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Three terminal Voltage regulators 1.Introduction: The voltage regulator is used to provide a stable dc voltage independent of the load current, temperature and ac line voltage regulation for powering the electronic circuits 2.Classification of IC voltage regulators: Fixed voltage Series regulators Adjustable voltage regulators General purpose regulators Switching voltage regulators 3.Three terminal fixed voltage regulators: The three terminal voltage has three terminals namely which is unregulated (Vin), regulated output and common or a ground potential. Vin o Input Voltage regulator o Ground Cin 4.Block diagram of three terminal voltage regulator Co Vo Output Vin o Series pass transistor o Vout R1 Thermal shutdown and current limiting Control signal R2 o Vref 5.Characteristics of three terminal IC voltage regulators 1. 2. 3. 4. Regulated output voltage ( specified by the manufacturer) |Vin|≥|Vo|+2 volts Io(max): It may vary from zero to rated maximum output current Thermal shutdown 6.Performance parameters 1. Line/input regulation: Percentage change in the output voltage for a change in the input voltage, expressed in mV or as a percentage of output voltage 2. Load Regulation: Change in the output voltage for a change in load current and is expressed in mV or as a percentage of Vo. 7.IC series of three terminal fixed voltage regulators The popular IC series are µA78XX (series of three terminal positive voltage regulators) and µA79XX (series of three terminal negative voltage regulators). The last two digits denoted as XX indicate the output voltage rating Device type 7805 7806 7808 Output voltage 5.0V 6.0V 8.0 Device type 7905 7906 7908 Output voltage -5.0V -6.0V -8.0 7812 7815 7818 7824 12.0V 15.0V 18.0V 24.0 7912 7915 7918 7924 -12.0V -15.0V -18.0V -24.0 In addition, two extra voltages -2V and -5.2 are also available with ICs 7902 and 7905.2 respectively 8.Fixed voltage as currents source I L I R IQ Where IL is the quiescent current and is about 4.2mA for 7805(data sheet) V I L R I Q I L ( I Q I L ) R R VR IL Drop out voltage Vin -Vo =2V Vin(input) o 1 IN GND OUT + 2 3 Ci IQ o IR Vo (output) R VR IL RL VL 9.Boosting IC regulator output current It is possible to boost the current of a three terminal regulator by connecting an external pass transistor in parallel with the regulator. When load current increases , the voltage drop across R1 increases. When this is approximately 0.7V, the transistor Q1 turns ON and supplies the extra current needed. Since V BE(ON) remains fairly constant, the excess current comes from Q1’s base after amplification by β. I L I c I o ; I C I B For regulator, Io Ii IQ I i ( I Q : small ) I B I i I R1 I o VEB (ON ) R1 Arranging for IL VEB (ON ) R1 The maximum current Io(max) for a 7805 regulator is 1A from the data sheet. I L (1 ) I o o Vin IR1 R1 IB 1 IN Ii GND IQ 3 OUT VEB Q1 IC 2 Io + o Vo (output) IL Load 10.Fixed regulator used as adjustable regulator The ground (GND) terminal of the fixed three terminal regulator is floating. The output voltage Vo VR V pot VR ( I Q I R 1 ) R2 Vo VR I Q R2 VR R2 R1 R Vo 1 1 VR R2 I Q R2 Vin(input) o 1 IN GND 3 OUT + 2 VR o R1 IR1 Vo (output) 0.1F IQ Vpot R2 11.Three terminal adjustable regulator: IC voltage regulators allows the adjustment of the output voltage from 1.2V to as high as 57V. The common terminal plays the role of control input and hence ADJUSTMENT(ADJ) terminal 12.Advantages of adjustable voltage regulator Improved line and load regulation by a factor of 10 or more Because of improved overload protection, greater load current can be drawn over the given operating temperature range Improved reliability for the power supply using these regulators Type LM 317 LM337 LM358 LM350 LM396 Three terminal adjustable regulators Output voltage Output current +1.2 to 57V 1.5A -1.2 to - 57V 1.5A +1.2 to 32V 5A +1.2 to 32V 3A +1.25 to 15V 10A 13.Connection diagram of LM 317 regulator LM317 IN o Vin Input o OUT ADJ IADJ Ground IR1 Vo Output R1 R2 The design of LM 317 is such that the current IADJ is very small and remains constant with line and load changes. Internally it develops a reference voltage of 1.25Vbetween OUT and ADJ terminals, which is denoted as Vref. The resistor R1 is called program resistor or current set resistor Vo VR1 VR 2 Vo I R1 R1 ( I R1 I ADJ ) R2 Vref 1.25 I R1 R1 R1 1.25 Vo 1.25 R2 I ADJ R2 R1 Vo 1.25 R1 R2 I ADJ R2 R1 The current IADJ is very small and hence the drop IADJR2 is also very small and is neglected R2 .Therefore Vo 1.251 R1 Thus the output voltage is the function of R1 and R2. Keeping R1 fixed and varying R2, the output voltage can be varied IN 4001 Vin Input up to 40V o LM317 IN OUT o ADJ C1=0.1F IN4001 1.25V R1=240 C3=10F R2 The diodes are necessary if output voltage is higher than 25V 14.Limitations of Linear voltage regulators The required input step down transformer is bulky and expensive Due to low line frequency(50Hz), large values of filter capacitors are required The efficiency is low Input must be greater than the output voltage As large is the difference between input and output voltage, more is the power dissipation in the series pass transistor For higher input voltages, efficiency decreases The need for dual supply, is not economical and feasible to achieve with the help of linear regulators Vo Output C2=1F