LTC1046CS8 datasheet(7/12 Pages) LINER

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LTC1046

APPLICATI

TYPICAL

Figure 7. Negative Voltage Converter

Negative Voltage Converter

Figure 7 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 GND (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 27

Ω

resistor. The 27

Ω output impedance is composed of two

terms: 1) the equivalent switched capacitor resistance

(see Theory of Operation), and 2) a term related to the ON

resistance of the MOS switches.

At an oscillator frequency of 30kHz and C1 = 10

µF, the first

term is:

f/ 2

EQUIV

OSC

()

•

••

•

–

15 10

10 10

Ω.

Notice that the equation for REQUIV is not a capacitive

reactance equation (XC = 1/ωC) and does not contain a 2π

term.

The exact expression for output impedance is complex,

but the dominant effect of the capacitor is clearly shown on

Figure 8. Voltage Doubler

Figure 9. Ultraprecision Voltage Divider

the typical curves of output impedance and power effi-

ciency versus frequency. For C1 = C2 = 10

µF, the output

impedance goes from 27

Ω at fOSC = 30kHz to 225Ω at

fOSC = 1kHz. As the 1/fC term becomes large compared to

switch ON resistance term, the output resistance is deter-

mined by 1/fC only.

Voltage Doubling

Figure 8 shows a two diode, capacitive voltage doubler.

With a 5V input, the output is 9.1V with no load and 8.2V

with a 10mA load.

V +

OSC

VOUT

BOOST

CAP +

GND

CAP –

LTC1046

µF

1046 F07

V+

1.5V TO 6V

VOUT = –V

REQUIRED FOR V+ < 3V

TMIN ≤ TA ≤ TMAX

V +

OSC

VOUT

BOOST

CAP +

GND

CAP –

LTC1046

µF10µF

1046 F08

V+

1.5V TO 6V

VOUT = 2

(VIN –1)

REQUIRED

FOR

V+ < 3V

Ultraprecision Voltage Divider

An ultraprecision voltage divider is shown in Figure 9. To

achieve the 0.0002% accuracy indicated, the load current

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

in accuracy, the load current can be increased.

V +

OSC

VOUT

BOOST

CAP +

GND

CAP –

LTC1046

µF

TMIN ≤ TA ≤ TMAX

IL ≤ 100nA

REQUIRED FOR V+ < 6V

1046 F09

V+

3V TO 12V

±0.002%

V+

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