4-48

TELCOM SEMICONDUCTOR, INC.

Figure 8. Positive Voltage Multiplier

Combined Negative Voltage Conversion

and Positive Supply Multiplication

Figure 9 combines the functions shown in Figures 3 and

8 to provide negative voltage conversion and positive volt-

age multiplication simultaneously. This approach would be,

for example, suitable for generating +9V and –5V from an

perform the pump and reservoir functions, respectively, for

plied 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.

Efficient Positive Voltage

Multiplication/Conversion

Since the switches that allow the charge pumping op-

eration are bidirectional, the charge transfer can be per-

formed backwards as easily as forwards. Figure 10 shows

a TC1044S transforming –5V to +5V (or +5V to +10V, etc.).

The only problem here is that the internal clock and switch-

drive section will not operate until some positive voltage has

been generated. An initial inefficient pump, as shown in

Figure 7. Lowering Oscillator Frequency

1

2

3

4

8

7

6

5

+

V

+

VOUT

C1

COSC

+

C2

TC1044S

Figure 9, could be used to start this circuit up, after which it

on), or else the diode and resistor shown dotted in Figure 10

can be used to "force" the internal regulator on.

Voltage Splitting

The same bidirectional characteristics used in Figure 10

can also be used to split a higher supply in half, as shown in

Figure 11. The combined load will be evenly shared be-

tween the two sides. Once again, a high value resistor to the

LV pin ensures start-up. 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

5, +15V can be converted (via +7.5V and –7.5V) to a nominal

–15V, though with rather high series resistance (~250

Ω).

1

2

3

4

8

7

6

5

V+

VOUT =

(2 V+) – (2 VF)

+

C2

D1

D2

+

C1

TC1044S

1

2

3

4

8

7

6

5

+

V +

VOUT =

(2 V +) – (2 VF)

C1

D1

+

+

C3

C4

VOUT = –V

+

C2

TC1044S

D2

+

Figure 9. Combined Negative Converter and Positive Multiplier

Negative Voltage Generation for

Display ADCs

The TC7106 is designed to work from a 9V battery. With

a fixed power supply system, the TC7106 will perform

conversions with input signal referenced to power supply

ground.

Negative Supply Generation for

4

¹⁄₂ Digit Data Acquisition System

The TC7135 is a 4

¹⁄₂ digit ADC operating from ±5V

supplies. The TC1044S provides an inexpensive –5V source.

(See AN16 and AN17 for TC7135 interface details and

software routines.)

TC1044S

CHARGE PUMP DC-TO-DC

VOLTAGE CONVERTER