B Y

A N I TA A T H AT T I L

S 5 E C A L P H A

RAJAGIRI SCHOOL OF ENGINEERING,KAKKANAD

IC VOLTAGE REGULATORS

INTRODUCTION

Integrated ICs with improved capabilities are appearing in an ever increasing numbers.

Often ,the use of specialized IC produces a simpler and more accurate circuits.

Examples: MF5(National ‘s switched capacitor filter), SE/NE 565(Signetics’Phase locked loop-PLL) , timer SE/NE 555 etc

Applications of specialized ICs:

*universal filters

*timers

*Phase locked loop(PLL)

*power amplifiers

*voltage regulators

*switching regulators

*voltage references

Topics to be dealt

78XX series79XX series317 variable regulators1723 switching regulators

IC Voltage Regulators4

Voltage regulator: is a circuit that supplies constant voltage regardless of changes in the load current.

Advantages of IC voltage regulator:

inexpensive,versatile,provides current /voltage boosting, internal short circuit current limiting, thermal shutdown, floating operation for high voltage applications.

Classification of IC voltage regulators: There are basically two kinds of IC voltage regulators:

Multi-pin type, e.g. LM723C 3-pin type, e.g. 78/79XX

Multi-pin regulators are less popular but they provide the greatest flexibility and produce the highest quality voltage regulation

3-pin types make regulator circuit design simple

Types of IC voltage regulators: Fixed output voltage regulators: positive fixed output regulator(78XX

series) and negative fixed output regulator(79XX series) Adjustable output voltage regulators: positive (LM317) and

negative(LM337) Switching regulators: motorola ‘s MC1723 NOTE: MC1723 is a general purpose regulator ;it can be used in many

ways as a fixed positive or negative output voltage regulator, variable output voltage regulator or as a switching regulator.Due to its flexibility it has become as a standard type in the electronics industry.

Basic power supply

• Transformer ;steps down high voltage AC mains to low voltage AC.

• Rectifier:converts AC to DC, but the DC output is varying

• Filter: smooths the DC from varying greatly to a small ripple.

• Regulator : eliminates ripple by setting DC output to a fixed voltage

Need for regulation

• Without stable potentials, circuit performance degrades and if the variations are large enough the components may get destroyed.

• In order to avoid this regulation is used

Performance parameters of voltage regulators

Line or input regulation: defined as a change in output voltage for a change in input voltage

Load regulation: defined as the change in output voltage for a change in load current.

Temperature stability or average temperature coefficient of output voltage(TCVO): defined as the change inoutput voltage per unit change intemperature and expressed in millivolts per degree celsius.

Ripple rejection: is the measure of regulator ‘s ability to reject ripple voltages.it is usually expressed in decibels.

Quiescent current Iq : current that flows into the regulator and does not include current drawn by load or internal resistor networks.

The smaller the value of these performance parameters ,the better the regulator.

FIXED VOLTAGE REGULATORa)positive voltage regulator

The 78XX voltage regulators Fig shows the connection diagram of 78XX series Proper operation requires a common ground between the input and output

voltages. The difference between the input and output voltages(Vin- Vout) called

dropout voltage must be 2V even during low point in the input ripple voltage.

Capacitor Ci, is required if the regulator is located an appreciable distance

from a power supply filter. Even though Co is not required, it may be used to

improve the transient response of the regulator.

Ci Co

Basic building block of +ve regulators

7805 as current source

The 7800 regulators can also be employed as current sources. A typical connection diagram of 7805 IC as a 0.5 A current source is depicted in figure.

• The current supplied to the load is given as IL = (VR / R )+ IQ

when Iq is quiescent current in amperes (4.3 m A typically for the 7805 IC)• In figure, VR = V23= 5 V and R = 10 ohms So IL = 5/10 = 0.5A• The output voltage with respect to ground is VOUT = VR + VL

• The load resistance, RL = 10 Ohms, therefore VL = 5 V Thus Vout = VR+ VL= 5 + 5 = 10 V • Minimum input voltage required,Vin = Vout + dropout voltage = 10 + 2=12V

78XX basic features

IC No Voltage

7805 5V

7806 6V

7808 8V

7809 9V

7810 10V

7812 12V

7815 15V

7818 18V

7824 24V

• Features

• 3 terminal positive voltage regulator with seven voltage options

• High Output Current - typically 1.5A

• Short circuit current limit - 750mA at 5v

• Internal thermal overload protection

• Low quiescent current - 6mA

• Max input voltage = 35v

• Minimum Input Voltage = Vout + 2.5

b)Negative voltage regulators

•       Negative voltage regulator IC's are available in 79XX series.

• These IC's are similar to the 78 series, but operating on negative voltage, and providing a regulated negative output voltage.

• The capacitors connected at the input

and output sides are used to provide

additional filter circuits of regulator circuits.

Features of 79XX

IC No Voltage

7902 -2V

7905 -5V

7905.5 -5.2V

7906 -6.2V

7908 -8V

7912 -12V

7915 -15V

7918 -18V

7924 -24V

• Same as that of 78XX series except that 79XX series are negative regulators

• They are available in same seven voltage options with two extra voltage options,-2V and -5.2V

• As shown in the figure

ADJUSTABLE VOLTAGE REGULATOR

• Adjustable voltage regulators are those who voltage can be varied and utilized.

• Advantages of adjustable voltage regulators:• * improved system performance• * improved overload protection• *improved system reliability• Example: LM317

ADJUSTABLE VOLTAGE REGULATOR

a)positive adjustable regulators-LM317

The LM317 is a three terminal positive voltage regulator, which can be oper ated with the output voltage regulated at any setting over the range of 1.2 V to 57 V.

The three terminals are Vin,Vout and ADJUSTMENT(ADJ).

Circuit of LM317

Resistors Rx and R2 set the output to any desired voltage over the adjustment range (1.2 to 57 V)

When configured as shown in figure LM317 develops a nominal voltage of 1.25V referred to as reference voltage Vref between output and adjustment terminal.

This reference voltage is

impressed across resistor R1,since voltage is constant current I is also constant

Since R1 sets current I its called current set or program resistor

Referring to figure,the output voltage Vo is Vo = R1 + R2 (I1 +Iadj)………….(1) Where I1= Vref /R1……….(2) R1 =current set resistor

R2 =output set resistor Iadj= adjustment pin current

Substituting (2) in (1) we get :Vout = VREF [1+ R2/R1] + IADJ R2

where VREF = 1.25 V, the reference voltage between the output and adjustment

terminals

the current IADJ is very small (100 micro Amperes) and constant. So the voltage drop across R2 due to IADJ is also very small and can be neglected so that equation  can be written as

Vout = 1.25 [1+ R2/R1]

The current set resistor Rx is usually 240 Ohms, and to achieve good load

regulation it should be tied directly to the output of the regulator rather than near the load.

The load regulation is 0.1 percent while the line regulation is 0.01 percent per volt.

LM317 with capacitors,protection diodes

No capacitors are needed unless LM317 is situated far from the power supply filter capacitors in which case a bypass capacitor Ci is used.

Output capacitor Co can be added to improve the transient response.

When external capacitors are used protection diodes are also used to prevent capacitors from discharging through low current points into the regulator.

b)Adjustable negative voltage regulators-LM337

• The LM337 series of voltage regulators are a complement of LM317 series.

• They are negative adjustable voltage regulators

• These negative voltage regulators are available in the same voltage and current options as the positive adjustable voltage regulator LM317.

Theory of switching regulators

• A basic switching regulator consists of four major components:• Voltage source Vin

• Switch S1• Pulse generator Vpulse

• Filter F1

• Voltage source: maybe dc supply,battery,unregulated or regulated supply

*Vin must supply required output power

*it must be large enough to supply sufficient dynamic range

*it may be required to store energy for a specified period of time during power failures.

Switch S1: is a transistor or thyristor connected as a power switch and is operated in saturated mode.The pulse generator output alternatively turns the switch on and off.

Pulse generator Vpulse : produces an asymmetrical square wave varying in either frequency or pulse width called frequency modulation or pulse width modulation. Frequency range is around 20khz.

*duty cycle:is the ratio of on time ton to the period T of the pulse waveform.

Duty cycle = ton = = ton = ton f

t0n + toff T

• Filter: converts the pulse waveform from output of the switch into a dc voltage.

• Since this switching mechanism allows the conversion similar to transformers, the switching regulator is referred to as dc transformer.

• Vo is expressed as ;• Vo = ton *Vin

• T • Case 1: if time period T is constant,V0 is directly proportional to the on time

ton , for a given value of Vin. This method of changing the output voltage by varying ton is called pulse width modulation

• Case 2: if ton is held constant , output voltage is inversely proportional to the period T or directly proportional to the frequency of the pulse waveform . This method of varying the output voltage is called frequency modulation

Switching regulator

• In switching regulator,the transistor acts as a switch.• When the transistor is off(switch is open) , no current flows,therefore no

power dissapation.• When the transistor is on,(switch is closed), high current flows but Vce

becomes low and therefore power dissapation is less.• Basic circuit of switching regulator• Bridge rectifier-switch-transformer-rectifier• Bridge rectifier converts ac input signal to unregulated dc• Switch : closed implies currrent is allowed to flow• open implies no current is allowed to flow• Transformer: the output of the switching network is coupled to the rectifier

through the transformer• Rectifier: provides final rectification and smoothing of the dc output

• Features• Delivers load current of about

150mA without an external pass transistor.

• Output voltage range 3Vdc to 37 Vdc

• .01%line regulation• .03%load regulation

MC1723

• The diagram shows motorola’s MC1723

• It is a general purpose regulator and can be used as fixed ,variable and switching regulator.

• The regulator requires an external transistor and a 1mh choke.

• To minimize its power dissipation during switching, external transistor used must be switching power transistor.

• The 1 mH choke smooths out current pulses to the load

• Capacitor c holds out output voltage at constant dc level.

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