IC Voltage Regulators: Fixed and Adjustable Output Regulators

2. INTRODUCTION • Integrated ICs with improved capabilities are appearing in an everincreasing numbers. • Often ,the use of specialized IC produces a simpler and moreaccurate • circuits. • Examples: MF5(National ‘s switched capacitor filter),SE/NE 565(Signetics’Phase locked loop-PLL) , timer SE/NE 555etc • Applications of specializedICs: • *universalfilters • *timers • *Phase lockedloop(PLL) • *poweramplifiers • *voltageregulators • *switchingregulators • *voltagereferences

3. Topics to bedealt • 78XX series • 79XXseries • 317 variableregulators • 1723 switchingregulators

4. IC VoltageRegulators • 4 • Voltage regulator: is a circuit that supplies constant voltage regardlessof changes in the loadcurrent. • Advantages of IC voltageregulator: • inexpensive,versatile,provides current /voltage boosting, internalshort circuit current limiting, thermal shutdown, floating operation for high voltage applications. • Classification of IC voltageregulators: • There are basically two kinds of IC voltageregulators: • Multi-pin type, e.g.LM723C • 3-pin type, e.g.78/79XX • Multi-pin regulators are less popular but they provide the greatestflexibility • and produce the highest quality voltageregulation • 3-pin types make regulator circuit designsimple

5. Types of IC voltageregulators: • Fixed outputvoltageregulators: positive fixed outputregulator(78XX • series) and negative fixed output regulator(79XXseries) • Adjustable outputvoltageregulators: positive (LM317) and negative(LM337) • Switching regulators: motorola ‘sMC1723 • 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 electronicsindustry.

6. Basic powersupply • Transformer ;steps down high voltage AC mains to lowvoltage AC. • Rectifier:converts AC to DC,but the DC output isvarying • Filter: smooths the DCfrom varying greatly to a small ripple. • Regulator : eliminates ripple by setting DC output to a fixed voltage

7. Need forregulation • Without stable potentials, circuit performance degrades and ifthe variations are large enough the components may getdestroyed. • In order to avoid this regulation isused

8. Performance parameters of voltage regulators • Line or input regulation:defined as a change in output voltage fora change in inputvoltage • Load regulation:defined as the change in output voltage for a changein loadcurrent. • Temperature stability or average temperature coefficient of output voltage(TCVO): defined as the change inoutput voltage per unitchange intemperature and expressed in millivolts per degreecelsius. • Ripple rejection: is the measure of regulator ‘s ability to rejectripple voltages.it is usually expressed indecibels. • Quiescent current Iq :current that flows into the regulator and doesnot include current drawn by load or internal resistornetworks. • The smaller the value of these performance parameters ,the betterthe • regulator.

9. FIXED VOLTAGEREGULATOR a)positive voltageregulator • Ci Co • The 78XX voltageregulators • Fig shows the connection diagram of 78XXseries • Proper operation requires a common ground between the input andoutput voltages. • The difference between the input and output voltages(Vin- Vout) calleddropout • voltage must be 2V even during low point in the input ripplevoltage. • 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 theregulator.

10. Basic building block of +veregulators

11. 7805 as currentsource • The7800regulatorscan also beemployedascurrentsources.Atypicalconnection diagramof 7805ICasa 0.5Acurrentsourceisdepictedinfigure. • The current supplied to the load is givenas • IL = (VR / R )+IQ • whenIqisquiescentcurrentin amperes(4.3mAtypicallyforthe7805IC) • Infigure, VR = V23= 5 V and R = 10ohms So IL = 5/10 =0.5A • The output voltage with respectto • groundis • VOUT = VR +VL • The load resistance, RL = 10 Ohms, therefore VL =5V Thus • Vout = VR+ VL= 5 + 5 = 10V • Minimum input voltagerequired, • Vin = Vout + dropout voltage = 10 +2=12V

12. 78XX basicfeatures • Features • 3 terminal positive voltageregulator • with seven voltageoptions • High Output Current - typically1.5A • Short circuit current limit - 750mAat 5v • Internal thermal overloadprotection • Low quiescent current -6mA • Max input voltage =35v • Minimum Input Voltage = Vout +2.5

13. b)Negative voltageregulators • • Negative voltage regulator IC's areavailable in 79XXseries. • These IC's are similar to the 78 series, but operating on negative voltage, and providinga regulated negative outputvoltage. • The capacitors connected at theinput and output sides are used toprovide additional filter circuits of regulatorcircu its.

14. Features of79XX • Same as that of 78XXseries except that 79XX series are negativeregulators • They are available in sameseven voltage options with two extra voltage options,-2V and-5.2V • As shown in thefigure

15. ADJUSTABLE VOLTAGEREGULATOR • Adjustable voltage regulators are those who voltage can be variedand utilized. • Advantages of adjustable voltageregulators: • • • • • • • Example:LM317 • improved systemperformance • improved overloadprotection • *improved systemreliability

16. ADJUSTABLE VOLTAGEREGULATOR a)positive adjustableregulators-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 57V. • The three terminals areVin,Vout andADJUSTMENT(ADJ).

17. Circuit ofLM317 • Resistors Rx and R2 set the output to any desired voltage over the adjustment range (1.2 to 57V) • When configured as shown in figure LM317 develops anominal voltage of 1.25V referred to as reference voltageVref between output and adjustmentterminal. • This reference voltage is impressed across resistorR1,since • voltage is constant current I isalso constant • Since R1 sets current I itscalled current set or programresistor

18. Referring to figure,the output voltage Vois •  Vo = R1 + R2 (I1+Iadj)………….(1) • Where I1= Vref/R1……….(2) • R1 =current set resistor R2 =output setresistor • Iadj= adjustment pincurrent • Substituting (2) in (1) we get :Vout = VREF [1+ R2/R1] + IADJR2 • 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 writtenas •  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 theload. • The load regulation is 0.1 percent while the line regulation is 0.01 percentper • volt.

19. LM317 with capacitors,protectiondiodes • No capacitors are needed unless LM317 is situated far from the power supply filter capacitors in which case a bypass capacitor Ciis used. • Output capacitor Co can beadded to improve the transientresponse. • When external capacitors areused protection diodes are also used to prevent capacitors from discharging through low current points into theregulator.

20. b)Adjustable negative voltageregulators-LM337 • The LM337 series of voltage regulators are a complementof LM317series. • They are negativeadjustable voltageregulators • These negative voltage regulators are available in the same voltage and current options as thepositive adjustable voltage regulator LM317.

21. Theory of switchingregulators • A basic switching regulator consists of four majorcomponents: • Voltage sourceVin • SwitchS1 • Pulse generatorVpulse • FilterF1

22. Voltage source:maybe dc supply,battery,unregulated or regulatedsupply • *Vin must supply required outputpower • *it must be large enough to supply sufficient dynamicrange • *it may be required to store energy for a specified period of timeduring • powerfailures. • Switch S1: is a transistor or thyristor connected as a power switch and is operated in saturated mode.The pulse generator output alternatively turnsthe switch on andoff. • Pulse generator Vpulse : produces an asymmetrical square wave varying in either frequency or pulse width called frequency modulation or pulsewidth modulation.Frequency range is around20khz. • *duty cycle:is the ratio of on time ton to the period T of thepulse waveform. • Dutycycle= ton ==ton = tonf • t0n + toff T

23. Filter:converts the pulse waveform from output of the switch into adc voltage. • Since this switching mechanism allows the conversion similar to transformers, the switching regulator is referred to as dctransformer. • Vo is expressed as; • Vo = ton*Vin • T • Case 1: if time period T is constant,V0 is directly proportional to the ontime ton , for a given value of Vin. This method of changing the output voltage by varying ton is called pulse widthmodulation • Case 2: if ton is held constant , output voltage is inversely proportional tothe period T or directly proportional to the frequency of the pulse waveform . This method of varying the output voltage is called frequencymodulation

24. Switchingregulator

26. In switching regulator,the transistor acts as aswitch. • When the transistor is off(switch is open) , no current flows,thereforeno powerdissapation. • When the transistor is on,(switch is closed), high current flows butVce becomes low and therefore power dissapation isless. • Basic circuit of switchingregulator • Bridgerectifier-switch-transformer-rectifier • Bridge rectifier converts ac input signal to unregulateddc • Switch : closed implies currrent is allowed toflow • open implies no current is allowed toflow • Transformer: the output of the switching network is coupled to therectifier through thetransformer • Rectifier: provides final rectification and smoothing of the dcoutput

30. Features • Delivers load current of about 150mA without an externalpass transistor. • Output voltage range 3Vdc to37 Vdc • .01%lineregulation • .03%loadregulation

31. MC1723 • The diagram showsmotorola’s MC1723 • It is a general purpose regulator and can be used as fixed,variable and switchingregulator. • The regulator requires anexternal transistor and a 1mhchoke. • To minimize its power dissipation during switching, external transistor used must be switching powertransistor. • The 1 mH choke smoothsout current pulses to theload • Capacitor c holds outoutput voltage at constant dclevel.