UNIT 4 SPECIAL FUNCTION ICS

555 Timer circuit – Functional block, characteristics & applications; 566-voltage

controlled oscillator circuit; 565-phase lock loop circuit functioning and

applications, Analog multiplier ICs.

Objectives:

• To know the functional blocks of 555 timer

• To study the characteristics and application of 555 timer

• To study the function of analog multiplier IC

History:

555 integrated circuit in the early 1970s that has ensured him a prominent position in the history of IC technology. If you are or have been a practicing electrical engineer anytime in the last 40 years, then most likely you have used the 555 timer/oscillator chip in your designs. Since the introduction of the 555 by Signetics in 1972, this integrated circuit has outsold all other IC types by a wide margin, with over 1 billion units sold worldwide in 2003 alone. The design has continued unchanged for over three decades and the range of applications has spanned such diverse areas as children’s toys and space craft electronics. This Oral History will discuss Hans’s pioneering 555 design and the legacy of this historic integrated. circuit.

he 555 Timer IC is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. The IC was designed by Hans R. Camenzind in 1970 and brought to market in 1971 by Signetics (later acquired by Philips). The original name was the SE555 (metal can)/NE555 (plastic DIP) and the part was described as "The IC Time Machine"[1]. It has been claimed that the 555 gets its name from the three 5 kΩ resistors used in typical early implementations,[2] but Hanz Camenzind has stated that the number was arbitrary[3]. The part is still in wide use, thanks to its ease of use, low price and good stability. As of 2003, it is estimated that 1 billion units are manufactured every year[3].

Depending on the manufacturer, the standard 555 package includes over 20 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-

in-line package (DIP-8).[4] Variants available include the 556 (a 14-pin DIP combining two 555s on one chip), and the 558 (a 16-pin DIP combining four slightly modified 555s with DIS & THR connected internally, and TR falling edge sensitive instead of level sensitive).

Ultra-low power versions of the 555 are also available, such as the 7555 and TLC555.[5] The 7555 requires slightly different wiring using fewer external components and less power.

Introduction:

555 timer IC

NE555 from Signetics in dual-in-line package Internal schematics

The 555 is an integrated circuit (chip) implementing a variety of timer and multivibrator applications. The IC was designed and invented by Hans R. Camenzind. It was designed in 1970 and introduced in 1971 by Signetics (later acquired by Philips). The original name was the SE555/NE555 and was called "The IC Time Machine".[1] The 555 gets its name from the three 5-kohm resistors used in typical early implementations.[2]

The 555 timer is one of the most popular and versatile integrated circuits ever produced. Depending on the manufacturer, it includes over 20 transistors, 2 diodes and 15 resistors on a silicon chip installed in an 8-pin mini dual-in-line package (DIP-8).[3]

The 556 is a 14-pin DIP that combines two 555s on a single chip.

The 558 is a 16-pin DIP that combines four slightly modified 555s on a single chip (DIS & THR are connected internally, TR is falling edge sensitive instead of level sensitive).

Also available are ultra-low power versions of the 555 such as the 7555 and TLC555. The 7555 requires slightly different wiring using fewer external components and less power.

Modes of operation

The 555 has three operating modes:

• Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, Frequency Divider,Capacitance Measurement, Pulse Width Modulation (PWM) etc

• Astable - Free Running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc.

• Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.

Specifications:

These specifications apply to the NE555. Other 555 timers can have better specifications depending on the grade (military, medical, etc).

Supply voltage (VCC) 4.5 to 15 V

Supply current (VCC = +5 V) 3 to 6 mA

Supply current (VCC = +15 V) 10 to 15 mA

Output current (maximum) 200 mA

Power dissipation 600 mW

Operating temperature 0 to 70 °C

The 555 has three operating modes:

• Monostable mode: in this mode, the 555 functions as a "one-shot". Applications include timers, missing pulse detection, bouncefree switches, touch switches, frequency divider, capacitance measurement, pulse-width modulation (PWM) etc

• Astable - free running mode: the 555 can operate as an oscillator. Uses include LED and lamp flashers, pulse generation, logic clocks, tone generation, security alarms, pulse position modulation, etc.

• Bistable mode or Schmitt trigger: the 555 can operate as a flip-flop, if the DIS pin is not connected and no capacitor is used. Uses include bouncefree latched switches, etc.

Schematic symbol

The connection of the pins is as follows:

Nr. Name Purpose

1 GND Ground, low level (0 V)

2 TRIG A short pulse high-to-low on the trigger starts the timer

3 OUT During a timing interval, the output stays at +VCC

4 RESET A timing interval can be interrupted by applying a reset pulse to low (0 V)

•

5 CTRL Control voltage allows access to the internal voltage divider (2/3 VCC)

6 THR The threshold at which the interval ends (it ends if the voltage at THR is at least 2/3 VCC)

7 DIS Connected to a capacitor whose discharge time will influence the timing interval

8 V+, VCC

The positive supply voltage which must be between 3 and 15 V

Monostable mode

Schematic of a 555 in monostable mode

The relationships of the trigger signal, the voltage on C and the pulse width in monostable mode

In the monostable mode, the 555 timer acts as a “one-shot” pulse generator. The pulse begins when the 555 timer receives a trigger signal. The width of the pulse is determined by the time constant of an RC network, which consists of a capacitor (C) and a resistor (R). The pulse ends when the charge on the C equals 2/3 of the supply voltage. The pulse width can be lengthened or shortened to the need of the specific application by adjusting the values of R and C.[6]

The pulse width of time t, which is the time it takes to charge C to 2/3 of the supply voltage, is given by

where t is in seconds, R is in ohms and C is in farads. See RC circuit for an explanation of this effect.

Astable mode

Standard 555 Astable Circuit

In astable mode, the '555 timer' puts out a continuous stream of rectangular pulses having a specified frequency. Resistor R1 is connected between VCC and the discharge pin (pin 7) and another (R2) is connected between the discharge pin (pin 7), and the trigger (pin 2) and threshold (pin 6) pins that share a common node. Hence the capacitor is charged through R1 and R2, and discharged only through R2, since pin 7 has low impedance to ground during output low intervals of the cycle, therefore discharging the capacitor.

In the astable mode, the frequency of the pulse stream depends on the values of R1, R2 and C:

[7]

The high time from each pulse is given by

and the low time from each pulse is given by

where R1 and R2 are the values of the resistors in ohms and C is the value of the capacitor in farads.

The 555 Timer

Following Forrest Mims in laying out the 555 Timer IC as a block diagram allows one to focus on the functions of the circuit.

Very popular for its versatility, the 555 Timer IC can operate in either astable or monostable multivibrator mode, resulting in a variety of applications.

This IC contains 23 transistors, 2 diodes and 16 resistors.

Supply voltage: 4.5 to 15 Supply current: 3 to 6 mA @5V 10 to 15mA@15V Output current: 200mA max Power dissipation: 600mW 8-pin mini DIP 556 is 14 pin dual 555.

Pinout diagram

R Nave

555 Astable Multivibrator

R Nave

555 Monostable Multivibrator

Index Electronics concepts IC Components Reference Mims, 555 Timer IC Circuits Tocci, Digital Systems Sec 5-24

Switched Capacitor

The switched capacitor filter allows for very sophisticated, accurate, and tuneable analog circuits to be manufactured without using resistors. This is useful for several reasons. Chief among these is that resistors are hard to build on integrated circuits (they take up a lot of room), and the circuits can be made to depend on ratios of capacitor values (which can be set accurately), and not absolute values (which vary between manufacturing runs). The Switched Capacitor Resistor To understand how switched capacitor circuits work, consider the circuit shown with a capacitor connected to two switches and two different voltages.

If S2 closes with S1 open, then S1 closes with switch S2 open, a charge (q is transferred from v2 to v1 with

. If this switching process is repeated N times in a time (t, the amount of charge transferred per unit time is given by

. Recognizing that the left hand side represents charge per unit time, or current, and the the number of cycles per unit time is the switching frequency (or clock frequency, fCLK) we can rewrite the equation as

. Rearranging we get

, which states that the switched capacitor is equivalent to a resistor. The value of this resistor decreases with increasing switching frequency or increasing capacitance, as either will increase the amount of charge transfered from v2 to v1 in a given time.

The Switched Capacitor Integrator Now consider the integrator circuit. You have shown (in a previous lab) that the input-output relationship for this circuit is given by (neglecting initial conditions):

We can also write this with the "s" notation (assuming a sinusoidal input, Aest, s=j )

If you replaced the input resistor with a switched capacitor resistor, you would get

Thus, you can change the equivalent ' of the circuit by changing the clock frequency. The value of ' can be set very precisely because it depends only on the ratio of C1 and C2, and not their absolute value.

The LMF100 Switched Capacitor Filter In this lab you will be using the MF100, or LMF100 (web page, datasheet, application note). This integrated circuit is a versatile circuit with four switched capacitor integrators, that can be connected as two second order filters or one fourth order filter. With this chip you can choose ' to either be 1/50 or 1/100 of the clock frequency (this is given by the ratio C1/C2 in the discussion above),. By

changing internal and external connections to the circuit you can obtain different filter types (lowpass, highpass, bandpass, notch (bandreject) or allpass).

2nd Order Filters

Filter Type Transfer Function

Low Pass

High Pass

Band Pass

Notch (Band Reject)

The pinout for the LMF100 is shown below (from the data sheet):

SWITCHED MODE POWER SUPPLY

A switched-mode power supply (also switching-mode power supply and SMPS) is an electronic power supply unit (PSU) that incorporates a switching

regulator. While a linear regulator maintains the desired output voltage by dissipating excess power in a pass power transistor, the switched-mode power supply switches a power transistor between saturation (full on) and cutoff (completely off) with a variable duty cycle whose average is the desired output voltage.

It switches at a much-higher frequency (tens to hundreds of kHz) than that of the AC line (mains), which means that the transformer that it feeds can be much smaller than one connected directly to the line/mains. Switching creates a rectangular waveform that typically goes to the primary of the transformer; typically several secondaries feed rectifiers, series inductors, and filter capacitors to provide various DC outputs with low ripple.

ADVANTAGE

The main advantage of this method is greater efficiency because the switching transistor dissipates little power in the saturated state and the off state compared to the semiconducting state (active region). Other advantages include smaller size and lighter weight (from the elimination of low frequency transformers which have a high weight) and lower heat generation due to higher efficiency.

DISADVANTAGE

Disadvantages include greater complexity, the generation of high amplitude, high frequency energy that the low-pass filter must block to avoid electromagnetic interference (EMI), and a ripple voltage at the switching frequency and the harmonic frequencies thereof.

Linear and Switching Voltage

Regulator Fundamentals

The most commonly used regulating modes are For linear regulators, the Standard, Low-Dropout, and Quasi Low-Dropout regulators will be covered (along with circuit examples). In the switching regulator section, the Buck, Buck-boost, Boost, and Flyback topologies will be detailed. Some examples will be given of products available for the design and implementation of switching converters. TUNED AMPLIFIERS

'Tuned' amplifiers are amplifiers involving a resonant circuit, and are intended for selective amplification within a narrow band of frequencies. Radio and TV amplifiers employ tuned amplifiers to select one broadcast channel from among the many concurrently induced in an antenna or transmitted through a cable. http://www-personal.engin.umd.umich.edu/~fmeral/ELECTRONICS%20II/01%25c3-Misc%20Notes/Tuned%20Circuit.pdf

ISOLATION AMPLIFIER

Isolation amplifiers are used as analog interfaces between systems with separated grounds.

Large-Signal Linearity

One of the main performance obstacles of isolation amplifiers is large-signal nonlinearity or distortion. The high-performance isolation amplifier applies linear optocouplers (LOCs) differentially to increase linearity over a large signal range. An LOC is like a generic optocoupler except that it has two matched, monolithic photodiodes, one for feedback to the LED driver and the other for the isolated output. LOCs are supplied by TI, Agilent, CP Clare, and Infineon (Siemens). The iso-amp uses a novel dual-feedback circuit topology to significantly reduce distortion.

The above distortion analyzer residual output was displayed on an oscilloscope for a single-LOC isoamp with a 4.8 V pk-pk, 100 kHz input, offset to -4 V. The prevalent second harmonic is caused by the LOC nonlinearity, which is an even function. The total harmonic distortion (THD) is 2.8 %, considerably worse than 0.16 % THD for the differential iso-amp.

VOLTAGE CONTROLLED OSCILLATOR

A voltage-controlled oscillator or VCO is an electronic oscillator designed to be controlled in oscillation frequency by a voltage input. The frequency of oscillation is varied by the applied DC voltage, while modulating signals may also be fed into the VCO to cause frequency modulation (FM) or phase modulation (PM); a VCO

with digital pulse output may similarly have its repetition rate (FSK, PSK) or pulse width modulated (PWM).

Control of frequency in VCOs

For high-frequency VCOs the voltage-controlled element is commonly a varicap diode connected as part of an LC tank circuit. For low-frequency VCOs, other methods of varying the frequency (such as altering the charging rate of a capacitor by means of a voltage controlled current source) are used. See Function generator.

Voltage-controlled crystal oscillators

A voltage-controlled crystal oscillator (VCXO) is used when the frequency of operation needs to be adjusted only finely. The frequency of a voltage-controlled crystal oscillator can be varied only by typically a few tens of parts per million (ppm), because the high Q factor of the crystals allows "pulling" over only a small range of frequencies.

There are two reasons for using a VCXO:

• To adjust the output frequency to match (or perhaps be some exact multiple of) an accurate external reference.

• Where the oscillator drives equipment that may generate radio-frequency interference, adding a varying voltage to its control input can disperse the interference spectrum to make it less objectionable. See spread-spectrum clock generation.

A temperature-compensated VCXO (TCVCXO) incorporates components that partially correct the dependence on temperature of the resonant frequency of the crystal. A smaller range of voltage control then suffices to stabilize the oscillator frequency in applications where temperature varies, such as heat buildup inside a transmitter.

VCO time-domain equations

• is called the oscillator gain. Its units are hertz per volt. • is the symbol for the time-domain waveform that is the VCO's

tunable frequency component. • is the symbol for the time-domain waveform that is the VCO's output

phase. • is the time-domain symbol of the control (input) voltage of the VCO;

it is sometimes also represented as

VCO freq-domain equations

Applications

VCOs are used in:

• electronic jamming equipment • function generators, • the production of electronic music, to generate variable tones, • phase-locked loops, • frequency synthesizers used in communication equipment.

Frequency synthesizer

• A frequency synthesizer is an electronic system for generating any of a range of frequencies from a single fixed timebase or oscillator.

• They are found in many modern devices, including radio receivers, mobile telephones, radiotelephones, walkie-talkies, CB radios, satellite receivers, GPS systems, etc.

• A frequency synthesizer can combine frequency multiplication, frequency division, and frequency mixing (the frequency mixing process generates sum and difference frequencies) operations to produce the desired output signal.

Types

Three types of synthesizer can be distinguished. The first and second type are routinely found as stand-alone architecture:

Direct Analog Synthesis (also called a mix-filter-divide architecture [1] as found in the 1960s HP 5100A) and by comparison the more modern

Direct Digital Synthesis (DDS) (Table-Look-Up). The third type are routinely used as communication system IC building-blocks: indirect digital (PLL) synthesizers including integer-N and fractional-N. [2]

Digiphase Synthesizer

It is in some ways similar to a DDS, but it has architectural differences. One of its big advantages is to allow a much finer resolution than other types of synthesizers with a given reference frequency. [3]

.

Coherent techniques generate frequencies derived from a single, stable master oscillator. In most applications, crystal oscillator are common, but other resonators and frequency sources can be used. Incoherent techniques derive frequencies from a set of several stable oscillators. [5] The vast majority of synthesizers in commercial applications use coherent techniques due to simplicity and low cost.

Synthesizers used in commercial radio receivers are largely based on phase-locked loops or PLLs. Many types of frequency synthesiser are available as integrated circuits, reducing cost and size. High end receivers and electronic test equipment use more sophisticated techniques, often in combination.

System analysis and design

In the System design of a frequency synthesizer, Mannassewitsch states that there are as many "best" design procedures as there are experienced synthesizer designers.

System analysis of a frequency synthesizer involves output frequency range (or frequency bandwidth or tuning range), frequency increments (or resolution or frequency tuning), frequency stability (or phase stability, compare spurious outputs), phase noise performance (e.g., spectral purity), switching time (compare settling time and rise time), and size, power consumption, and cost. [8][9] James A. Crawford says that these are mutually contradictive requirements[9]

Trial-and-error superseded by calculation and control theory

The trial and error method was once the work-horse for designers of frequency synthesizers [4].

This began to change with the works of Floyd M. Gardner (his 1966 Phaselock

techniques) and Venceslav F. Kroupa (his 1973 Frequency Synthesis)[4]. Mannassewitsch calls this the Brute-force approach.[10] Techniques and formulae have been provided by Dean Banerjee [11].

Gearbox approach

Surpisingly sophisticated mathematical techniques analogous to mechanical gear ratio relationships can be employed in frequency synthesis when the frequency synthesis factor is composed of multiplicative integers in the numerator and denomenator. [4] This method allows for effective planning of distribution and suppression of spectral spurs.

Modulo-N approach

Principle of PLL synthesizers

See main article: Phase-locked loop

A phase locked loop does for frequency what the Automatic Gain Control does for voltage. It compares the frequencies of two signals and produces an error signal which is proportional to the difference between the input frequencies. The error signal is then low pass filtered and used to drive a voltage-controlled oscillator (VCO) which creates an output frequency. The output frequency is fed through a frequency divider back to the input of the system, producing a negative feedback loop. If the output frequency drifts, the error signal will increase, driving the frequency in the opposite direction so as to reduce the error. Thus the output is locked to the frequency at the other input. This input is called the reference and is derived from a crystal oscillator, which is very stable in frequency. The block diagram below shows the basic elements and arrangement of a PLL based frequency synthesizer.

The key to the ability of a frequency synthesizer to generate multiple frequencies is the divider placed between the output and the feedback input. This is usually in the form of a digital counter, with the output signal acting as a clock signal. The counter is preset to some initial count value, and counts down at each cycle of the clock signal. When it reaches zero, the counter output changes state and the count value is reloaded. This circuit is straightforward to implement using flip-flops, and because it is digital in nature, is very easy to interface to other digital components or a microprocessor. This allows the frequency output by the synthesizer to be easily controlled by a digital system.

Example

Suppose the reference signal is 100 kHz, and the divider can be preset to any value between 1 and 100. The error signal produced by the comparator will only be zero when the output of the divider is also 100 kHz. For this to be the case, the VCO

must run at a frequency which is 100 kHz x the divider count value. Thus it will produce an output of 100 kHz for a count of 1, 200 kHz for a count of 2, 1 MHz for a count of 10

and so on. Note that only whole multiples of the reference frequency can be obtained with the simplest integer N dividers. Fractional N dividers are readily available .

A Flexible Compander IC for Wireless Microphone Applications

A new IC for implementing companding noise reduction in professional wireless microphone applications is described. Unlike existing devices designed primarily for the cordless telephone market, the new design allows straightforward, repeatable implementation of companding schemes incorporating ratios greater or less than 2 to 1, level-dependent ratios, limiters, and noise gates. The overall device architecture and design and performance of individual functional blocks is described. Several examples of encoder and decoder implementations are presented. The design techniques used to maintain wide dynamic range while minimizing power consumption are described.

The LM565 is a general purpose Phase-Locked Loop IC containing a stable, highly

linear voltage controlled oscillator (VCO) for low distortion FM demodulation,

and a double balanced phase detector with good carrier suppression. The VCO

frequency is set with an external resistor and capacitor, and a tuning range of 10:1

can be obtained with the same capacitor. The characteristics of the closed loop

system--bandwidth, response speed, capture and pull in range--may be adjusted

Phase

Comparator

Low Pass

Filter

Amplifier

VCO

÷ N

Network

Input

Signal

PLL

Output

Signal

over a wide range with an external resistor and capacitor. The loop may be broken

between the VCO and the phase detector for insertion of a digital frequency

divider to obtain frequency multiplication.A Phase-Locked Loop has basically three

states:

1. Free-running. 2. Capture. 3. Phase-lock

Before the input is applied the PLL is in the free running mode. Once the input

frequency is applied, the VCO frequency continues to change until it equals the

input frequency, and the PLL is then in the phase locked state. When phase locked

the loop tracks any change in the input frequency. The range over which the loop

system will follow changes in the input frequency is called the lock range. On the

other hand, the frequency range in which the loop acquires phase-lock is the

capture range, and is never greater than the lock range.A low-pass filter is used to

control the dynamic characteristics of the phase-locked loop. If the difference

between the input and VCO frequencies is significantly large, the resultant signal is

out of the capture range of the loop. Once the loop is phase-locked, the filter only

limits the speed of the loop's ability to track changes in the input frequency.

2 marks questions

1. Mention some areas where PLL is widely used:

2. Mention some applications of 555 timer:

3. List the applications of 555 timer in monostable mode of operation:

4. List the applications of 555 timer in Astable mode of operation:

5. List the basic building blocks of PLL:

6. What are the three stages through which PLL operates?

7. Define lock-in range of a PLL:

8. Define capture range of PLL:

9. Define Pull-in time.

10. Give the classification of phase detector:

11. What are the problems associated with switch type phase detector?

12. What is a voltage controlled oscillator?

13. On what parameters does the free running frequency of VCO depend

on?

14. Give the expression for the VCO free running frequency.

15. Define Voltage to Frequency conversion factor.

16. What is the purpose of having a low pass filter in PLL?

17. Discuss the effect of having large capture range.

18. Mention some typical applications of PLL:

19. What is a compander IC? Give some examples.

20. What are the merits of companding?.

21. List the applications of OTA:

16 marks questions

1. Briefly explain the block diagram of PLL and derive the expression

for Lock range and capture range.

2. With a neat functional diagram, explain the operation of VCO. Also

derive an expression for fo.

3. Analyze the analog multiplier IC with a neat circuit diagram. Discuss

its applications.

4. discuss the applications of PLL:

5. What is 555 timer? What are the features of 555 timer? Explain the

monostable mode in detail?

6. Explain the Astable mode of operation using 555 timer.

7. discuss the applications of 555 monostable timer.