In my previous post I had discussed about the simple layout and uses of a LM723 IC. Now we shall consider on how to implement a basic low voltage regulators using LM 723. Firstly voltage regulators using 723 voltage regulators are divided into two-
- High voltage regulators
- Low voltage regulators
This classification is based on the voltage ranges that the IC is used to regulate. As discussed in the previous post, the control voltage remains at a constant value of 7.15V irrespective of the input voltage at V+ and V-. The regulator circuit, in simple terms, works by comparing the output voltage to its reference voltage and altering the output based on the result. So a low voltage regulator circuit is used to create regulated voltages up to 7.15V at the output and high voltage regulators to get output voltages from 7.15v to 37V.
Low Voltage Regulator – Circuit Diagram
A simple positive low voltage regulator can be made using 723 as shown in the above schematic. In order to understand the circuit operation let us consider the internal circuit diagram as show below.
The voltage at the NI terminal of the error amplifier due to R1 R2 divider is,
The difference between Vni and the output voltage Vo which is directly fed back to the INV terminal is amplified by the error amplifier. The output of error amplifier drives the pass transistor Q1 so as to minimize the difference between the NI and INV inputs of the error amplifier. Since Q1 is acting as an emitter follower
If the output voltage becomes low, the voltage at the INV terminal of the error amplifier also goes low. This makes the output of the error amplifier to become more positive, thereby driving transistor Q1 more into conduction. This reduces the voltage across Q1 and drives more current into the load causing voltage across load to increase. So the initial drop in the load voltage has been compensated. Similarly, any increase in the load voltage, or changes in the input voltage gets compensated.
As mentioned earlier, the reference voltage is typically 7.15V. So the output voltage Vo is,
which will always be less than 7.15v. So this circuit can be used as a low voltage regulator.
In addition to the two resistors mentioned above, a stabilizing capacitor C1 of approximately 100pF has to be connected between the INV (inverting terminal of error amplifier) and frequency compensation pin along with a resistor R3 between the non inverting terminal of the error amplifier and the output terminal to minimize temperature drift. For maximum efficiency, put