POWER

SUPPLY

PRE

REGULATOR

LOAD

CHARGE

PUMP

POST

REGULATOR

LM7705

OSCILLATOR

OUT=V-

S2

S1

S4

S3

Ó1

Ó2

LM7705

SNVS420A – NOVEMBER 2008 – REVISED JUNE 2009

www.ti.com

Figure 3. 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 4. The external power supply and load are added in this diagram as well.

Figure 4. LM7705 Architecture

The architecture given in Figure 4 shows that the LM7705 contains 3 functional blocks:

•

Pre-regulator

•

Charge pump inverter

•

Post-regulator

The output voltage is stabilized by:

•

Controlling the power supplied from the power supply to the charge pump input by the pre-regulator

•

The power supplied from the charge pump output to the load by the post-regulator.

A more detailed block diagram of the negative bias generator is given in Figure 5. The control of the pre-

regulator is based on measuring the output voltage of the charge pump. The goal of the post-regulator is to

provide an accurate controlled negative voltage at the output, and acts as a low pass filter to attenuate the output

voltage ripple. The voltage ripple is a result of the switching behavior of the charge pump and is dependent of the

output current and the values of the used capacitors.

10

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