General Purpose Linear IC 723 Regulator

General Purpose Linear IC 723 Regulator

Dec.-03,04,05,06,07,08,10,12,14,15, May-07,08,12,13,14

The popular general purpose precision regulator is IC 723. It is a monolithic linear integrated circuit in different physical packages.

The pin diagram alongwith the various packages is shown in the Fig. 5.8.1 (a), (b) and (c).

 

1. Important Features of IC 723

1) It works as voltage regulator at output voltage ranging from 2 to 37 volts at currents upto 150 mA.

2) It can be used at load currents greater than 150 mA with use of suitable NPN or PNP external pass transistors.

3) Input and output short-circuit protection is provided.

4) It has good line and load regulation (0.03 %)

5) Wide variety of applications of series, shunt, switching and floating regulator.

6) Low temperature drift and high ripple rejection.

7) Low standby current drain.

8) Small size, lower cost

9) Relative ease with which power supply can be designed.

10) It provides a choice of supply voltage.

 

2. Internal Structure of IC 723

The functional block diagram of IC 723 can be divided into four major blocks

1) Temperature compensated voltage reference source, which is zener diode.

2) An op-amp circuit used as an error amplifier.

3) A series pass transistor capable of a 150 mA output current.

4) Transistor used to limit output current. 

The functioning of the above blocks can be explained with the help of a simplified functional block diagram of IC 723 as shown in the Fig. 5.8.2.

Temperature compensated zener diode, constant current source and reference amplifier constitutes the reference element.

Key Point 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, V_ref 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 Q₁ which acts as variable resistor. The series pass transistor is a small power transistor having about 800 mW dissipation. The unregulated power supply source (< 36 V 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 R_sc 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. 

The internal structure can be represented in more simplified form as shown in the Fig. 5.8.3.

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.

 

3. Applications of IC 723

The various regular circuits as per the requirement can be achieved using IC 723. Some of them are discussed below :

a. Basic Low Voltage Regulator (V_o = 2 to 7 volts)

The resistor, R_sc is connected between CL and CS pins. The current limit transistor remains non-conductive unless drops across R_sc is 0.6 V (equal to V_BE drop). The value of R_sc can be found out by following equation

R_sc = V_sense / I_limit = 0.6 / I_limit   …. (5.8.1)

I_limit can be selected as 1.2 to 1.5 times the maximum load circuit. Potential divider made up of R₁ and R₂ is connected between Vref and non-inverting terminals.

V_{non –inverting} = V_ref × R₂ / R₁ + R₂ ... (5.8.2)

As the series pass transistor is working as emitter follower.

b. Low Voltage High Current Regulator

Output voltage from +2 to +7V and load current can be more than 150 mA. For this one transistor is connected externally, shown as Q₁ in the Fig. 5.8.5.

The functional equations are similar to that of basic low voltage regulator circuit.

c. Basic Positive High Voltage Regulator

For this type, output voltage varies from +7 V to +37 V and IL ≤ 150 mA.

The non-inverting terminal connected to Vref through R3. Due to this arrangement the error amplifier acts as non-inverting amplifier.

This is also called basic high voltage low current regulator.

d. Positive High Voltage High Current Regulator

For this type, output voltage from +7 V to +37 V and load current I_L > 150 mA. For this a external transistor Q₁ is connected, as shown in the Fig.5.8.7.

For this different expressions are similar to basic high voltage regulator and reproduced for the convenience.

While the power dissipation of transistor Q1 and the IC is given by the same expressions as given by the equations (5.8.5) and (5.8.6).

e. Negative Voltage Regulator

Connections for getting negative voltage regulator are shown in Fig. 5.8.8. An external PNP transistor, Q₁ is connected. Resistances can be from 1 kΩ to 10 kΩ

If magnitude of -V_i is less than 9 V, connect V_CC+ and V_C to a positive supply such that V_CC+ to V_CC- is greater than 9 V, for proper functioning of the IC. 

 

Example 5.8.1 Design a regulator using IC 723 to meet the following specifications :

V_o = 5 V; I_o = 100mA. V_in = 15 ± 20 %

I_sc = 150 mA; V_sense = 0.7 V

Solution : The given specifications are, V_o = 5 V; I_o = 100 mA, V_in = 15 ± 20 %,

Neglecting input bias current of an error amplifier we can write,

Use 1.5 kΩ (standard) resistor.

The designed regulator is shown in the Fig. 5.8.9.

 

Example 5.8.2 Draw the circuit diagram of IC 723 based positive voltage regulator, to give + 8 V output at 200 mA. Incorporate short circuit protection current limit circuit to operate at 400 mA. Find all resistor values and calculate their wattage.

Solution : V_o = + 8 V,

I_L = 200 mA, I_sc = 400 mA

As output is more than 7 V and IL is more than 150 mA, it is positive high voltage high current regulator. It is shown in the Fig. 5.8.10.

The equations of operation are,

All the resistor type can be of metal film resistors.

For the power rating of R₁ and R₂, assume the input current to the inverting terminal zero.

 

Example 5.8.3 Design an adjustable voltage regulator (5 V to 15 V) with a short circuit current limit of 50 mA using a 723 regulator.

Solution : Let us design it as a basic high voltage regulator but by changing V_ref from 7 V to V'_ref less than 5 V. The circuit is shown in the Fig. 5.8.11.

By varying R_1b, continuous adjustment of V_o from 5 V to 15 V can be achieved.

 

Example 5.8.4 Design a voltage regulator using IC 723 regulator to satisfy the following specifications :

i) V_o = 12V, ii) I_o - 500 mA, iii) V_in = 18 + 20 %  iv) I_sc = 600 mA,

and. v) V_sense = 0.7 V. Give the complete schematic diagram. (Assume and justify if any data required)

Solution :

The output transistor is required to satisfy

I_o = 500 mA

The designed circuit is shown in the Fig. 5.8.12.

Review Questions

1. List the important features of IC 723.

2. Explain the following applications of IC723,

i. Basic low voltage regulator

ii. Basic positive high voltage regulator.

3. Discuss the internal structure of 723 voltage regulator.

4. Design a low voltage regulator using 723 for 5 V and explain its characteristics.

Dec.-05, Marks 8

5. Explain the current limiting feature and current boosting in 723 IC.

May-08, Marks 10

6. Draw and explain the functional block diagram of a 723 IC regulator and make the necessary changes to make it as low voltage regulator.