Three Terminal Fixed Voltage Regulators

Three Terminal Fixed Voltage Regulators

May-04,17. Dec.-07,10,16

As the name suggests, three terminal voltage regulators have three terminals namely input which is unregulated (V_in), regulated output (V_o) and common or a ground terminal. These regulators do not require any feedback connections. The Fig. 5.9.1 shows the basic three terminal voltage regulator.

The capacitor C_in is required if regulator is located at appreciable distance, more than 5 cm from a power supply filter. The output capacitor C_o may not be needed but if used it improves the transient response of the regulator i.e. regulator response to the transient changes in the load. This capacitor also reduces the noise present at the output. The difference between and Vo (V_in - V_o) is called as dropout voltage and it must be typically 2.0 V even during the low point on the input ripple voltage, for the proper functioning of the regulator.

 

1. Block Diagram of Basic Three Terminal IC Regulator

The Fig. 5.10.2 shows the functioal block diagram of basic three terminal IC regulator.

This is basically a series voltage regulator circuit. A part of output voltage is taken with the help of potential divider formed by R₁ and R₂. This is compared with reference voltage, Vref internally generated with the help of zener diode. After comparison, a control signal is generated which is applied through protective circuit to the series pass transistor working as control element. This element works as a variable resistance. The control signal adjusts the control element in such a way that output voltage remains constant.

Thermal shutdown means that the chip will automatically turn itself off if the internal temperature exceeds, typically, 175 °C. The current limiting circuit will protect the chip from excessive load current. Because of the thermal shutdown and current limiting, the IC voltage regulator chip is almost indestructible.

 

2. Datasheet Specifications of IC Linear Regulators

A number of performance parameters are generally specified for the IC voltage regulators by the manufacturers. These are as follows :

i) Line regulation : It is defined as the change in the output voltage for a given change in the input voltage. It is expressed as a percentage of output voltage or in millivolts.

ii) Load regulation : As defined earlier, it is the change in output voltage over a given range of load currents i.e. from full load to no load. It is usually expressed in millivolts or as a percentage of output voltage.

iii) Ripple rejection : It indicates regulator's ability to reject ripple voltage present in the input. It is defined as the ratio of the r.m.s. input ripple voltage to the r.m.s. output ripple voltage. It is expressed in decibels (dB).

iv) Dropout voltage : It is the minimum voltage that must exist between input and output terminals. As mentioned earlier, it is the difference between input voltage and output voltage Vo. For most of the regulators it is 2 to 3 V.

v) Output resistance (R_o) : It is the rate of change of output voltage with respect to the output current. It should be as small as possible.

vi) Maximum input voltage (V_{in max}) : This is the maximum input voltage that can be applied to the regulator safely.

vii) Maximum power dissipation (P_Dmax) : This is the maximum power which regulator can dissipate without damage. The actual power dissipated is approximately equal to the voltage drop between the input and the output terminals multiplied by the current through the regulator. For many regulators it is internally limited. 

viii) Quiescent current (I_Q) : It is also called as standby current. This is the supply current drawn by the regulator without any load. It can also be defined as the current that must flow from the ground terminal of the regulator to operate satisfactorily.

ix) Rated output current (I_o) : It is the maximum value of the output current above which current limiting occurs.

x) Output noise voltage : It indicates the tendency of the output voltage to fluctuate above its prescribed d.c. value, over a specified frequency range.

xi) Maximum operating junction temperature : It is the maximum value of junction temperature above which thermal shutdown occurs.

Out of these parameters maximum input voltage, maximum power dissipation, maximum operating junction temperature etc. are called as absolute maximum ratings of IC regulator, while other ratings are called as electrical characteristics of IC regulator.

 

3. LM 78XX and 79XX Three Terminal IC Voltage Regulators

The popular IC series of three terminal regulators is pA 78XX and µA 79XX. The series µA 78XX is the series of three terminal positive voltage regulators while µA 79XX is the series of three terminal negative voltage regulators. The last two digits denoted as XX, indicate the output voltage rating of the IC.

Such series is available with seven voltage options as indicated in Table 5.10.1.

The 79XX series voltage regulators are available with same seven options as 78XX series, as indicated in Table 5.10.1. In addition, two extra voltages -2 V and -5.2 V are also available with ICs 7902 and 7905.2 respectively. 

These ICs are provided with adequate heat sinking and can deliver output currents more than 1 A. These ICs do not require external components. These are provided with internal thermal protection, overload and short circuit protection.

The two series are available in various versions like low power and high power versions. The low power versions are available in plastic or metal packages, like small signal transistors. The higher power versions are packaged in TO-3 type metal cans or in TO-220 type moulded plastic packages like power transistors.

These are shown in the Fig. 5.10.3 (a) and (b).

 

4. Typical Connection of IC 7805 Regulator

The Fig. 5.10.4 shows the typical connection of IC pA 7805 regulator. The maximum input voltage which can be applied to IC 7805 is 35 V. The maximum dropout voltage is 2.5 V. The output of 7805 regulator is 5 V fixed. Hence minimum input voltage required is 5 + 2.5 = 7.5. So input range of IC 7805 regulator is 7.5 V to 35 V to get fixed 5 V as regulated output. This is indicated in the Fig. 5.10.4.

 

5. Positive 5 V Power Supply using IC 7805

A 5 V output voltage supply system using full wave bridge rectifier, capacitor filter and IC regulator 7805 is shown in the Fig. 5.10.5. The a.c. line voltage is 230 V which is stepped down to 15 V using a transformer. A full wave rectifier alongwith the capacitor voltage provides the unregulated voltage input to IC 7805 regulator. This input contains a.c. ripple of few volts. The IC 7805 regulator provides the regulated output of 5 V.

 

6. Adjustable Regulator using 78XX Series

Though IC 78XX series regulators have fixed value of the regulated output voltage, by connecting two resistances externally, an adjustable output voltage can be obtained.

The typical connection of 78XX IC regulator to obtain variable output voltage is shown in the Fig. 5.10.6.

V_out = V_reg [1+ R₂ / R₁]

where V_reg = Regulated fixed voltage of IC

By varying R₂, variable output voltage can be obtained.

 

7. Applications of IC 78XX and 79XX

These ICs are regulator ICs and are basically used to provide constant d.c. voltages to various components in complex electronic circuits.

The IC 7805 is typically used to provide constant 5 V supply to the digital circuits. 

The IC 7812 and 7912 are used to provide dual supply of ± 12 V to operational amplifiers used in the electronic circuits.

 

Example 5.10.1 Calculate the output voltage of the adjustable regulator shown in the Fig. 5.10.7.

If R₂ is varied from 1 kΩ to 10 kΩ find the range of output voltage.

Solution : R₁ = 5 kΩ, R₂ = 10 kΩ

Thus the V_out can be varied from 9.6 V to 24 V, by varying R₂ from 1 kΩ to 10 kΩ .

 

8. Boosting Regulator Output Current

For limiting the cost and size, if output current as high as 10 A is required then it is convenient to boost a regulator output current, which is of less capacity, with the help of external circuit.

The three terminal regulators can be boosted by connecting an external pass transistor in parallel with the regulator itself. Due to this the maximum output current of 78XX regulator which is 1 A, can be boosted. The boosting is possible because the job of supplying the output current is shared by the regulator and the transistor. The IC 7805 regulator with the current boosting arrangement is shown in the Fig. 5.10.8.

For the low load currents (I_L), the drop across R₁ is insufficient (less than 0.7 V) to conduct the transistor Q₁. Hence the entire load current is supplied by the regulator itself.

When the load current I_L increases, the drop across R₁ also increases, when it becomes greater than 0.7 V then the transistor Qi starts conducting. For example if R₁ is 7 Q then for load current above 100 mA, V_R1 becomes 100 mA × 7 i.e. 0.7 V and Q₁ starts conducting. The excess current required by the regulator is supplied by Q₁, by amplifying I_B as per the relation I_C = β  I_B. Hence

Thus the current boosting is achieved which depends on the value of β  and (I_o)_max of the regulator.

 

Example 5.10. 2 For a 7805 IC voltage regulator, a current boosting is provided with a transistor having V_BE = 1 V and β  = 15. Calculate maximum load current it can deliver. Assume R₁ as 7 Ω.

Solution : The (I_o )_max of 7805 regulator is 1 A.

Thus IC which can supply maximum of 1 A can supply maximum load of 13.85 A, with the help of the current boosting arrangement.

 

Example 5.10. 3 The simplified block diagram of an 1C voltage regulator is shown in Fig. 5.10.9. Calculate its output voltage V_o.

Solution : As the gain of differential amplifier A → ∞, the virtual ground is applicable to it.

Hence V_B = V_ref.

V_B =5V

This is shown in the Fig. 5.10.9 (a).

The current I remains same through both the resistors as no current flows into the op-amp.

 

Example 5.10.4 Using 7805 design a current source to deliver a 0.2 A current to a 22 Ohm 10 W load.

Solution : The current source using 7805 is shown in the Fig. 5.10.10.

The current through RL is given by,

Review Questions

1. Explain the block diagram of basic three terminal IC regulator.

2. Draw and explain the typical connection of IC 7805.

3. How to achieve adjustable regulator using 78XX series ?

4. How can the current drive capability be increased while using three terminal voltage regulators ?

5. Draw the circuit diagram and explain the operation of any one negative voltage regulator.

Dec.-07, Marks 8

6. What do you mean by fixed voltage regulators ? List its applications.