introduction - rmd engineering collegeintroduction integrated ics with improved capabilities are...
TRANSCRIPT
INTRODUCTION
Integrated ICs with improved capabilities are appearing in an ever increasing
numbers.
Often ,the use of specialized IC produces a simpler and more accurate
circuits.
Examples: MF5(National ‘s switched capacitor filter), SE/NE
565(Signetics’Phase locked loop-PLL) , timer SE/NE 555 etc
Applications of specialized ICs:
*universal filters
*timers
*Phase locked loop(PLL)
*power amplifiers
*voltage regulators
*switching regulators
*voltage references
Topics to be dealt
78XX series
79XX series
317 variable regulators
1723 switching regulators
IC Voltage Regulators4
Voltage regulator: is a circuit that supplies constant voltage regardless of
changes in the load current.
Advantages of IC voltage regulator:
inexpensive,versatile,provides current /voltage boosting, internal short
circuit current limiting, thermal shutdown, floating operation for high voltage
applications.
Classification of IC voltage regulators:
There are basically two kinds of IC voltage regulators:
Multi-pin type, e.g. LM723C
3-pin type, e.g. 78/79XX
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
Types of IC voltage regulators:
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)
Switching regulators: motorola ‘s MC1723
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.
Basic power supply
• 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
Need for regulation
• 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
Performance parameters of voltage regulators
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.
FIXED VOLTAGE REGULATORa)positive voltage regulator
The 78XX voltage regulators
Fig shows the connection diagram of 78XX series
Proper operation requires a common ground between the input and output
voltages.
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.
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.
Ci Co
Basic building block of +ve regulators
7805 as current source
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
IL = (VR / R )+ IQ
when Iq is quiescent current in amperes (4.3 m A typically for the 7805 IC)
• In figure, VR = V23= 5 V and R = 10 ohms
So IL = 5/10 = 0.5A
• The output voltage with respect to
ground is
VOUT = VR + VL
• The load resistance, RL = 10 Ohms,
therefore VL = 5 V Thus
Vout = VR+ VL= 5 + 5 = 10 V
• Minimum input voltage required,
Vin = Vout + dropout voltage = 10 + 2=12V
78XX basic features
IC No Voltage
7805 5V
7806 6V
7808 8V
7809 9V
7810 10V
7812 12V
7815 15V
7818 18V
7824 24V
• Features
• 3 terminal positive voltage regulator with seven voltage options
• High Output Current - typically 1.5A
• Short circuit current limit - 750mA at 5v
• Internal thermal overload protection
• Low quiescent current - 6mA
• Max input voltage = 35v
• Minimum Input Voltage = Vout + 2.5
b)Negative voltage regulators
• Negative voltage regulator IC's are available
in 79XX series.
• These IC's are similar to the 78 series, but
operating on negative voltage, and providing a
regulated negative output voltage.
• The capacitors connected at the input
and output sides are used to provide
additional filter circuits of regulator circuits.
Features of 79XX
IC No Voltage
7902 -2V
7905 -5V
7905.5 -5.2V
7906 -6.2V
7908 -8V
7912 -12V
7915 -15V
7918 -18V
7924 -24V
• Same as that of 78XX series
except that 79XX series are
negative regulators
• They are available in same seven
voltage options with two extra
voltage options,-2V and -5.2V
• As shown in the figure
ADJUSTABLE VOLTAGE REGULATOR
• Adjustable voltage regulators are those who voltage can be varied and
utilized.
• Advantages of adjustable voltage regulators:
• * improved system performance
• * improved overload protection
• *improved system reliability
• Example: LM317
ADJUSTABLE VOLTAGE REGULATOR
a)positive adjustable regulators-LM317
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.
The three terminals are Vin,Vout
and ADJUSTMENT(ADJ).
Circuit of LM317
Resistors Rx and R2 set the output to any desired voltage over the adjustment range (1.2 to 57 V)
When configured as shown in figure LM317 develops a nominal voltage of 1.25V referred to as reference voltage Vref between output and adjustment terminal.
This reference voltage is
impressed across resistor R1,since
voltage is constant current I is also constant
Since R1 sets current I its called current set or program resistor
Referring to figure,the output voltage Vo is
Vo = R1 + R2 (I1 +Iadj)………….(1)
Where I1= Vref /R1……….(2)
R1 =current set resistor
R2 =output set resistor
Iadj= adjustment pin current
Substituting (2) in (1) we get :Vout = VREF [1+ R2/R1] + IADJ R2
where VREF = 1.25 V, the reference voltage between the output and adjustment
terminals
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
Vout = 1.25 [1+ R2/R1]
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 nearthe load.
The load regulation is 0.1 percent while the line regulation is 0.01 percent per volt.
LM317 with capacitors,protection diodes
No capacitors are needed unless
LM317 is situated far from the
power supply filter capacitors in
which case a bypass capacitor Ci is
used.
Output capacitor Co can be added
to improve the transient response.
When external capacitors are used
protection diodes are also used to
prevent capacitors from
discharging through low current
points into the regulator.
b)Adjustable negative voltage regulators-LM337
• 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.
Theory of switching regulators
• A basic switching regulator consists of four major components:
• Voltage source Vin
• Switch S1
• Pulse generator Vpulse
• Filter F1
• Voltage source: maybe dc supply,battery,unregulated or regulated supply
*Vin must supply required output power
*it must be large enough to supply sufficient dynamic range
*it may be required to store energy for a specified period of time during power failures.
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.
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.
*duty cycle:is the ratio of on time ton to the period T of the pulse waveform.
Duty cycle = ton = = ton = ton f
t0n + toff T
• 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.
• Vo is expressed as ;
• Vo = ton *Vin
• T
• 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
Switching regulator
• In switching regulator,the transistor acts as a switch.
• When the transistor is off(switch is open) , no current flows,therefore no power dissapation.
• When the transistor is on,(switch is closed), high current flows but Vce
becomes low and therefore power dissapation is less.
• Basic circuit of switching regulator
• Bridge rectifier-switch-transformer-rectifier
• Bridge rectifier converts ac input signal to unregulated dc
• Switch : closed implies currrent is allowed to flow
• open implies no current is allowed to flow
• Transformer: the output of the switching network is coupled to the rectifier through the transformer
• Rectifier: provides final rectification and smoothing of the dc output
• Features
• Delivers load current of about
150mA without an external pass
transistor.
• Output voltage range 3Vdc to 37
Vdc
• .01%line regulation
• .03%load regulation
MC1723
• The diagram shows motorola’s
MC1723
• It is a general purpose regulator
and can be used as fixed ,variable
and switching regulator.
• The regulator requires an external
transistor and a 1mh choke.
• To minimize its power dissipation
during switching, external
transistor used must be switching
power transistor.
• The 1 mH choke smooths out
current pulses to the load
• Capacitor c holds out output
voltage at constant dc level.