By<br /> Anita a thattil<br /> S5 ec alpha<br />Rajagiri school of engineering,kakkanad<br />IC VOLTAGE REGULATORS<br />
INTRODUCTION<br />Integrated ICs with improved capabilities are appearing in an ever increasing numbers.<br />Often ,the use of specialized IC produces a simpler and more accurate circuits.<br />Examples: MF5(National ‘s switched capacitor filter), SE/NE 565(Signetics’Phase locked loop-PLL) , timer SE/NE 555 etc<br />Applications of specialized ICs:<br /> *universal filters<br /> *timers<br /> *Phase locked loop(PLL)<br /> *power amplifiers<br /> *voltage regulators<br /> *switching regulators<br /> *voltage references<br />
Topics to be dealt<br />78XX series<br />79XX series<br />317 variable regulators<br />1723 switching regulators<br />
IC Voltage Regulators<br />4<br /><ul><li>Voltage regulator: is a circuit that supplies constant voltage regardless of changes in the load current.
Advantages of IC voltage regulator:</li></ul> inexpensive,versatile,provides current /voltage boosting, internal short circuit current limiting, thermal shutdown, floating operation for high voltage applications. <br />Classification of IC voltage regulators:<br /><ul><li> There are basically two kinds of IC voltage regulators:
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</li></li></ul><li>Types of IC voltage regulators:<br /><ul><li>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)
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.</li></li></ul><li>Basic power supply<br /><ul><li>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</li></li></ul><li>Need for regulation<br /><ul><li>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</li></li></ul><li>Performance parameters of voltage regulators<br /><ul><li>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.</li></li></ul><li>FIXED VOLTAGE REGULATORa)positive voltage regulator <br />Ci<br />Co<br />The 78XX voltage regulators <br />Fig shows the connection diagram of 78XX series <br />Proper operation requires a common ground between the input and output voltages.<br />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.<br />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.<br />
7805 as current source<br /><ul><li>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</li></ul> IL= (VR/ R )+ IQ<br />when Iq is quiescent current in amperes (4.3 m A typically for the 7805 IC)<br /><ul><li>In figure, VR= V23= 5 V and R = 10 ohms</li></ul> So IL= 5/10 = 0.5A<br /><ul><li>The output voltage with respect to</li></ul>ground is<br /> VOUT= VR+ VL<br /><ul><li>The load resistance, RL= 10 Ohms, </li></ul>therefore VL= 5 V Thus <br />Vout= VR+ VL= 5 + 5 = 10 V <br /><ul><li>Minimum input voltage required,</li></ul>Vin = Vout + dropout voltage = 10 + 2=12V<br />
Example: LM317</li></li></ul><li>ADJUSTABLE VOLTAGE REGULATORa)positive adjustable regulators-LM317<br />The LM317 is a three terminal positive voltage regulator, which can be operated with the output voltage regulated at any setting over the range of 1.2 V to 57 V. <br />The three terminals are Vin,Vout and ADJUSTMENT(ADJ).<br />
Circuit of LM317<br />Resistors Rx and R2 set the output to any desired voltage over the adjustment range (1.2 to 57 V)<br />When configured as shown in figure LM317 develops a nominal voltage of 1.25V referred to as reference voltage Vrefbetween output and adjustment terminal.<br />This reference voltage is impressed across resistorR1,since voltage is constant current I is also constant<br />Since R1 sets current I its called current set or program resistor<br />
Referring to figure,the output voltage Vo is<br />Vo = R1 + R2 (I1 +Iadj)………….(1)<br />Where I1= Vref/R1……….(2)<br />R1 =current set resistor<br />R2 =output set resistor <br />Iadj= adjustment pin current <br />Substituting (2) in (1) we get :Vout = VREF [1+ R2/R1] + IADJR2<br />where VREF = 1.25 V, the reference voltage between the output and adjustment terminals<br />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<br />Vout = 1.25 [1+ R2/R1]<br />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.<br />The load regulation is 0.1 percent while the line regulation is 0.01 percent per volt. <br />
LM317 with capacitors,protection diodes<br />No capacitors are needed unless LM317 is situated far from the power supply filter capacitors in which case a bypass capacitor Ci is used.<br />Output capacitor Co can be added to improve the transient response.<br />When external capacitors are used protection diodes are also used to prevent capacitors from discharging through low current points into the regulator. <br />
b)Adjustable negative voltage regulators-LM337<br /><ul><li>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.</li></li></ul><li>Theory of switching regulators<br /><ul><li>A basic switching regulator consists of four major components:
Filter F1</li></li></ul><li><ul><li>Voltage source: maybe dc supply,battery,unregulated or regulated supply</li></ul> *Vin must supply required output power<br /> *it must be large enough to supply sufficient dynamic range<br /> *it may be required to store energy for a specified period of time during power failures.<br />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.<br />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.<br /> *duty cycle:is the ratio of on time ton to the period T of the pulse waveform.<br />Duty cycle =ton ==ton =ton f<br /> t0n + toffT <br />
<ul><li>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.
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 </li></li></ul><li>Switching regulator<br />