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LTC1044A

12V CMOS Voltage Converter

Linear Technology 

LTC1044A

Applications Information

Capacitor Selection

External capacitors C1 and C2 are not critical. Matching is

not required, nor do they have to be high quality or tight

tolerance. Aluminum or tantalum electrolytics are excellent

choices with cost and size being the only consideration.

Negative Voltage Converter

Figure 6 shows a typical connection which will provide

a negative supply from an available positive supply. This

circuit operates over full temperature and power supply

ranges without the need of any external diodes. The LV

pin (pin 6) is shown grounded, but for V+ ≥ 3V it may be

floated, since LV is internally switched to ground (pin 3)

for V+ ≥ 3V.

The output voltage (pin 5) characteristics of the circuit

are those of a nearly ideal voltage source in series with

an 80Ω resistor. The 80Ω output impedance is composed

of two terms:

1. The equivalent switched-capacitor resistance (see

Theory of Operation).

2. A term related to the on-resistance of the MOS

switches.

At an oscillator frequency of 10kHz and C1 = 10µF, the

first term is:

REQUIV

=

(fOSC

1

/ 2)

•

C1

=

5

•

103

•

1

10

•

10

–

6

=

20Ω

Notice that the above equation for REQUIV is not a capaci-

tive reactance equation (XC = 1/C) and does not contain

a 2π term.

1

8

2

7

3 LTC1044A 6

10µF

4

5

V+ (1.5V TO 12V)

REQUIRED FOR V+ < 3V

VOUT = – V+

10µF

TMIN ≤ TA ≤ TMAX

1044a F06

Figure 6. Negative Voltage Converter

The exact expression for output resistance is extremely

complex, but the dominant effect of the capacitor is

clearly shown on the typical curves of Output Resistance

and Power Efficiency vs Frequency. For C1 = C2 = 10µF,

the output impedance goes from 60Ω at fOSC = 10kHz to

200Ω at fOSC = 1kHz. As the 1/(f • C) term becomes large

compared to the switch-on resistance term, the output

resistance is determined by 1/(f • C) only.

Voltage Doubling

Figure 7 shows a two-diode capacitive voltage doubler.

With a 5V input, the output is 9.93V with no load and 9.13V

with a 10mA load. With a 10V input, the output is 19.93V

with no load and 19.28V with a 10mA load.

VIN

(1.5V TO 12V)

1

8

2

7

3 LTC1044A 6

+

Vd

1N5817

Vd

1N5817

+

4

5

REQUIRED

FOR V+ < 3V

+

VOUT = 2(VIN – 1)

10µF

10µF

1044a F07

Figure 7. Voltage Doubler

Ultra-Precision Voltage Divider

An ultra-precision voltage divider is shown in Figure 8. To

achieve the 0.002% accuracy indicated, the load current

should be kept below 100nA. However, with a slight loss

in accuracy the load current can be increased.

1

8

2

7

+ C1

3 LTC1044A 6

10µF 4

5

V+ (3V TO 24V)

V+/2 ±0.002%

TMIN ≤ TA ≤ TMAX

IL ≤ 100nA

+

REQUIRED FOR

C2

V+ < 6V

10µF

1044a F08

Figure 8. Ultra-Precision Voltage Divider

1044afa

8

For more information www.linear.com/LTC1044A

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