Survey on LCD

Design of a Rail-to-Rail Class-B Buffer with DC Level-Shifting Current Mirror for LCD Column Drivers

2012-13

CHAPTER-1

INTRODUCTION

1.1GENERAL INFORMATION

A liquid crystal display (LCD) is a flat panel display, electronic visual

display, or video display that uses the light modulating properties of liquid crystals.

Liquid crystals do not emit light directly.

LCDs are available to display arbitrary images (as in a general-purpose computer

display) or fixed images which can be displayed or hidden, such as preset words, digits,

and 7-segment displays as in a digital clock. They use the same basic technology,

except that arbitrary images are made up of a large number of small pixels, while other

displays have larger elements.

LCDs are used in a wide range of applications including computer

monitors, televisions, instrument panels, aircraft cockpit displays, and signage. They are

common in consumer devices such as video players, gaming

devices, clocks, watches, calculators, and telephones, and have replaced cathode ray

tube (CRT) displays in most applications. They are available in a wider range of screen

sizes than CRT and plasma displays, and since they do not use phosphors, they do not

suffer image burn-in. LCDs are, however, susceptible to image persistence.

The LCD screen is more energy efficient and can be disposed of more safely than a

CRT. Its low electrical power consumption enables it to be used in battery-

powered electronic equipment. It is an electronically modulated optical device made up

of any number of segments filled with liquid crystals and arrayed in front of a light

source (backlight) or reflector to produce images in colour or monochrome. Liquid

crystals were first developed in 1888.

M.tech VLSI Design and Embedded System, RNSIT Page 2


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CHAPTER 2

LITRATURE SURVEY

Sl. No

Year Title Method Result

1 2011 A RAIL-TO-RAIL

CLASS-B

BUFFERWITH DC

LEVEL-SHIFTING

CURRENT

MIRROR AND

DISTRIBUTED

MILLER

COMPENSATION

FOR LCD

COLUMN

DRIVERS

A rail-to-rail

class-B buffer

with a dc

level-shifting

current mirror

and the

proposed

distributed

Miller

compensation

is presented

for liquid

crystal display

By using a dc

level-shifting

current

mirror, the

rail-to-rail

input

stage directly

turns on/off

the rail-to-rail

push-pull

output stage

without extra

comparators.

It results in a

low-power

rail-to-rail

class-B

buffer.

2 2010 A NEW COMPACT

LOW-POWER

HIGH-SPEED

RAIL-TO-RAIL

CLASS-B BUFFER

FOR LCD

APPLICATIONS

Proposes a

two-stage

class-B buffer,

which

includes an

input rail-to-

The proposed

output buffer

can

obtain fast

driving

capabilities



2012-13

rail

differential

amplifier

embodying the

current

comparators

with a limited

quiescent

current

dissipation by

exploiting

two current

comparators.

3 1999 A CLASS-B

OUTPUT

BUFFER FOR

FLAT-PANEL-

DISPLAY

COLUMN

DRIVER

Presents a low

static power,

large output

swing, and

wide operating

voltage range

class-B output

buffer

amplifier for

driving the

large column

line

capacitance in

flat-panel

display.

A comparator

is used in the

negative

feedback path

to eliminate

quiescent

current in the

output stage.

4 2004 A HIGH-SPEED

DRIVING

SCHEME AND

COMPACT

HIGH-SPEED

It uses

one

differential

amplifier, two

output driving

The zero

compensation

technique is

used for the

stability but



2012-13

LOW-POWER

RAIL-TO-RAIL

CLASS-B

BUFFER

AMPLIFIER

FOR LCD

APPLICATIONS

stages and the

compensation

resistor

the slew rate

and the

settling time

are improved

by the second

output stage.

5 2009 A RAIL-TO-

RAIL UNITY

GAIN BUFFER

AMPLIFIER

FOR LOW-

COST HIGH

RESOLUTION

TFT LCD

PANELS

A structural

rail-to-rail

high voltage

CMOS buffer

amplifier for

driving

gamma

correction

reference

voltage of

TFT LCD

panel is

presented.

A class AB

output stage

with

common-

source

transistors is

used to

achieve rail-

to-rail output.

loss exists

6 2003 LOW POWER

HIGH-SPEED

CLASS AB

BUFFER

AMPLIFIER

FOR LCD

APPLICATIONS

To improve

the speed of

the buffer

amplifier, a

new slew-

enhancement

technique

The proposed

Class AB

buffer

amplifier

improves the

transient

response



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without

consuming

any static

current is

proposed.

without

consuming

any

additional

static current

programs

7 2010 A

SUBTHRESHOL

D TO ABOVE-

THRESHOLD

LEVEL

SHIFTER

COMPRISING A

WILSON

CURRENT

MIRROR

A novel level

shifter circuit

that is capable

of converting

subthreshold

to above-

threshold

signal is

proposed.

The circuit

does not have

a static

current path

between the

supply rails

and therefore

offers a

reduced static

power

dissipation.

CHAPTER 3



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PROJECT DESCRIPTION

OBJECTIVE:

The aim of the project is to a design rail-to-rail class-B buffer with a dc level-shifting

current mirror for liquid crystal display (LCD) column drivers with low power dissipation

and rail to rail swing.

MOTIVATION:

For high-end colour panels in portable applications, a lot of buffers are needed.

The requirements of the buffers for high-quality displays are low power dissipation,

large driving capability, small area, and large output swing.

Class b buffers have low static power, large output swing, large driving capability and

wide operating voltage range.

LCD DISPLAY:



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SYSTEM LEVEL BLOCK DIAGRAM:

Fig.1 Block diagram of LCD system

Liquid Crystal Displays (LCDs) are expected to be the dominant technology for flat panel

televisions. Television applications use much larger panels than those for computer monitors,

and the colour must be more accurate.

Computer monitors use 6 or 8 bits per colour; television panels require 10 bits per colour. To

maintain colour purity, the system also needs to compensate for temperature, ambient

lighting, and variations in panel manufacturing.

Fig.1 is a block diagram of an LCD system. A horizontal line of pixel data from the video

processor is retimed by the timing controller and distributed to the column drivers. The

column drivers are essentially 420-output digital-to-analog converters (DACs). The timing

controller also clocks the row drivers to scan the image from the top to the bottom of the

panel. The system also contains voltage references for the column drivers, power supply

components, and the LCD back plane driver.

Generally, a liquid crystal driving (LCD) system is composed of column drivers, row

drivers, a timing controller, and a reference source.



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A column driver includes shift registers, input registers, data latches, level shifters,

Digital-to-Analog Converters (DACs), and output buffers.

Among these components, the DACs occupy the largest area. For different display

panels, the output buffer must meet various requirements in the column drivers. For large-

size display panels, the buffers must drive a large column line capacitance.

For high-end colour panels in portable applications, a lot of buffers are needed. The

size and power dissipation of these buffers must be minimized to save the die area and battery

power. For the panels with higher gray levels, the buffers should offer a rail-to-rail

input/output operation.

In summary, the requirements of the buffers for high-quality displays are low power

dissipation, large driving capability, small area, and large output swing.

LCD COLUMN DRIVER:



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Fig.2 LCD column driver

Fig.2 shows the conventional column driver architecture. The column driver supplies

high analog voltages to the LCD panel. To reduce power consumption, the digital circuit uses

a low voltage power supply.

Digital display data is fed to the RGB inputs and sampled into the input registers. A

wide data latch presents one row of serial input pixel data to the level shifters inputs. The

level shifters then boost the digital signals to higher levels. The DAC of each channel outputs

a voltage level corresponding to a digital sub-pixel code. The output buffers drive the highly

capacitive data lines of the LCD panel. For a dot inversion operation, the DACs offer

voltages with positive and negative polarities for the same digital input code.



2012-13

Fig. 3 The layout of column driver.

Figure 3 depicts the layout of a conventional column driver. One resistor string is put

in the middle of the chip to supply the reference voltages to all channels. Each channel needs

a decoder to route the reference voltage, corresponding to the digital input code, to the

corresponding output buffer. Since several hundreds of channels are built into a single chip,

the die area of the routing lines connecting the resistor string and the decoders is very large.

For example, a 10-bit column driver IC requires 2048 metal lines. Hence, these metal

lines and the decoders occupy a very large percentage of the column driver IC’s area,

especially for high colour depth displays.

To drive the data lines of the LCD panel, each channel needs an output buffer. The

output buffers, which are usually made of operational amplifiers, drive highly capacitive data

lines.



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CLASS B BUFFER FOR LCD COLUMN DRIVER:

BLOCK DIAGRAM OF CLASS B BUFFER:

Fig. 4 Rail-to-Rail Class-B Buffer with DC Level-Shifting Current Mirror

A simple class B buffer with DC level-shifting current mirror block diagram is

shown in Fig. 4. Class B buffers have low static power, large output swing and wide

operating voltage range.

The input stage consists of biasing circuit, differential amplifier and a dc level

shifting current mirror. The rail to rail output stage consists of a charging and discharging

transistors.

The biasing circuit provides the bias voltage to the differential amplifier in the input

stage. The differential amplifier it consists of a biasing circuit, two complementary input

pairs and two constant current sources.

In order to reduce the quiescent current, all the transistors of the input stage adopt the

large channel length and small aspect ratios.

The large line capacitance is modelled as a load capacitor to which the output is

given. The output capacitance value ranges from 10 to 1200 pF.



2012-13

CIRCUIT DIAGRAM:

Fig. 5 Circuit diagram of Rail-to-Rail Class-B Buffer with DC Level-Shifting Current Mirror

The transistors operate in the saturation region and the device sizes of Mp4, Mn4,

Mp6 and Mn6 are identical with those of Mp5, Mn5, Mp7 and Mn7 respectively. In Fig. 5,

Mp3 and Mn3 have the currents equal to mIB, where IB is the current in the biasing circuit.

When the output voltage is equal to the input voltage, the currents of Mp1, Mp2, Mn1 and

Mn2 is equal to mIB/2.

The transistors Mp1, Mp2, Mn4, Mn5 and Mn1, Mn2, Mp4, Mp5 constitute the

folded cascade differential amplifier. The transistors Mp6, Mp7, Mn6 and Mn7 form the dc

level shifting stage.

The output stage is a rail-to-rail output stage, which consists of a charging transistor

(Mp8) and a discharging transistor (Mn8). In order to reduce the quiescent current, all the

transistors of the input stage adopt the large channel length and small aspect ratios. It also

increases the output impedance and decreases the current mismatch. Since this rail-to-rail

folded-cascoded differential amplifier has a low bias current, the channel-length modulation

effect is neglected in the following discussions.



2012-13

The drain currents ID4 and ID6 are given by

Since MP6 andMN6 are biased by the drain voltages of the diode-connected transistors

MN4 and MP4, respectively, the voltages VSG_MP6 and VGS_MN6 are derived as

In Fig. 2, the drain current of two diode-connected transistors MN7 and MP7 is given

by

Assume ID6=ID7, the voltages VSG_MP7 and VGS_MN7 are given by



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By (3), (5) and (6), the following equality is given as

Assume that the transistors MP6 and MN6 operate at the boundary between saturation

and triode regions, i.e.,

By using (5) and (9), the voltages VSD_MP5 and VDS_MN5 are derived as

From (10), both the transistors MP5 and MN5 will also operate at the boundary between

saturation and triode regions. By utilizing this dc level-shifting current mirror, the voltages

VSD_MP5 and VDS_MN5 are lowered. Since VSD_MP5= VSG_MP8 and VDS_MN5=VGS_MN8, if

VSD_MP5 < Vtp and VDS_MN5 < Vtn, the output stage, MP8 and MN8, will be turned off in the

steady state. A dc level-shifting current mirror is used to achieve a class-B operation without

extra comparators.

By (1), (2), and (7), the class-B buffer with a dc level-shifting current mirror is shown

in Fig. 5. The output stage is a rail-to-rail output stage, which consists of a charging transistor

MP8 and a discharging transistor MN8. The input stage is a rail-to-rail folded-cascoded

differential amplifier, and it consists of a biasing circuit (MB1–MB4), two complementary

input pairs (MP1 –MP2 and MN1–MN2), two constant current sources (MP3 and MN3), and a dc

level-shifting current mirror (MP4–MP5, MN4–MN5, MP6–MP7, and MN6–MN7). In order to

reduce the quiescent current, all the transistors of the input stage adopt the large channel

length and small aspect ratios. It also increases the output impedance and decreases the

current mismatch.



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The following equality is obtained as

From (11), following two equalities are derived as

From (12)–(13), when VDD increases, ID6 increases. The quiescent currents of the self-

biased class-B buffer are dependent on the power supply. To work under a larger supply

voltage, the channel lengths of MP4, MN4, MP6, and MN6 must be enlarged to have a low

quiescent current.



2012-13

CHAPTER 4

SPECIFIC REQUIREMENTS

SOFTWARE REQUIREMENTS:

Cadence virtuoso.

Cadence spectre.

The software which are used in project are noted above and the respective

tools are used for

CADENCE VIRTUOSO: It is used as circuit simulator.

CADENCE SPECTRE: It is used to post layout simulation.



2012-13

CHAPTER 5

PROJECT PLAN

Module Description Result Output Start Date End Date

Finalizing the project Done 10th sep 2012 18th sep 2012

PHASE 1

Literature Survey Done

Study of LCD, Column drivers,

Different techniques for column driver.

18th sep 2012 1 nov 2012

study and choosing the proper type of buffer for

LCD column

driver for low power

consumption

Done Class B buffer is chosen 1st oct 2012 15th oct 2012

study and choosing the proper level shifter for low power

consumption

Done

Level shifter Comprising a

Wilson Current Mirror is chosen

6th oct 2012 10th oct 2012

study of In other methods which 12th oct 2012



2012-13

other low power

technique which can be used.

progress can be used to lower the power

Phase I report Done 13 nov 2012 16 nov 2012

PHASE 2

Studying of tools.

In progress 10th dec 12 20th dec 12

Designing the Level

shifter block 20th dec 12 20th jan 13

Designing the

differential amplifier

20th jan 13 1st feb 13

Designing bias circuit and rail to rail output

block

2nd feb 13 26th feb 13

PHASE 3

integration of all the blocks

1st march 13 14th march 13

verifying and

comparing with the existing

14th march 13 1st april 13



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models

making the report 2nd april 13 28th april 13

submitting report.

29th april 13



2012-13

REFERENCES

[1] P. C. Yu and J. C. Wu, “A class-b output buffer for flat-panel-display column driver,”

IEEE J. Solid-State Circuits, vol. 34, pp. 116–119, Jan. 1999.

[2]D. Marano, G. Plumbo, and S. Pennisi, “A new compact low-power high-speed rail-to-rail

class-b buffer for LCD applications,” IEEE J. Display Technol., vol. 6, no. 5, pp. 184–190,

May 2010.

[3]C. W. Lu, “A rail-to-rail class-ab amplifier with an offset cancellation for LCD drivers,”

IEEE J. Solid-State Circuits, vol. 44, pp. 525–537,Feb. 2009.

[4]M. C. Weng and J. C. Wu, “A compact low-power rail-to-rail class-b buffer for LCD

column driver,” IEICE Trans. Electron., vol. E85-C, no. 8, pp. 659–1663, Aug. 2002.

[5]TFT-LCD source drivers NT39360, NT3982, and NT3994. Novatek. [Online]. Available:

http://www.novatek.com.tw/

[6]Chih-Wen Lu (2009). TFT-LCD Driver IC Design, New Developments in Liquid Crystals,

Georgiy V Tkachenko (Ed.), ISBN: 978-953-307-015-5, InTech, Available from:

http://www.intechopen.com/books/newdevelopments-in-liquid-crystals/tft-lcd-driver-ic-

design

[7] C. W. Lu, C. C. Shen, and W. C. Chen, “An area-efficient fully R-DACbased TFT-LCD

column driver,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 57, no. 10, pp. 2588–2601,

Oct. 2010


http://www.intechopen.com/books/newdevelopments-in-liquid-crystals/tft-lcd-driver-ic-design

http://www.intechopen.com/books/newdevelopments-in-liquid-crystals/tft-lcd-driver-ic-design

http://www.novatek.com.tw/


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Documents

Survey on LCD