low power step down dc-dc converter
TRANSCRIPT
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>).
26
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.
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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.
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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.
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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.
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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.
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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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