Download Linear Voltage regulators

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)
0.1F
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  1.251 
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=0.1F
IN4001
1.25V
R1=240
C3=10F
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=1F