Subject: Liner Integrated Circuits

Topic: Special Purpose Integrated Circuits

Difficulty: Medium

IC 723 as a Low Voltage Regulator

The LM723 is the adjustable voltage regulator IC designed for series regulator application, with a current output of 150mA without external pass transistor.

The above figure shows the Functional Diagram of IC 723.
IC 723 has two sections:

1. A constant current Source:- A zener diode and a reference amplifier producing a constant voltage of about 7 volts at Vref end. The zener diode is being compelled to operate at fixed point (so that zener output voltage is a fixed voltage) by a constant current Source.
2. An error amplifier: The second section of IC 723 comprises of an error amplifier, a series pass transistor Q1 and a current limiting transistor. The error amplifier compares a sample of output voltage applied at Inverting input terminal to the reference voltage Vref applied at the Non-Inverting input terminal.
   The conduction of the transistor Q1 is controlled by the error signal.

Temperature compensated zener diode, constant current source and reference amplifier constitutes the reference element. In order to get a fixed voltage from zener diode, the constant current source forces the zener to operate at a fixed point.

Output voltage is compared with this temperature compensated reference potential of the order of 7 volts. For this Vref is connected to the non-inverting input of the error amplifier.

This error amplifier is high gain differential amplifier. It’s inverting input is connected to the either whole regulated output voltage or part of that from outside. For later case a potential divider of two scaling resistors is used. Scaling resistors help in getting multiplied reference voltage or scaled up reference voltage.

Error amplifier controls the series pass transistor Qi, which acts as variable resistor. The series pass transistor is a small power transistor having about 800 mW dissipation. The unregulated power supply source (< 36V d.c.) is connected to collector of series pass transistor.

Transistor Q2 acts as current limiter in case of short circuit condition. It senses drop across lc placed in series with regulated output voltage externally.

The frequency compensation terminal controls the frequency response of the error amplifier. The required roll-off is obtained by connecting a small capacitor of 100 pF between frequency compensation and inverting input terminals.

Both noninverting and inverting terminals of the error amplifier are available on outside pins of IC 723. Due to this, device becomes versatile and flexible to use. Only restriction is that internal reference voltage is 7 volts and therefore we have to use two different circuits for getting regulated outputs of below 7 volts and above 7 volts.

To design it as a low voltage regulator for Vo = 5V, 200mA

Vo = Vref x R2 / (R1 + R2)

For Vo = 5V, Vref = 7.15V

we get R2 = 2.32 x R1

Assuming, R1 = 2.2K, R2 = 5.1K

R3= R1 || R2
R3= 2.2K || 5.1K
R3= 1.5KΩ

Rsc = V_sense / I_SC

Assume V_SENSE = 0.5V and I_SC=10mA

RSC=1.5KΩ

Selection of Output transistor Q3

PDT3=VCE3×IC3
PDT3=(Vin−VO)×IC3
Assume Vin = 12V
PDT3=(12−5)×200mA
PDT3=1.4W
β=IL/IB=200mA/100mA
Assume IB = 100mA

β=2

Select a transistor with Power dissipation ≥ 1.4W and beta ≥ 2

RL=VO/IL
RL=5/200mA
RL=25Ω