SNVS420D November 2008 – May 2018 LM7705
As indicated in Functional Description, 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 28. The external power supply and load are added in this diagram as well.
The architecture given in Figure 28 shows that the LM7705 contains 3 functional blocks:
The output voltage is stabilized by:
A more detailed block diagram of the negative bias generator is given in Figure 29. 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 lowpass 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.
In Charge Pump Theory, 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.