CHAPTER 1

INTRODUCTION

1.1 GENERAL

Battery life is an important issue in all portable electronic devices. The

matter becomes even more crucial when the necessary portable devices are medical

implants.An implant is a medical device manufactured to replace a missing

biological structure, support a damaged biological structure, or enhance an existing

biological structure such as artificial pacemaker and cochlear implants. In these

devices, life itself might become dependent on the battery life. Naturally, as with

all battery-powered devices, the battery of an implant must be replaced after a

certain period of time. Afrequent change of an implant’s battery is not desired

because it requires surgical procedure. This has driven researchers to develop

powering solutions for implants. Whether the implant is powered by a battery,

inductive link, piezoelectric source, or a combination of these sources, it is

important to have circuits with ultra-low-power consumption that wouldefficiently

use these energy resources. Reducing the power dissipation in these circuits also

helps to reduce the risk of damaging surrounding tissues due to dissipated heat.

One method of reducing power consumption in complementary metal–oxide

semiconductor (CMOS) circuits is the dynamic voltage scaling (DVS) technique.

1

Theself-timed systems referred to as asynchronous systems can lead to further

reduction in power consumption .The DVS technique explored in this paper uses

Switched Capacitors(SC) to obtain a dc-dc converter. This type of converter is

suitable for implants because it is efficient and can be integrated. In addition, since

this type of dc–dc converter does not have any inductor, it is less affected by

electromagnetic interference, and can be used in implants that utilize inductive

links.To reduce switching losses at light loads, the proposed asynchronous dc–dc

converter is able to select the number of switches to operate while it keeps

additional switches OFF.

1.2 LITERATURE REVIEW

This converter is a SC DC-DC converter with variable conversion ratio. It is

designed to provide three different conversion ratios 1, 1/2, and 2/3. The input

battery voltage can be varied from 2.8V to 5V for an output of 1.8V, while the

maximum load current for this converter is 100mA and the conversion efficiency is

in the range from 85% to 65%. Finally, the output ripple was measured to be less

than 10mV. [1]

It uses three integrated 400pF capacitors; this design was able to step-down

the input voltage from 5V to 1V. A pulse width modulation (PWM) control

scheme with 25MHz switching frequency was adopted to regulate the output

voltage. It achieves 62% efficiency when the switching losses are included, while

the theoretical maximum efficiency was 80%. Neither the maximum load power

nor maximum load current was provided. [2]

This design is a fully integrated SC DC-DC converter targeting systems that

can apply the DVS technique to reduce power consumption. This work describes a

SC network, consisting of four capacitors, that can achieve five different

2

conversion ratios (1, 3/4, 1/2, 2/3 and 1/3). With the use of PFM control and an

automatic frequency scaling (AFS) block, this converter achieved conversion

efficiency in the range of 80-50% for load powers in the range of 5µW to 1µW. the

converter can regulate output voltages between 1.1 V to 0.3 V. [3]

This design is built on two time-interleaved SC DC-DC converters to

produce lower output voltage ripple and faster load transient. The configuration of

this converter allows it to choose between conversion ratio of 1, 1/2, 2/3 and 1/3.

The converter takes input voltages in the range of 2.1 V to 3.3 V and output

voltages in the range of 0.9 V to 1.8 V, with a maximum efficiency of 76%. [4]

The authors here design another SC DC-DC converter with adjustable

conversion ratios to work with (1, 1/2ad 2/3). With a PFM controller operating

with a base frequency of 1MHz, the converter steps down input voltages in the

range of 5-15V to an output voltage of 2V. The whole design is reported to have

efficiency in the range of 28% to 42%. [5]

1.3ORGANISATION OF THE THESIS

The thesis consists of six chapters including introduction as the first chapter,

which gives introduction to implantable devices and about the existing system.

Chapter 1 describes the papers referred and technical information inferred from the

literature surveys.

Chapter 2 deals with the block diagram of our project and general description of

switched capacitor dc-dc conversion network.

Chapter 3 deals with the simulation of switched capacitor dc-dc conversion

network.

Chapter 4 describes about the overall hardware description of our project.

Chapter 5 deals with conclusion and future scope.

3

CHAPTER 2

SWITCHED CAPACITOR DC-DC CONVERTER

2.1 INTRODUCTION

A switched capacitor is an electronic circuit element used for discrete

time signal processing. It works by moving charges into and out of capacitors

when switches are opened and closed. Usually, non-overlapping signals are used to

control the switches, so that not all switches are closed

simultaneously. Filters implemented with these elements are termed 'switched-

capacitor filters'. Switched capacitor filters depend only on the ratios between

capacitances. This makes them much more suitable for use within integrated

circuits, where accurately specified resistors and capacitors are not economical to

construct.

2.2 BLOCK DIAGRAM OF ASYNCHRONOUS STEP DOWN DC-

DC CONVERTER

A12 V DC supply is given to the switched capacitor dc-dc converter which

steps down the voltage in the range between 0.9 V to 1.5 V. A 5V dc supply is

given to the controller, which produces pulse signals which are given to the driver

4

unit. The driver unit is provided with a 12 V dc supply which amplifies the pulse

signals inorder to drive the mosfet switches.

Fig.2.1Block diagram of asynchronous step down dc-dc

converter for implantable devices

The block diagram shown has controller unit driver unit and a switched

capacitor dc-dc conversion network.

2.2.1 SWITCHED CAPACITOR DC-DC CONVERTER

The simplest switched capacitor (SC) circuit is the switched capacitor

resistor, made of one capacitor C and two switches S1 and S2 which connect

thecapacitor with a given frequency alternately to the input and output of the SC.

5

DC Supply[12V]

Switched capacitor DC-

DC conversion network

Load

Driver Unit

Controller Unit

12V DC Supply

5V DC Supply

Each switching cycle transfers a charge   from the input to the output at the

switching frequency. Recall that the charge q on a capacitor C with a

voltage V between the plates is given by:

q=CV (2.1)

where V is the voltage across the capacitor. Therefore, when S1 is closed while 

S2 is open, the charge stored in the capacitor CS is:

qIN=CsVIN (2.2)

When S2 is closed, some of that charge is transferred out of the capacitor, after

which the charge that remains in capacitor CS is:

qOUT=CsVout (2.3)

Thus, the charge moved out of the capacitor to the output is:

q=qIN-qOUT=Cs (VIN-VOUT) (2.4)

Because this charge q is transferred at a rate f, the rate of transfer of charge per unit

time is:

I=qf (2.5)

Note that we use I, the symbol for electric current, for this quantity. This is to

demonstrate that a continuous transfer of charge from one node to another is

equivalent to a current. Substituting for q in the above, we have:

I=Cs(VIN-VOUT)f (2.6)

Let V be the voltage across the SC from input to output. So:

V=VOUT-VIN (2.7)

So the equivalent resistance R (i.e., the voltage–current relationship) is:

R=V/I =1/Csf (2.8)

6

Thus, the SC behaves like a lossless resistor whose value depends on

capacitance CS and switching frequency f.The SC resistor is used as a replacement

for simple resistors in integrated circuits because it is easier to fabricate reliably

with a wide range of values. It also has the benefit that its value can be adjusted by

changing the switching frequency.

2.3 CIRCUIT DIAGRAM OF SWITCHED CAPACITOR DC-DCCONVERTER

Fig.2.2 General Circuit Diagram of SC DC-DC converter

The Fig 2.2 illustrates the circuit diagram of switched capacitor dc-dc

conversion network. In this network we are using five mosfet switches .Switch

7

Soacts as direct switch. It consists of two converter sections with two switches each.

Switches S1 and S1are complementary switches, similarly switches S2 and S2of

converter 2 are complementary. There are three modes of operation.

2.4 MODES OF OPERATION

2.4.1 MODE 1

In this mode switches So ,S1 ,S2 will be in ON state and switches S1,S2 will

be kept OFF. The output is charged directly from the input via the direct switch,

this makes the charge up time very short. The two converters will be

simultaneously charged via switches S1and S2

Fig.2.3Circuit Diagram of MODE1 operation

8

2.4.2 MODE 2

In this mode switches S1 and S2 are kept ON and the switches S0,S1 ,S2 are

switched OFF. The output is charged from the converter 1.The converter 2 will

keep on charging through switchS2 .

Fig.2.4Circuit Diagram of MODE 2 operation

2.4.3 MODE 3

In this mode, only switch S2 will be in ON state and all other switches are

kept OFF. The output will be charged by converter 2.

9

Fig.2.5Circuit Diagram of MODE 3 operation

2.5 CONTROLLER UNIT

The controller unit is used to provide pulse signals to the switched capacitor

dc-dc conversion network. A 5 V dc supply is provided to the controller unit by

means of a power supply unit. The controller used is PIC 16F877A. It is a forty pin

IC.

2.6 DRIVER UNIT

The driver unit is used to amplify the signals from the microcontroller to

drive the mosfet switches. It is provided with a 12 V dc supply. It uses TTL logic

in amplification of the signals. The driver unit consists of optocoupler which is

used in isolating the controller and the driver unit.

10

2.7 CONCLUSION

The following details can be inferred from this chapter:As only five switches

are used which is much lesser when compared with the existing system, the power

losses are reduced. Also the electromagnetic interference can be reduced as no

inductors are used. Thus switched capacitor dc-dc conversion structure is explained

in detail in this chapter.

11

CHAPTER 3

SIMULATION USING MATLAB

3.1 INTRODUCTION

Simulations are abstractions of reality. Often they deliberately emphasize

one part of reality at the expense of other parts. Sometimes this is necessary due to

computer power limitations. Sometimes it's done to focus your attention on an

important aspect of the simulation.

Simulation has become a very powerful tool on the industry application as

well as in academics, nowadays. It is now essential for an electrical engineer to

understand the concept of simulation and learn its use in various applications.

Simulation is one of the best ways to study the system or circuit behavior without

damaging it .The tools for doing the simulation in various fields are available in the

market for engineering professionals. Many industries are spending a considerable

amount of time and money in doing simulation before manufacturing their product.

In most of the research and development (R&D) work, the simulation plays a very

important role. Without simulation it is quiet impossible to proceed further. It

should be noted that in power electronics, computer simulation and a proof of

concept hardware prototype in the laboratory are complimentary to each other.

However computer simulation must not be considered as a substitute for hardware

12

prototype. The objective of this chapter is to describe simulation of switched

capacitor dc-dc converter using MATLAB tool with R load.

3.2 ABOUT MATLAB

MATLAB is a high-level language and interactive environment that enables

you to perform computationally intensive tasks faster than with traditional

programming languages such as C, C++, and Fortran. MATLAB is a high-

performance language for technical computing. It integrates computation,

visualization, and programming in an easy-to-use environment where problems

and solutions are expressed in familiar mathematical notation. Typical uses

includes-Math and computation, Data acquisition, Algorithm development and

Modeling, simulation, and prototyping.

Simulations were performed by using MATLAB-Simulink to verify the

proposed BDC full bridge converter can practically be implemented to improve the

efficiency of converter.

Simulink is an environment for multidomain simulation and Model-Based

Design for dynamic and embedded systems. It provides an interactive graphical

environment and a customizable set of block libraries that let you design, simulate,

implement, and test a variety of time-varying systems, including communications,

controls, signal processing, video processing, and image processing.

3.3 SIMULATION OF SWITCHED CAPACITOR DC-DC CONVERTER NETWORK

In simulation of switched capacitor dc-dc conversion network, we go for

five mosfet switches. Switch So which is made to act as direct switch is operated at

13

a frequency of 50 Hz, and zero delay is provided. Switch S1 and S1 are operated at

a high frequency of 1.9MHz and a small phase delay is provided. As the switches

S1 and S1 are complementary, NOT gate is used to provide pulses to switch S1

.Switches S2 and S2are operated at the same frequency as that of S1 andS1 , but the

delay provided to the switches is much greater than that provided to S1 andS1 .

14

Fig.3.1Simulation of SC dc-dc conversion network

Switch S0 is a direct switch and its switching pattern is shown below.The

pulse pattern shows that there is zero delay provided for this switch. The pulses are

given at an interval of about 0.02 seconds and its frequency is about 50 Hz.

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.040

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

TIME(SEC)

Am

plitu

de

Pulse to switch So

Fig.3.2 Pulse to switch S0

15

Switches S1 and S1are high frequency switches and their switching pattern is

given below.

The pulse pattern shows that there is a small delay provided to this switch

and it is about 0.175 µs.

3.054 3.055 3.056 3.057 3.058 3.059 3.06

x 10-3

0.5472

0.5472

0.5472

0.5472

0.5472

0.5472

PULSE TO SWITCH S1

TIME(SEC)

AM

PLI

TUD

E

Fig.3.3 Pulse to switch S1

16

Switches S2 and S2are high frequency switches and their switching pattern

is given below.

The pulse pattern shows that there is a small delay provided to this switch

and it is about 0.263 µs.

3.896 3.8965 3.897 3.8975 3.898 3.8985 3.899 3.8995 3.9 3.9005 3.901

x 10-3

0.5162

0.5162

0.5162

0.5162

0.5162

0.5162

0.5162

0.5162

0.5162

TIME(sec)

AM

PLI

TUD

E

Pulse to switch S2

Fig.3.4 Pulse to switch S2

17

The output voltage of the converter is shown below and it is about 1.28 V for an input voltage of 12 V.A small amount of ripple can be seen in the result.

0 0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.040

2

4

6

8

10

12

14

TIME PERIOD(SEC)

VO

LTA

GE

(V)

Fig.3.5 Output voltage of converter network

3.4 CONCLUSION

The Simulink models for switched capacitor dc-dc converter network has

been simulated to produce the desired output voltage for given input voltage using

MATLAB. The simulation result shows an efficiency of 84%.

18

CHAPTER 4

HARDWARE DESCRIPTION

4.1 INTRODUCTION

This chapter talks in detail about the overall circuit diagram and various

other hardware components used in low power asynchronous step down dc-dc

converter.

4.2 OVERALL CIRCUIT DIAGRAM

Fig 4.1 illustrates the overall circuit diagram of asynchronous step down dc-

dc converter which shows the power supply unit, controller unit, driver unit and the

switched capacitor dc-dc conversion unit.

Initially 12 V ac supply is provided to the controller unit and driver unit via

230/12 V step down transformer. The microcontroller produces pulse signals

which are given to the driver circuit. The driver circuit uses TTL logic to amplify

the drive signals and is used to drive the mosfet switches. An optocoupler is used

19

to isolate the driver and controller unit. The microcontroller used here is PIC

16F877A.

20

S 2

500mA

C 1

1 n

500mA

R 4

1 0 0

R 2

1 0 0

500mA

J 1

12V

AC1

2

Q 2

S 0

M C T2 E

U 2

C 30 . 1 u F

U 1

O P -0 7 C / 3 0 1 / T1

R 5

1 0 0

230/12V

S

R 7

1 K

U 1

O P -0 7 C / 3 0 1 / T1

R 31 K

R 31 K

- +

D 51

2

3

4

R 6

1 K

230/12V

R 2

1 0 0

C 14 7 0 u F

D 1

D 1 N 1 9 0

C 1

1 n

R 7

1 K

Y 1

C R Y S TA L

D A TA

U 1

O P -0 7 C / 3 0 1 / T1

R 4

1 0 0

M C T2 E

U 2

Q 3

G

S

Q 3

R 5

RESI

STOR

VAR

13

2 Q 1

B D X3 7

R 7

1 K

C L

V o u t

-S 1

F R O M M IC R OC O N T R O LLER

D 1

D 1 N 1 9 0

U 1L 7 8 0 5 / TO 3

1

3

2V I N

GND

V O U T

R 2

1 0 0

M C T2 E

U 2

R 7

1 K

C 1

1 n

S W 1

S W P U S H B U TTO N

F R O M M IC R OC O N T R O LLER

R 5

1 0 0

R 2

1 0 0

230/12V

R 1

1 K

C L K

Q 3

S

M C T2 E

U 2

R 1

R E S I S TO R

R 31 K

R 1

1 K

R B 3

V P P

F R O M M IC R OC O N T R O LLER

GQ 3

D 1

D 1 N 1 9 0

U 1

O P -0 7 C / 3 0 1 / T1

500mA

G

R 4

1 0 0

S

D 1

D 1 N 1 9 0

R 5

1 0 0

C 1

1 nS 1

VDD

C 6C A P

230/12V

Q 2

R 6

1 K

U 1

O P -0 7 C / 3 0 1 / T1

Q 2

F R O M M IC R OC O N T R O LLER

R 2220o

hm

Q 2

R B 0

Q 1

B D X3 7

M C T2 E

U 2 Q 1

B D X3 7

R 31 K

Q 1

B D X3 7

C 5C A P

R 1

1 K

500mA

Q 1

B D X3 7

R 5

1 0 0

Q 3

R 3

R E S I S TO R

V in

12V

DC-

+

D 1

D 1 N 1 9 0

D 7

L E D

R L

Q 2

S

R 6

1 K

C 21 0 0 u F

U 6

P I C 1 6 F 8 7 7 a

2 4

2 1

2 3

12

1 31 4

1 51 61 71 8

1 9

456

91 0

11

3 43 3

32 31

3 02 92 82 7

32 3 9

3 83 73 63 5

78

1

2 52 6

2 0

2 2

4 0

R C 5 / S D O

R D 2 / P S P 2

R C 4 / S D I / S D A

VSS

O S C 1 / C L K IO S C 2 / C L K O

R C O / T1 0 S 0 / T1 C K IR C 1 / T1 O S I / C C P 2R C 2 / C C P 1R C 3 / S C K / S C L

R D 0 / P S P 0

R A 2 / A N 2 / V R E F -R A 3 / A N 3 / V R E F +R A 4 / TO C K I

R B 1 / A N 6 / W RR B 2 / A N 7 / C S

VDD

R B 1R B 0 / I N T

VDD

VSS

R D 7 / P S P 7R D 6 / P S P 6R D 5 / P S P 5R D 4 / P S P 4

R A 1 / A N 1R A 0 / A N 0 R B 6 / P G C

R B 5R B 4

R B 3 / P G MR B 2

R A 5 / A N 4 / S SR B 0 / A N 5 / R D

M C L R / V P P

R C 6 / TX/ C KR C 7 / R X/ D T

R D 1 / P S P 1

R D 3 / P S P 3

R B 7 / P G D

R 1

1 K

F R O M M IC R OC O N T R O LLER

R 1

1 K

R 31 K

C 4

C A P

R 4

1 0 0

R 2

1 0 0

G

R 6

1 K

G

230/12V

R 6

1 K

C 1

1 n

R 7

1 K

-S 2

R 5

1 0 0

R 4

1 0 0

Fig 4.1 overall circuit diagram of asynchronous step down dc-dc converter

21

4.3 CONTROLLER UNIT

S W 1

S W P U S H B U TTO N

D A TA

C 21 0 0 u F

- +

D 51

2

3

4

VDD

C 5C A P

D 7

L E D

C 14 7 0 u F

U 1L 7 8 0 5 / TO 3

1

3

2V I N

GN

D

V O U T

R 1

R E S I S TO R

N

U 6

P I C 1 6 F 8 7 7 a

2 4

2 1

2 3

12

1 31 4

1 51 61 71 8

1 9

456

91 0

11

3 43 3

32 31

3 02 92 82 7

32 3 9

3 83 73 63 5

78

1

2 52 6

2 0

2 2

4 0

R C 5 / S D O

R D 2 / P S P 2

R C 4 / S D I / S D A

VS

S

O S C 1 / C L K IO S C 2 / C L K O

R C O / T1 0 S 0 / T1 C K IR C 1 / T1 O S I / C C P 2R C 2 / C C P 1R C 3 / S C K / S C L

R D 0 / P S P 0

R A 2 / A N 2 / V R E F -R A 3 / A N 3 / V R E F +R A 4 / TO C K I

R B 1 / A N 6 / W RR B 2 / A N 7 / C S

VD

D

R B 1R B 0 / I N T

VD

D

VS

S

R D 7 / P S P 7R D 6 / P S P 6R D 5 / P S P 5R D 4 / P S P 4

R A 1 / A N 1R A 0 / A N 0 R B 6 / P G C

R B 5R B 4

R B 3 / P G MR B 2

R A 5 / A N 4 / S SR B 0 / A N 5 / R D

M C L R / V P P

R C 6 / TX/ C KR C 7 / R X/ D T

R D 1 / P S P 1

R D 3 / P S P 3

R B 7 / P G D

C 30 . 1 u F

R B 0

C 4

C A P

R 5

RE

SIS

TO

R V

AR

13

2

C L K

2 3 0 / 1 2 V

TR A N S F O R M E R

1 5

4 8230 V

V P P

P

R 3

R E S I S TO R

Y 1

C R Y S TA L

R 2220o

hm

C 6C A P

R B 3

Fig.4.2 PIC microcontroller 16F877A

22

4.3.1 POWER CIRCUIT FOR MICROCONTROLLER

Power supply is a reference to a source of electrical power. A device or

system that supplies electrical or other types of energy to an output load or

group of loads is called a power supply unit or PSU. The term is most

commonly applied to electrical energy supplies, less often to mechanical ones,

and rarely to others

The operation of power supply circuits built using filters, rectifiers, and

then voltage regulators. Starting with an AC voltage, a steady DC voltage is

obtained by rectifying the AC voltage, then filtering to a DC level, and finally,

regulating to obtain a desired fixed DC voltage. The regulation is usually

obtained from an IC voltage regulator Unit, which takes a DC voltage and

provides a somewhat lower DC voltage, which remains the same even if the

input DC voltage varies, or the output Load connected to the DC voltage

changes.

4.3.2 VOLTAGE REGULATOR

A voltage regulator is an electrical regulator designed to automatically

maintain a constant voltage level. A voltage regulator may be a simple "feed-

forward" design or may include negative feedback control loops. It may use an

electromechanical mechanism, or electronic components. Depending on the design,

it may be used to regulate one or more AC or DC voltages.

The 78xx (sometimes LM78xx) is a family of self-contained fixed linear

voltage regulator integrated circuits. The 78xx family is commonly used in

electronic circuits requiring a regulated power supply due to their ease-of-use and

low cost. For ICs within the family, the xx is replaced with two digits, indicating

23

the output voltage (for example, the 7805 has a 5 volt output, while the 7812

produces 12 volts).

4.3.3 BRIDGE RECTIFIER

A diode bridge is an arrangement of four (or more) diodes in a bridge

circuit configuration that provides the same polarity of output for either polarity of

input. When used in its most common application, for conversion of an  alternating

current (AC) input into direct current a (DC) output, it is known as a bridge

rectifier. A bridge rectifier provides full-wave rectification from a two-wire AC

input, resulting in lower cost and weight as compared to a rectifier with a 3-wire

input from a transformer with a center-tapped secondary winding.

4.3.4 FEATURES OF PIC CONTROLLER

HIGH-PERFORMANCE RISC CPU

Only 35 single-word instructions to learn

All single-cycle instructions except for program branches, which are two-

cycle

Operating speed: DC – 20 MHz clock input DC – 200 ns instruction cycle

Up to 8K x 14 words of Flash Program Memory, Up to 368 x 8 bytes of

Data Memory (RAM), Up to 256 x 8 bytes of EEPROM Data Memory

Pin out compatible to other 28-pin or 40/44-pin

PIC16CXXX and PIC16FXXX microcontrollers

PERIPHERAL FEATURES

Timer0: 8-bit timer/counter with 8-bit prescaler

Timer1: 16-bit timer/counter with prescaler, can be incremented during

Sleep via external crystal/clock

24

Timer2: 8-bit timer/counter with 8-bit period register, prescaler and

postscaler

o Two Capture, Compare, PWM modules

o Capture is 16-bit, max. resolution is 12.5 ns

o Compare is 16-bit, max. resolution is 200 ns

PWM max. resolution is 10-bit

Synchronous Serial Port (SSP) with SPI™ (Master mode) and I2C™

(Master/Slave)

Universal Synchronous Asynchronous Receiver Transmitter (USART/SCI)

with 9-bit address detection

Parallel Slave Port (PSP) – 8 bits wide with external RD, WR and CS

controls (40/44-pin only)

Brown-out detection circuitry for Brown-out Reset (BOR)

ANALOG FEATURES

10-bit, up to 8-channel Analog-to-Digital Converter (A/D)

Brown-out Reset (BOR)

Analog Comparator module with:

o Two analog comparators

o Programmable on-chip voltage reference (VREF) module

o Programmable input multiplexing from device inputs and internal

voltage reference

o Comparator outputs are externally accessible

25

SPECIAL MICROCONTROLLER FEATURES

100,000 erase/write cycle Enhanced Flash program memory typical

1,000,000 erase/write cycle Data EEPROM memory typical

Data EEPROM Retention > 40 years

Self-reprogrammable under software control

In-Circuit Serial Programming™ (ICSP™) via two pins

Single-supply 5V In-Circuit Serial Programming

Watchdog Timer (WDT) with its own on-chip RC oscillator for reliable

operation

Programmable code protection

Power saving Sleep mode

Selectable oscillator options

In-Circuit Debug (ICD) via two pins

4.3.5 PORTC AND THE TRISC REGISTER

PORTC is an 8-bit wide, bidirectional port. The corresponding data direction

register is TRISC. Setting a TRISC bit (= 1) will make the corresponding PORTC

pin an input (i.e., put the corresponding output driver in a High-Impedance mode).

Clearing a TRISC bit (= 0) will make the corresponding PORTC pin an output

(i.e., put the contents of the output latch on the selected pin). PORTC ismultiplexed

with several peripheral functions (Table 4-5). PORTC pins have Schmitt Trigger

input buffers. When the I2C module is enabled, the PORTC<4:3> pins can be

configured with normal I2C levels, or with SMBus levels, by using the CKE bit

(SSPSTAT<6>).

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4.4 DRIVER UNIT

D 1

D 1 N 1 9 0

R 5

1 0 0

U 1

O P -0 7 C / 3 0 1 / T1

2 3 0 / 1 2 V

TR A N S F O R M E R

1 5

4 8R 7

1 K

Q 3G

R 1

1 K 2 3 0 V

R 31 K Q 2

R 6

1 K

R 2

1 0 0

5 0 H z

C 1

1 n

S

R 4

1 0 0

M C T2 E

U 2 Q 1

B D X3 7

F R O M M IC R OC O N T R O LLER

Fig..4.3 Driver Circuit

4.4.1 BUFFER

A buffer amplifier (sometimes simply called a buffer) is one that provides

electrical impedance transformation from one circuit to another. Two main types of

buffer exist: the voltage buffer and the current buffer.

A current buffer amplifier is used to transfer a current from a first circuit,

having a low output impedance level, to a second circuit with a high input

impedance level.

4.4.2 OPTOCOUPLER

There are many situations where signals and data need to be transferred from

one subsystem to another within a piece of electronics equipment, or from one

piece of equipment to another, without making a direct ohmic electrical

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connection. Often this is because the source and destination are (or may be at

times) at very different voltage levels, like a microprocessor, which is operating

from 5V DC but being used to control a triac that is switching 240V AC. In such

situations the link between the two must be an isolated one, to protect the

microprocessor from over voltage damage.

Relays can of course provide this kind of isolation, but even small relays

tend to be fairly bulky compared with ICs and many of today’s other miniature

circuit components. Because they’re electro-mechanical, relays are also not as

reliable and only capable of relatively low speed operation. Where small size,

higher speed and greater reliability are important, a much better alternative is to

use an optocoupler. These use a beam of light to transmit the signals or data across

an electrical barrier, and achieve excellent isolation.

Optocoupler typically come in a small 6-pin or 8-pin IC package, but are

essentially a combination of two distinct devices: an optical transmitter, typically a

gallium arsenide LED (light-emitting diode) and an optical receiver such as a

phototransistor or light-triggered diac. The two are separated by a transparent

barrier which blocks any electrical current flow between the two, but does allow

the passage of light. The basic idea is shown, along with the usual circuit symbol

for an optocoupler. Usually the electrical connections to the LED section are

brought out to the pins on one side of the package and those for the phototransistor

or diac to the other side, to physically separate them as much as possible. This

usually allows optocouplers to withstand voltages of anywhere between 500V and

7500V between input and output. Optocouplers are essentially, digital or switching

devices, so they’re best for transferring either on-off control signals or digital data.

Analog signals can be transferred by means of frequency or pulse-width

modulation.

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4.4.2.1 OPTOCOUPLER OPERATION

Basically the simplest way to visualize an optocoupler is in terms of its two

main components: the input LED and the output transistor or diac. As the two are

electrically isolated, this gives a fair amount of flexibility when it comes to

connecting them into circuit. All we really have to do is work out a convenient way

of turning the input LED on and off, and using the resulting switching of the

phototransistor/ diac to generate an output waveform or logic signal that is

compatible with our output circuitry.

This means you can arrange for the LED, and hence the optocoupler, to be

either on or off, for a logic high (or low) in the driving circuitry. In some circuits,

there may be a chance that at times the driving voltage fed to the input LED could

have reversed polarity (due to a swapped cable connection, for example). This can

cause damage to the device, because optocoupler LED’s tend to have quite a low

reverse voltage rating: typically only 3 - 5V. So if this is a possibility, a reversed

polarity diode should be connected directly across the LED as shown in Fig.3. On

the output side, there are again a number of possible connections even with a

typical optocoupler of the type having a single phototransistor receiver (such as the

4N25 or 4N28). In most cases the transistor is simply connected as a light-operated

switch, in series with a load resistor RL (see Fig.4). The base of the transistor is

left unconnected, and the choice is between having the transistor at the top of the

load resistor or at the bottom. i.e., in either pull-up or pull-down mode. This again

gives plenty of flexibility for driving either logic gates or transistors, as shown in

Fig.5. If higher bandwidth is needed, it can be achieved by using only the collector

and base connections, and by using the transistor as a photodiode. This lowers the

optocoupler’s CTR and transfer gain considerably, but can increase the bandwidth

to 30MHz or so.

29

An alternative approach is still to use the output device as a phototransistor,

but tie the base down to ground (or the emitter) via a resistor Rb, to assist in

removal of stored charge (Fig.6B). This can extend the opto’s bandwidth usefully

(although not dramatically), without lowering the CTR and transfer gain any more

than is necessary. Typically you’d start with a resistor value of 1MW, and reduce it

gradually down to about 47kW to see if the desired bandwidth can be reached.

A variation on the standard optocoupler with a single output phototransistor

is the type having a photo- Darlington transistor pair in the output, such as the

6N138. As mentioned earlier this type of device gives a much higher CTR and

transfer gain, but with a significant penalty in terms of bandwidth. Connecting a

base tieback resistor can again allow a useful extension of bandwidth without

sacrificing too much in terms of transfer gain.

4.4.3 TRANSISTOR-TRANSISTOR LOGIC

Transistor–transistor logic (TTL) is a class of digital circuits built from

bipolar junction transistors (BJT) and resistors. It is called transistor–transistor

logic because both the logic gating function (e.g., AND) and the amplifying

function are performed by transistors (contrast this with RTL and DTL).

TTL is notable for being a widespread integrated circuit (IC) family used in

many applications such as computers, industrial controls, test equipment and

instrumentation, consumer electronics, synthesizers, etc. The designation TTL is

sometimes used to mean TTL-compatible logic levels, even when not associated

directly with TTL integrated circuits, for example as a label on the inputs and

outputs of electronic instruments.

30

The input to a TTL circuit is always through the emitter(s) of the input

transistor, which exhibits a low input resistance.  The base of the input transistor,

on the other hand, is connected to the Vcc line, which causes the input transistor to

pass a current of about 1.6 mA when the input voltage to the emitter(s) is logic '0',

i.e., near ground. Letting a TTL input 'float' (left unconnected) will usually make it

go to logic '1', but such a state is vulnerable to stray signals, which is why it is

good practice to connect TTL inputs to Vcc using 1 kohm pull-up resistors.

The most basic TTL circuit has a single output transistor configured as an

inverter with its emitter grounded and its collector tied to Vcc with a pull-up

resistor, and with the output taken from its collector. Most TTL circuits,

However, use a totem pole output circuit, which replaces the pull-up resistor

with a Vcc-side transistor sitting on top of the GND-side output transistor. The

emitter of the Vcc-side transistor (whose collector is tied to Vcc) is connected to

the collector of the GND-side transistor (whose emitter is grounded) by a

diode.  The output is taken from the collector of the GND-side transistor. Figure

1 shows a basic 2-input TTL NAND gate with a totem-pole output.

31

CHAPTER 5

CONCLUSION

5.1 GENERAL

The importance of low-power circuit techniques in portable devices and

biomedical implants drove researchers to develop new design methods of reducing

the power consumption of these devices. One of the challenges that SC DC-DC

convertersfacesis the low conversion efficiency atlight loads. In this work, we have

demonstrated an approach for efficient power deliveryin ultra-low-power devices

using SC DC-DC converters. By switching only when required, SC DC-DC

converter reduces the switching power losses. In contrast to the

methods that weredeveloped previously in this field, we proposed an

asynchronous control strategy that would minimize the switching power losses.

After reviewing the basics of the SC DC-DC converters, we have shown

that a SC DC-DC converter should use different conversion ratios under different

output voltagesto maximize the conversion efficiency. The proposed design used

three different topologies to realize three different conversion ratios. This

converter can be helpful in supplying power to ultra-low power circuits that are

found in bio-medical implants.

32

5.2 FUTURE SCOPE

The proposed converter still has room for improvements. A closed loop

structure can be designed which would help in the scaling of output voltage and

thus can change according to the output changes. This can improve the efficiency

of the whole structure. Thus according to the load changes the input would change.

The output voltage ripple which would lead to unnecessary switching and

irregularities in operating frequencies can be avoided by adopting a hysteresis

comparator approach.

33

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[3] Y. Ramadass and A.Chandrakasan, Voltage scalable switched capac-

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[4] Chowdhury, I.; DongshengMaAn, integrated reconfigurable switched-

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1) www.fairchildsemi.com

2) www.datasheetreference.com

3) www.seminartopics.com

4) www.en.wikipedia.org

5) www.cindybob.com

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