Download Voltage regulators 723

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
Vnoninverting  Vref 
R1  R2
Since the series pass transistor is working as emitter follower. Hence
R2
Vnoninverting  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
100F
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