POWER

SUPPLY

PRE

REGULATOR

LOAD

CHARGE

PUMP

POST

REGULATOR

LM7705

OSCILLATOR

OUT=V-

S2

S1

S4

S3

Ó1

Ó2

LM7705

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SNVS420B – NOVEMBER 2008 – REVISED MARCH 2013

APPLICATION INFORMATION

This applications section will give a description of the functionality of the LM7705. The LM7705 is a switched

capacitor voltage inverter with a low noise,

−0.23V fixed negative bias output. The part will operate over a supply

voltage range of 3 to 5.25 Volt. Applying a logical low level to the SD input will activate the part, and generate a

fixed

−0.23V output voltage. The part can be disabled; the output is switched to ground level, by applying a

logical high level to the SD input of the part.

FUNCTIONAL DESCRIPTION

The LM7705, low noise negative bias generator, can be used for many applications requiring a fixed negative

voltage. A key application for the LM7705 is an amplifier with a true zero output voltage using the original parts,

while not exceeding the maximum supply voltage ratings of the amplifier.

The voltage inversion in the LM7705 is achieved using a switched capacitor technique with two external

storage capacitors. This switched capacitor technique is given in Figure 22.

Figure 22. Voltage Inverter

The internal oscillator generates two anti-phase clock signals. Clock 1 controls switches S1 and S2. Clock 2

Due to the switched capacitor technique a small ripple will be present at the output voltage, with a frequency of

the oscillator. The magnitude of this ripple will increase for increasing output currents. The magnitude of the

ripple can be influenced by changing the values of the used capacitors.

In the next section a more detailed technical description of the LM7705 will be given.

TECHNICAL DESCRIPTION

As indicated in the functional description section, the main function of the LM7705 is to supply a stabilized

negative bias voltage to a load, using only a positive supply voltage. A general block diagram for this charge

pump inverter is given in Figure 23. The external power supply and load are added in this diagram as well.

Figure 23. LM7705 Architecture

The architecture given in Figure 23 shows that the LM7705 contains 3 functional blocks:

•

Pre-regulator

•

Charge pump inverter

•

Post-regulator

The output voltage is stabilized by:

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