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OKI SEMICONDUCTOR

INTRODUCTION

With the continuing demand for smaller and more efficient systems, designers are seeking new ways to reduce power

and increase integration. This paper describes how good use of OKI Semiconductor’s monolithic display drivers can

reduce power consumption and component count in systems that incorporate liquid crystal displays (LCDs). Infor-

mation in this paper is applicable both to small LCD panels (with a dozen or less segmented digits) and to large LCD

panels (such as used for laptop screens).

LCD elements are capacitive in nature and dissipate virtually no power, whether selected or deselected. When an

LCD element is selected, a voltage is applied across the liquid crystal that varies the element’s reflectivity. Removing

the voltage deselects the LCD element and returns the liquid crystal to its original state.

In general, an alternating-current (AC) source is required to illuminate the individual elements in an LCD panel. A

direct-current (DC) source is not suitable for driving an LCD panel because of electrolytic reactions in the LCD’s

liquid. Placing a potential difference across the liquid in an LCD panel causes ion migration, which gradually erodes

the anode terminal and causes destructive deposition on the cathode terminal. The electrolytic reaction could be

diminished by plating the electrodes with a non-reactive conductive element, such as gold, for example; however,

the high cost of gold and similar non-reactive elements makes this solution impractical for all but the most esoteric

applications.

A far simpler way to eliminate electrolytic corrosion is to use an AC source to drive the LCD panel, eliminating uni-

directional ion migration and the associated problems. AC configurations for driving LCDs fall in two main catego-

ries, which are:

•

Static driver configurations

•

Multiplexed driver configurations

The next two sections describe these two configurations. The third section in this application note addresses power-

related issues. This application note concludes with some complete circuit examples.

USING STATIC DRIVERS

In configurations using static drivers, a separate driver signal (SEG) provides an AC source for each element, and all

elements use a single shared common (COM) terminal.

LCD elements are often

segments

of an alphanumeric digit. In larger configurations, LCD elements are individual

dots

that emulate the appearance of a CRT monitor. Static driver configurations are simple to use and are generally

suitable for LCDs with less than 80 segments.

OKI Semiconductor supplies a range of single-chip solutions for static LCD driver configurations, some of which

also include built-in RC oscillation circuits, in small-outline IC (SOIC) and plastic quad flat pack (PQFP) packages.

The table below lists the main characteristics of single-chip static LCD drivers from OKI Semiconductor.

Figure 1

below shows static driver connections to a single-digit, seven-segment alphanumeric display. Separate driv-

ers, SEG1 - SEG8, power each segment. All segments share a common ground connection, COM.

LCD Drivers for Static Configurations

Part Number

Segments

On-Chip Oscillator

Drive Voltage (V

LCD

)

Package

MSM5219B

√

4.0 - 7.0

60-lead PQFP

MSM5221

3.0 - 7.0

80-lead PQFP

MSM5265

√

3.0 - 6.0

100-lead PQFP

––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––––

OKI SEMICONDUCTOR

Figure 2

below shows the AC waveforms for driving the illustrated display. When an element is deselected, the com-

bined COM and SEG signals negate each other, as shown for the (COM – SEG1) signal illustrated below. When an

element is selected, the combined COM and SEG signals constructively reinforce each other, as shown for the (COM

– SEG2) signal below.

In static drive configurations, the AC frequency used to drive each segment is identical. This AC frequency is called

the

frame frequency

. In static drive configurations, a single element is selected or deselected in any one individual

frame, as shown in

Figure 2

above.

Static driver configurations generally use a frame frequency in the 20-200 Hz range. Lower frame frequencies can

cause visible flicker. Higher frame frequencies do not provide sufficient time for charging the capacitive LCD ele-

ments.

SEG7

SEG6

SEG1

SEG2

SEG3

SEG5

SEG4

SEG8

Figure 1. Segment Connections for Static-Drive Conﬁgurations

COM

Segment connections over

the liquid crystal.

KEY:

Common base connections

under the liquid crystal.

Figure 2. Functional Waveforms for Static Drive Conﬁgurations

COM

SEG1

SEG2

COM – SEG2

LCD

-V

LCD

COM – SEG1

(Selected)

(Deselected)

LCD

1 Frame

(Deselected)

(Selected)

OKI SEMICONDUCTOR

USING MULTIPLEXED DRIVERS

For configurations requiring more than about 80 drivers, it is more efficient to multiplex the COM and SEG signals

than to use a static driver configuration. In multiplexed configurations, each SEG driver signal powers more than one

segment, and the circuit uses more than one COM signal. The SEG and COM signals actually form a grid, with each

segment driven by a unique SEG/COM node. The multiplexed drive method reduces the number of driver circuits

and the number of connections between the circuit and the display cell. This reduces cost when driving many display

elements.

Figure 1

below illustrates this reduction in driver count by comparing static and multiplexed drive configurations for

a six-digit display. The static driver configuration requires 49 connections to the LCD, whereas the multiplexed con-

figuration requires only 21 connections. Increasing the degree of multiplexing can further reduce the number of con-

nections; however, increased multiplexing also reduces the circuit’s tolerance to voltage variation.

A range of multiplexed configurations are possible, distinguished by:-

•

Bias

, indicating the number of voltage levels used to power the LCD display.

•

Duty Cycle

, indicating the number of segments driven by each individual output driver.

•

Frame Frequency Type,

indicating whether the COM signal alternates over one frame (Type A) or two frames

(Type B).

For example, in a 1/2 bias, 1/2 duty-cycle configuration, each individual output driver uses two voltage levels to drive

two segments. Similarly, in a 1/3 bias, 1/3 duty-cycle configuration, each driver uses three voltage levels to drive

three segments (

Figure 1

above is a 1/3 bias, 1/3 duty cycle configuration).

Frame frequency determines the degree of flickering and vividness. For a high degree of multiplexing, the type-B

configuration can make the display more vivid, but can also introduce flickering at lower clock frequencies.

Figure 4

ComA

ComB

ComC

Common

Static

Multiplex

1/3 bias,

1/3 duty cycle

Figure 3. Static versus Multiplexed Conﬁgurations

OKI SEMICONDUCTOR

and

Figure 5

below illustrate the difference between type-A and type-B configurations for the LCD driver network

shown in

Figure 1

above.

Type B configurations are more common, as the maximum required frequency is lower.

Figure 6

through

Figure 10

illustrate various multiplexed configuration, all of which use a Type B configuration to reduce frame frequency.

Figure 6

on the next page illustrates how a 1/2 bias, 1/2 duty cycle, Type B configuration can drive 62 outputs. This

particular example is suitable for systems using the MSM6660. Full V

LCD

voltage is applied across the selected seg-

ment for display and less than full V

LCD

voltage is applied across the deselected segment.

COM

SEG

COM – SEG

1 Frame

Figure 4. A-Type Waveforms for a 1/3 Duty Cycle, 1/3 Bias Conﬁguration

SEG

COM – SEG

1 Frame

Figure 5. B-Type Waveforms for a 1/3 Duty Cycle, 1/3 Bias Conﬁguration

COM

OKI SEMICONDUCTOR

Figure 7

below shows the waveforms for the 1/2 bias, 1/2 duty cycle configuration shown in

Figure 6

above.

Figure 8

illustrates a 1/3 bias, 1/3 duty cycle, Type B LCD network, suitable for connection to OKI’s MSM6606.

SEG1

SEG2

SEG3

SEG4

COMA

COMB

Figure 6. A 1/2 Bias, 1/2 Duty Cycle, Type B Conﬁguration

COM1

LC1

LC3

COM2

LC1

LC3

SEGn

LC1

LC3

On Off On Off On Off On Off On Off On Off On

Figure 7. Waveforms for 1/2 Bias, 1/2 Duty Cycle, Type B Conﬁguration

Note:When 1/2 duty is selected and 1/2 bias is used, perform the following:

When the code is -01, short VLC1 and VLC2 to supply the bias voltage.

When the code is -02 or -03, externally short VLC1 and VLC2.

COM2 - SEGn

LCD

-V

LCD

OKI SEMICONDUCTOR

Figure 9

below depicts waveforms for the 1/3 bias, 1/3 duty cycle, Type B network in

Figure 8

above.

SEG1

SEG2

SEG3

COM1

COM3

Figure 8. 1/3 Duty Cycle, 1/3 Bias, Type B Conﬁguration

COM2

COM1

LC1

LC2

LC3

On Off On Off On Off On Off On Off On Off On

COM2

LC1

LC2

LC3

COM3

LC1

LC2

LC3

SEGn

LC1

LC2

LC3

COM2 - SEGn

-V

LCD

Figure 9. 1/3 Duty Cycle, 1/3 Bias, Type B Waveforms

OKI SEMICONDUCTOR

Figure 10, below, illustrates a 1/5 bias, 1/16 duty cycle, Type B configuration, together with COM and SEG wave-

forms. This example uses the dot-matrix configuration for display.

COM 0

LCD

COM 0

SEG 0

LCD

-V

LCD

-V

LCD

1 Frame

COM 0

- SEG 0

(Select Waveform)

COM 0

- SEG 0

(Non-Select Waveform)

= V

LCD

= V

LCD

= V

LCD

= V

LCD

= V

LCD

Figure 10. A 1/5 bias, 1/16 duty cycle, Type B Conﬁguration and Waveforms

OKI SEMICONDUCTOR

Larger configurations are simply extensions of the above principles. OKI Semiconductor supplies a range of single-

chip and multi-chip solutions for multiplexed LCD driver configurations supporting any required number of dots or

segments. The table below lists single-chip LCD drivers, indicating major differences for each part.

For larger dot configurations, such as found in laptop displays for example, common and segment drivers are located

on separate chips.

The table below shows critical features for ICs that provide common drivers only. Note the use of thin quad flat packs

(TQFPs) and Tape Automated Bonding (TAB) for devices with higher lead counts.

The next table, opposite, shows the same critical features for ICs that provide segment drivers only.

[1] On-chip bias resistors.

Single-Chip LCD Drivers for Multiplexed Configurations

Part Number

Dots

Duty Cycle Ratio

On-Chip

Oscillator

Drive Voltage

LCD

)

Package

MSM6544

1/2

√

3.0 - 6.0

56-lead PQFP

MSM6606

1/2

4.5 - 5.5

64-lead PQFP

MSM6660-01

1/2

4.0 - 6.0

80-lead PQFP

MSM6660-02

1/3

[1]

4.0 - 6.0

80-lead PQFP

MSM6660-03

1/3

[1]

4.0 - 6.0

80-lead PQFP

MSM5265

1/2

√

3.0 - 6.0

100-lead PQFP

MSM5260

1/32 - 1/64

8.0 - 18.0

100-lead PQFP

MSC5301B-01

16 COM

64 SEG

1/16

4.0 - 16.0

100-lead PQFP

MSC5301B-02

8 COM

64 SEG

1/8

4.0 - 16.0

100-lead PQFP

Common Drivers for Multiplexed Configurations

Part Number

Drivers

Duty Cycle Ratio

Max. Dots

Drive Voltage

LCD

)

Package

MSM5238

1/32 - 1/64

2,048

3.0 - 16.0

44-lead PQFP

MSM5298A

1/64 - 1/256

17,408

8.0 - 28.0

80-lead TQFP

MSM6368

1/256 - 1/480

38,400

25.0 - 40.0

100-lead TQFP

Segment Drivers for Multiplexed Configurations

Part Number

Drivers

Duty Cycle

Ratio

Max. Dots

Inputs

Drive Voltage

LCD

)

Package

MSM5259

1/8 - 1/16

640

3.0 - 6.0

56-lead PQFP

MSM5839B

1/32 - 1/128

5,120

8.0 - 18.0

56-lead PQFP

MSM5839C

1/3 - 1/64

2,560

4.0 - 11.0

56-lead PQFP

MSM5299A

1/64 - 1/256

20,480

8.0 - 28.0

100-lead PQFP

OKI SEMICONDUCTOR

POWER SUPPLY

The table below shows the relationship between the required number of driving biases and the display duty ratios.

Either passive or active bias generation circuitry can provide the required voltage levels, as described in the next two

subsections.

Passive Bias Generation

A resistor ladder is the most common way to generate the required bias voltage levels. Figure 11 below shows a typ-

ical resistor ladder. V

REF

may be tied to V

, or, for displays with larger voltage bias ranges, V

REF

may be tied to a

negative power source.

MSM5299A-01

1/64 - 1/256

20,480

8.0 - 28.0

100-lead TQFP

MSM5299C

1/64 - 1/256

20,480

8.0 - 28.0

100-lead PQFP

MSM6669

1/100 - 1/256

20,480

14.0 - 28.0

0.25mm TAB

Relationship between Duty Cycle and Driving Bias

Duty Ratio

Drive Bias

Voltage Levels

Static

–

1/2

1/3

1/4

1/3

1/7

1/4

1/8

1/4

1/11

1/4

1/12

1/4

1/14

1/5

1/16

1/5

1/24

1/5

1/32

1/5

1/64

1/5

Segment Drivers for Multiplexed Configurations (Continued)

Part Number

Drivers

Duty Cycle

Ratio

Max. Dots

Inputs

Drive Voltage

LCD

)

Package

OKI SEMICONDUCTOR

The required operating margin and power consumption determines the appropriate resistor values. Because the LCD

load is capacitive, the current during element charging and discharging distorts the waveform. Generally, the value

of resistor R is 1 k

Ω

to 10 k

Ω

. Lower resistor values reduce distortion but increase power dissipation. Larger LCD

panels exhibit greater capacitance, and so resistor values may be decreased proportionally as the display size

increases.

No capacitor is required, but a 1-

F capacitor can be used if necessary. Connecting a capacitor in parallel to the resis-

tors can reduce waveform distortion during the charge and discharge periods, but only to a limited degree. Larger

capacitor values generate a voltage level shift and reduce the operating margin. Figure 12 below illustrates how to

connect capacitors in a passive bias network.

In large arrays, the LCD source and common signals form a matrix configuration that complicate the path of the

charge/discharge current through the load. Moreover, current varies according to the demand for the power consump-

tion of the equipment in which the LCD is incorporated. As a result, it is not possible to generate precise equations

for calculating resistor and capacitor values.

Active Bias Generation

In larger displays, such as graphic displays, the liquid crystal is larger and the duty cycle ratio is smaller. Stability of

the liquid crystal’s drive level is therefore more important for a large display than for a small display.

Because graphic displays are large and contain many picture elements, the LCD driver’s impedance produces distor-

tion in the drive waveforms and degrades display quality. For this reason, the impedance of the LCD driver bias

sources should be reduced with operational amplifiers. Figure 13 below shows examples of op amp configurations

to provide this reduced impedance.

LCD

VLC1

VLC2

VLC3

For 1/2 bias (when 1/2 duty is selected)

LCD

VLC1

VLC2

VLC3

For 1/3 bias (when 1/3 duty is selected)

LCD

Figure 11. Network for Simple Passive Bias Generation

REF

LCD

VLC1