I = f

B

A

CHARGE

PUMP

INVERTOR

PRE

REGULATOR

POST

REGULATOR

LM7705

SNVS420B – NOVEMBER 2008 – REVISED MARCH 2013

www.ti.com

•

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 24. 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.

Figure 24. Charge Pump Inverter with Input/Output Control

In the next section a simple equation will be derived, that shows the relation between the ripple of the output

current, the frequency of the internal clock generator and the value of the capacitor placed at the output of the

LM7705.

Charge Pump Theory

This section uses a simplified but realistic equivalent circuit that represents the basic function of the charge

pump. The schematic is given in Figure 25.

Figure 25. Charge Pump

(1)

When the switch changes between A and B at a frequency f, the charge transfer per unit time, or current is:

(2)

The switched capacitor network can be replaced by an equivalent resistor, as indicated in Figure 26.

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