HIGH FREQUENCY 7660 DC-TO-DC

VOLTAGE CONVERTER

TC7660H-2 10/1/96

© 2001 Microchip Technology Inc.

DS21466A

2

2

= 2.12

Ω,

n (number of devices)

Paralleling Devices

Any number of TC7660H voltage converters may be

paralleled to reduce output resistance (Figure 4). The reser-

resistance would be approximately:

Cascading Devices

The TC7660H may be cascaded as shown in (Figure 4)

to produce larger negative multiplication of the initial supply

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

the practical limit is probably 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 TC7660H

Changing the TC7660H Oscillator Frequency

It may be desirable in some applications (due to noise or

other considerations) to increase or decease the oscillator

frequency. This can be achieved by overdriving the oscilla-

tor from an external clock, as shown in Figure 6. In order to

prevent possible 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

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.

Simple Negative Voltage Converter

Figure 3 shows typical connections to provide a nega-

tive supply where a positive supply is available. A similar

scheme may be employed for supply voltages anywhere in

the operating range of +1.5V to +10V, keeping in mind that

pin 6 (LV) is tied to the supply negative (GND) only for supply

voltages below 3.5V.

The output characteristics of the circuit in Figure 3 are

those of a nearly ideal voltage source in series with 70

Ω.

Thus, for a load current of – 10 mA and a supply voltage of

+5V, the output voltage would be – 4.3V.

The dynamic output impedance of the TC7660H is due,

primarily, to capacitive reactance of the charge transfer

output for only 1/2 of the cycle, the equation is:

1

2

3

4

8

7

6

5

TC7660H

1.0

µF

+

V

+

1.0

µF

+

1. VOUT = –n V

+

for 1.5V

V

+

10V

NOTES:

*

C1

C2

Figure 3. Simple Negative Converter

Figure 4. Increased Output Voltage by Cascading Devices

1

2

3

4

8

7

6

5

V

+

1

2

3

4

8

7

6

5

1.0

µF

1.0

µF

"n"

"1"

1.0

µF

VOUT

1. VOUT = –n V

+

for 1.5V

V

10V

+

NOTES:

*

*

+

+

+

TC7660H

TC7660H