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General purpose voltage regulators 1.Introduction: The popular general purpose precision regulator is 723. It is monolithic IC available in different physical packages. 2.Important features of IC723 It works as voltage regulator at output voltage ranging from 2 to 37 volts at currents upto 150mA It can be used at load currents greater than 150mA with the use of external pass transistors Input and output short circuit protection is provided Wide variety of series, shunt , switching and floating regulator Good load and line regulation Low standby current drain It provides choice of supply voltage 3. Functional block diagram The major blocks are Temperature compensated voltage reference source An op amp circuit used as error amplifier A series pass transistor capable of a 150mA output current Transistor used to limit output current Vcc( unregulated input) o Frequency compensation o Temperature compensated zener diode o Vc Inverting Vref input __ Q1 o Non inverting input Constant current source + o Vo Q2 6.2V o CS Current sense o -Vcc o CL Current limit 4.Applications of IC 723 4.1 Basic low voltage regulator(Vo=2 to 7 volts) The resistor, Rsc is connected between CL and CS pins. The current limit resistor remains non-conductive unless drop across Rsc is equal the VBE drop(≈0.7V). The value of Rsc is given by the equation V 0.7 Rsc sense I limit I limit Ilimit can be selected as 1.2 to 1.5 times the maximum load current. Potential divider made up of R1 and R2 is connected between Vref and non-inverting terminal. R2 Vnoninverting Vref R1 R2 Since the series pass transistor is working as emitter follower. Hence R2 Vnoninverting Vref R1 R2 o VZ R1 and R2 can be between 1kΩ and 10kΩ. R3=R1||R2. Maximum load current can be 150mA. +Vi o 12 11 10 6 Rsc o 2 IC 723 3 R1 R3 4 5 Cref 100pF R2 7 13 4.2 Low voltage High current regulator: Regulated output Vo +Vi o 12 11 Q1 10 6 Rsc 2 IC 723 o Regulated output Vo 3 R1 R3 4 5 100pF R2 13 7 Output voltage ranging from 2 to 7 Volts and the load current of more than 150mA can be obtained by connecting a transistor Q1 as shown in Figure. The functional equations are Vo Vref Rsc R2 R1 R2 0 .7 I lim it Power dissipation of the transistor= Vi (max) Vo (min) I L (max) Power dissipation of IC= Vi (max) Vo (min) I L (max) h fe(min) of (Q1) 4.3 Basic positive high voltage regulator Output varies from +7 V to +37V and IL ≤150mA. The non-inverting terminal connected to Vref through R3. It acts as a non-inverting amplifier. Hence the output voltage is R Vo Vref 1 1 R2 Rsc 0.7 Vsence ; R3 R1 || R2 I lim I sc +Vi o 12 11 10 6 Rsc o 2 IC 723 3 R3 4 R1 5 7 13 R2 100pF This voltage regulator is also called high voltage low current regulator 4.4 Positive high voltage high current regulator Regulated output Vo +Vi o 12 11 Q1 10 6 2 IC 723 3 R3 Rsc 5 o 4 R1 13 7 Regulated output Vo 500pF R2 For this, output voltage ranges from +7V to +37V and load current IL>150mA. For this an transistor is connected. R1 R 2 V o V ref R2 V sense R1 R 2 0.6 R sc ; R3 I lim it I sc R1 R 2 Power dissipation of the transistor= Vi (max) Vo (min) I L (max) Power dissipation of IC= Vi (max) Vo (min) I L (max) h fe(min) of (Q1) 4.5 Negative voltage regulator: An external PNP transistor Q1 is connected. Resistances can be varied from 1kΩ to 10kΩ R3 R4 Vref R1 R2 Vout R1 2 If magnitude of –Vi is less than 9V, connect Vcc+ and Vc to the positive supply such that Vcc+ to Vcc- is greater than 9V, for proper functioning of the IC o Vo 2k R2 12 11 9 Q1 6 2 R4 IC 723 3 5 4 R3 R1 7 13 100F 5.Protection circuits in Regulators Constant current limiting(Short circuit protection) Fold back current limiting Over voltage protection Thermal shutdown 5.1 Constant current limiting circuit o Regulated output Vo Under normal working condition and rated load current, the drop across the R4is insufficient to turn on the transistor Q3 and hence pass transistor Q1 continues to supply the rated load current Due to overloading or short circuit protection, the drop across R4 becomes sufficient to turn on the transistor Q3, the collector current of Q3 flows through R3 and decreases the base voltage of Q1, thus decreases the load voltage. This decrease in output voltage prevents the large load current VBE I SL R4 where ISL = load current when load terminals shorted I SL The minimum load resistance Rmin VBE R4 + Q1 Vreg I SL R4 IE1 + Vout o Q3 R3 IB3 R1 RL Vin Reference voltage Comparator circuit R2 __ o __ 5.2 Fold back current Limiting The fold back technique allows to provide the necessary load current at rated voltage but reducing the short circuit current Let VA be the voltage at the point A and the current flowing through R 4 is almost IL + Q1 VA R4 + o o R1 R1 Q3 I + - VBE3 R6 RL Vo Vin Reference voltage __ Comparator circuit VF(feed back voltage) R2 o o V A I L R4 Vo Neglecting the base current of IB, the current flowing through R5 and R6 is same as I VA VB 3 R6 R5 R6 ( I L R 4 Vo ) R6 R5 R6 R6 R5 R6 k ( I L R4 Vo ) Let k VB 3 VE 3 Vo VBE3 VB3 VE 3 k ( I L R4 Vo ) Vo Arranging for IL V (1 k )Vo I L BE3 kR4 Thus if the output terminal is shorted, the output voltage Vo reduces to zero.Hence V I sc I L BE3 kR4 (1 k )Vo The rated current can be written as I L I sc kR4 Where IL= Rated load current Isc=Short circuit current __ The rated load current IL is also called knee current(Iknee). Rated current is more than the short circuit current