Technical Resources

Application Notes

1. Basic Operation of an LCD
2. LCD Direct Drive Techniques
3. Multiplexing LCD's
4. Multiplexed Liquid Crystal Displays
5. LCD Direct Drive Circuit
6. Sample Multiplex Drive Circuit
7. Temperature Compensation (With Schematic)
8. Ghosting in LCD's
9. Long Life / High Reliability LCD's
10. LVDS Signal System for TFT Displays
11. Glossary of Terms

1. Basic Operation of an LCD

A liquid crystal display consists of two substrates that form a "flat bottle" that contains the liquid crystal mixture. The inside surfaces of the bottle or cell are coated with a polymer that is buffed to align the molecules of liquid crystal. The liquid crystal molecules align on the surfaces in the direction of the buffing. For twisted nematic devices, the two surfaces are buffed orthogonal to one another, forming a 90º twist of the liquid crystal from one surface to the other. See FIGURE 1.



FIGURE 1

This helical structure has the ability to control light. A polarizer is applied to the front and an analyzer/reflector is applied to the back of the cell. When randomly polarized light passes through the front polarizer, it becomes linearly polarized. It then passes through the front glass and is rotated by the liquid crystal molecules and passes through the rear glass. If the analyzer is rotated 90º to the polarizer, the light will pass through the analyzer and be reflected back through the cell. The observer will see the background of the display, which in this case, is the silver-gray of the reflector.

When an appropriate drive signal is applied to the cell electrodes, an electric field is set up across the cell. The liquid crystal molecules will re-align with the electric field perpendicularly to the glass surface. The incoming linearly polarized light passes through the cell unaffected and is absorbed by the rear analyzer. The observer sees a black character on a sliver-gray background. See FIGURE 2. When the electric field is turned off, the molecules relax back to their 90º twist structure. This is referred to as a Positive Image, Reflective Viewing Mode.



FIGURE 2

2. LCD Direct Drive Techniques

LCD's require an AC drive voltage with minimal DC component. Prolonged DC operation may cause electrochemical reactions inside the displays which will cause significantly reduced life. The initial indications of display degradation because of excessive DC current is an electro-plating of liquid crystal components onto the surface electrodes. These will appear as a mirror-like "burn-in" of the electrode pattern that was energized with DC. Elevated temperature will accelerate this effect.

Because an LCD is made up of several dielectric layers, the Equivalent Circuit shown below is a series of capacitors and a shunted resistor.



There are also series resistances to consider, including the resistivity of the indium oxide electrode paths and the crossover resistance. When voltages are specified for LCD operation, they refer to RMS (Root-Mean-Square) voltage, measured across the glass. For a static drive LCD, this is from an energized pin to the common plane.

Drive frequencies for direct drive displays are typically between 60Hz and 150Hz. Depending on the display size and design, displays can be operated at higher frequencies, but this will result in increased power consumption. Operation below 60Hz may result in display flicker. Electroplating may also occur at low frequencies when the speed of response of the liquid crystal is faster than the drive frequency. The net voltage that the liquid crystal would see would then have a DC component with respect to the speed of the fluid.

LCD's can be overdriven by a combination of voltage and frequency, which will result in cross talk or "ghosting" Ghosting is the appearance or partial activation of an "off" segment. At a specified viewing direction, the liquid crystal is formulated to begin turning on at a specific voltage, which is designated as the voltage threshold. By convention, this is the 10% on-state, or the condition where the contrast is 10% of the full-on contrast. Ghosting occurs when the effective RMS voltage across the display exceeds the voltage threshold of the liquid crystal.

On large displays with specific geometric designs, the backplane impedance could become a significant issue, where the voltage and current are out of phase. High drive voltage and/or frequency can contribute to produce voltages across segments that are at or above the voltage threshold. This will cause ghosting. This ghosting will always occur first at the off-axis condition called out as the optimum viewing angle. Since the current is directly proportional to the frequency, there is a voltage-frequency product which must not be exceeded. These values are very dependent on the design and layout of any given part, so proper display design and choice of driving conditions are important. It is also very important that all unused segments be connected to the backplane, and not allowed to float.

An actual drive circuit will be a symmetrical square wave with less than 50mV DC offset. The low DC offset drive circuit is best designed using a CMOS "exclusive OR" gate. Refer to the bottom part of the figure below:



Please note that normal TTL devices are not suitable for drivers, as they typically have enough DC offset in their output signal to destroy an LCD. Today's commercially available display drivers are designed to produce the optimum signal output.

The top part of the above figure shows the output waveforms of our "exclusive OR" circuit. Plot A is the 40 Hz, 50% duty cycle square wave input to the XOR gate, or "clock" signal. Plot B is the control signal waveform which will turn a segment "on". Plot C is the output of the XOR gate. Plot D shows the RMS voltage as seen by the LCD segment.

Note that when the control input (B) is at logic 0, the resultant output signal to the segment is in phase with the clock output, resulting in a 0V RMS voltage at the segment. It will therefore remain in the "off" state. When the control input to the XOR goes high, the output shifts 180^o out of phase to the backplane, resulting in an RMS value at the segment equal to the supply voltage. The segment will turn "on".

The drawing below shows a typical segment plane and common plane layout for a seven segment digit. From the drawing, it can be seen that the common plane electrode is made as large as possible to keep the resistance low and thereby eliminate "ghosting".


3. Multiplexing LCD's

The details of designing a drive circuit for a multiplexed LCD are covered in our App note Multiplexed Liquid Crystal Displays. We will only cover the highlights here. We also have a sample drive circuit on in our Application Notes section, entitled A Sample Multiplex Drive Circuit. There are also numerous design guides available from the many LSI chip vendors, which contain excellent design tips and guidelines.

To aid in explaining the relationship between duty cycle and bias levels, we will choose a multiplex level of 64, which is a common multiplex duty cycle for a graphics display. To understand why specific bias levels are chosen, we will look at possible bias levels, and demonstrate why there is an optimum bias. What effect on the viewed display will switching between an LCD Bias of 1/9 and 1/8 have? Why is LCD Bias limited to 1/9 and 1/8 for Duty = 64?

The duty cycle defines the recommended bias. There are optimum bias levels for each duty cycle that will define the largest on/off voltage ratio to the LCD. The following formulas define the on and off voltages that the display will see for a given mux level and bias configuration.

V_{rms OFF} = ^V_b ⁄ _S [((N-1) + (S-2)²)/N]^1/2 where V_b is the bias (supply) voltage

V_{rms ON} = ^V_b ⁄ _S [((N-1) + S²)/N]^1/2

S = bias => 1 + √N (nearest integer)

N = multiplex level

As an example, for 1/64 duty, the following bias levels will give these corresponding on/off voltage ratios.

Bias configuration	On/Off ration
7	1.128
8	1.133
9	1.134
10	1.133
11	1.130

Note that Bias = 9 gives the largest on/off ratio, to optimize the contrast.

The necessary bias voltages are usually generated by the use of a resistor dividing network, and example of which is shown below. V_cc is usually 5 volts, and the number of resistors in the ladder is determined from the table above.

The values of the resistors is determined mostly by power requirements and waveform distortion. Because an LCD is a capacitive load, the values should be decreased to decrease distortion. However, this increases power consumption. These values should therefore be made as large as possible, keeping in mind that very large displays have very large capacitance, and may require lowering the resistor values.


4. Multiplexed Liquid Crystal Displays

Retaining the desirable liquid crystal display characteristics of ultra low power consumption and high contrast ratio under high ambient light levels, multiplexed liquid crystal displays greatly reduce the number of external connections necessary for numeric and mixed numeric descriptor displays and make possible dot matrix displays for full alphanumeric and graphic capability. Unlike conventional LCDs which have separate external connections for each segment and descriptor location plus common plane, multiplexed LCD's have their segments arranged as intersections of an X-Y grid. This reduction in the number of external connections enhances device reliability and potential display density. Their liability is the increased complexity of drive circuitry (or perhaps microprocessor software) necessary for their operation.

Unlike other display technologies that respond to peak or average voltage and current, Liquid Crystal Displays are sensitive to the RMS voltage across the interplane capacitance at a given segment location. In general, for a given liquid crystal fluid operating in a multiplex mode, as the number of "common planes" (i.e. electrodes common to all character locations - the "X" dimension of the matrix) is increased, the angle of view over which good display contrast is visible, decreases. When the multiplex rate is greater than about 16, it is necessary to switch to an STN display to maintain viewing angle and contrast. This applications note is applicable to both TN and STN displays, although for simplicity, we will show examples of a segmented display, and a 5x7 dot matrix only.

Three matrix formats have emerged as popular configurations. The 2 x 4 format, shown in FIGURE 1 below, is useful in numeric displays utilizing seven segments plus decimal point at each character location. FIGURE 1(a) shows the interconnections on the common planes. Note that the same segments at each digit are bussed together. FIGURE 1(b) shows the interconnections of the segment planes, rendering the 2 x 4 matrix arrangement for each character location as illustrated in FIGURE 1(c). FIGURE 1(d) shows the method by which four descriptors may be handled as a 1 x 4 subgroup in the same display.



FIGURE 1





FIGURE 2 below illustrates the 5x7 dot matrix format. Seven common plane rows plus five segment plane columns per character form a 35 dot field which can be used for full alphanumeric capability. With this format, the matrix is physically arranged as a rectangular grid.

FIGURE 2



Other multiplex formats are possible for application to custom display requirements, consistent with limiting one dimension of the matrix to a practical value, and consistent with that which is possible in the geometric sense, i.e. there must be room to get all of the leads to the edge of the glass without crossing each other. Our technical question What is Direct Drive Multiplexing? shows a simple routing scheme.

The method of drive for multiplexed displays is essentially a time division multiplex with the number of time divisions equal to twice the number of common planes used in a given format. As is the case with conventional LCDs, in order to prevent irreversible electrochemical action from destroying the display, the voltage at all segment locations must be caused to reverse polarity periodically so that zero net DC voltage is applied. This is the reason for the doubling in time divisions: Each common plane must be alternately driven with a voltage pulse of opposite polarity.

Refer to the figure at the right, which is representative of the 2 x 4 format, where the waveforms required for the generation of character "4" are given. In (a), (b), (c), and (d), the invariant common plane drive waveforms are shown. They can be thought of as bipolar strobe or select pulses. The segment plane waveforms corresponding to the specific characters "4" are presented in (e) and (f). The algebraically summed voltage at segment locations a, b, c, and d.p. for on and off optimized voltages are shown at (g), (h), (i), and (j), respectively (Refer to FIGURE 1). Voltage level VB is the datum reference for the display. To meet the symmetry criterion, no net DC voltage on the display, note that: (1 ) (VD - VB) = - (0-VB) (1a) VB = VD/2 (2) (VC - VB) = - (VA - VB). This convention of voltage levels is chosen consistent with the requirement that the display be driven from a logic system. (CMOS, capable of switching analog voltage levels) operating from a unipolar supply of magnitude >= VD.

To determine the optimum ratio of (V_D - V_B) to (V_C - V_B), or identically (0 - V_B) to (V _A - V_B), requires analysis of the mathematical expressions for the net RMS voltages impressed upon segment locations for both the on and off conditions.

In general, for a multiplexing scheme of N common planes and the conventions specified above:

(3) VRMS (OFF SEGMENT) = [(3+N)/4N * VD2 - (1 + N)/N VCVD + VC2] 1/2

(4) VRMS (ON SEGMENT) = [(N - 1)/4N * VD2 + (1 - N)/N * VCVD+VC2] 1/2

Optimum display contrast will occur when the ratio of the above expressions is maximized. This is determined by setting the derivative of the RMS ratio equal to zero and solving for V_C in terms of V_D. Our technical question What is Contrast shows how contrast is measured. Applying the formulae:

(5) d/dx u/v = (v du/dx -u dv/dx) /v2

and (6) d/dx un = nun -1 du/dx renders

(7) VC = VD/2 (1 + 1/N1/2 )

for maximum RMS voltage ratio and hence, contrast.

Substituting equation (7) into equations (3) and (4) provides expressions for optimized RMS on and off voltages:

(8) VRMS OFF(optimized) = VD ((1 - 1/ N 1/2) /2N ))1/2

(8a) VD = .VRMS OFF ((2N/(1 - 1/N1/2))1/2

and (9) VRMS 0N(optimized) = VD((1+1/ N1/2)/2N))1/2

FIGURE 4

FIGURE 4 at left is a histogram plot showing normalized values of VRMSon and VRMS off (i.e. VD = 1.000) showing the interdependence of available drive voltage and the number of common planes. Note that as the number of common planes is increased, the contrast versus voltage requirement for the liquid crystal fluid used in such a format becomes more stringent. To actually use a given liquid crystal fluid in a given multiplex format requires matching of the fluid's turn on threshold voltage to the drive system's RMS off voltage. Utilizing equation (8a) by substituting the liquid crystal fluid's threshold voltage for V RMS Off will specify a minimum value of VD and thus a minimum supply voltage for the driving logic.

Figure 5 below shows a typical voltage divider network suitable for providing the analog voltage levels to a multiplexed display via CMOS analog switching. Resistors R_A, R_B, R_C and R_D set the voltage ratios. Potentiometer R_E sets the magnitude of V_D to match the liquid crystal requirements. Capacitor C_bypassensures that node V_D is of low source impedance at the drive frequency (i.e. at f_drive X_C << R_D), providing symmetric source impedances at the taps of the divider. Under these conditions, the highest impedance tap is at V_B and is approximately equal to:

(10) (R_A+R_B)/2 = (R_C + R_D)/2 = Z_sourceV_B



FIGURE 5

As is the case with non multiplexed displays, the drive frequency should be chosen to be above the flicker-fusion rate, i.e. >30 Hz. Since increasing the drive frequency significantly above this value increases current demand by the CMOS drive electronics, and to prevent problems due to the finite conductivity of the display segment and common electrodes, an upper drive frequency limit of 60-90 Hz is recommended. With these frequency limits in mind, the approximate source impedance of the divider network can be determined as a function of the net capacitance of the display to be driven. Because this capacitance is a function of the dielectric anisotropy of the liquid crystal fluid, it is non-linear with voltage and should be taken at its maximum value for the fully "on" condition.

Since the exact loading at each divider network voltage tap is a complex function of the instantaneous common plane drive, segments energized, and segments unenergized, the following rule of thumb is suggested as a method of choosing appropriate network source impedances. The RC time constant formed by the source impedance of node V_B and the total "on" capacitance of the display should be an insignificant fraction (e.g. <1/100) of the common plane pulse width.

Recalling equation (10), the rule of thumb becomes:

(11) ((RC + RD)/2) CON <= 1/50 t

where t is the common plane pulse width and is equivalent to:

(12) 1/2N f Drive

since (13) (VC - VB) = (RC/(RC + RD))(VD - VB)

substituting equations (1a) and (7) yields:

(14) RD = RC (N1/2 - 1),

and combining (11), (12), and (14) results in:

(15) RC < 1/(50 N3/2 fDrive Con )

For example: let

N = 3 (e.g. 3 x 3 format), f_Drive = 40Hz (suggested nominal), C_on = 3.8 nf, V_TH = 2.3 V_RMS ( LC fluid threshold)

Thus, by equation (15) RC = RB = 25.3K ohm

Utilizing equation (14) RD = RA = 18.5 K ohm

(16) |Xc BYPASS| = RD/100 @ fDrive or CBYPASS = 21..5 uf

Utilizing equation (8a) VD = VRMS off ((2N/(1 - 1/N1/2))1/2 = (2.3V) (3.77) = 8.67 V

As is the specification for non-multiplexed displays, the net DC component to which a segment location can safely be subjected is <50mV. A worst case analysis of resistor tolerances shows that if R_A, R_B, R_C and R_D are 1% film units, the criterion is met.

If a multiplexed LCD is to be operated over a temperature range in excess of the normal indoor ambient range of 18^oC to 25^oC, temperature compensation such that V_RMSoff tracks the fluid threshold voltage V_th, may be required. Depending upon the particular liquid crystal fluid, the temperature coefficient of V_th is typically in the range of -5 to -15 mV/^oC. Re-inspection of equation (8a) shows the relation between the temperature coefficient of V_RMS "OFF" and that of V_D: The right hand term of equation (8a) must be multiplied by the temperature coefficient of V_th to determine the temperature coefficient of V_D. Note that this resultant is dependent upon the particular multiplex format utilized.

Thus, in summary, considerations of:

1. symmetric drive 2. optimized voltage ratio 3. optimized source impedance 4. optimized drive frequency and 5. temperature compensation of threshold voltage

are the constituents of the successful application of multiplexed LCDs.

Advanced Multiplex Derivation

Let us assume a matrix arrangement of segments N x m. In the drawing on the right, the horizontal lines labeled CP1, CP2, etc. represent the common planes of a display multiplexed by N, and the vertical lines labeled SEG1, SEG2, etc., represent the N segments in the display.

Let us further assume a time division scheme whereby time sequential "select" or "strobe" pulses are applied to the common planes (N dimension of matrix) at some arbitrary amplitude Vs. Refer to drawing on left. On the segment lead, we place an arbitrary voltage +/- VD, so that when it is summed with strobe pulse Vs sum (VS + VD) causes a segment location to turn on, and sum (VS-VD) causes a segment location to turn off. Refer to drawing on right. Symmetric levels +/- VD are chosen as a consequence of the (N -1) time intervals when the strobe pulse is not present. If asymmetrical levels are chosen, a higher residual voltage will be present on unaddressed segments.

Twisted nematic liquid crystals are RMS sensitive. RMS voltage is proportional to the power that a voltage waveform can impart to its load:



The net RMS voltage seen by segment locations within the display can be calculated:

For an "on" segment: VRMS = [((VS + VD)2 + (N - 1)(VD)2)/N]1/2

For an "off" segment: VRMS = [((VS - VD)2 + (N - 1)(VD)2)/N]1/2

The optimum drive situation occurs when the above expressions achieve their maximum ratio. A little differential calculation is used to determine this maximum, which will yield:

VRMS = ((VS)2/N)1/2 * VS/N1/2

REMOVAL OF D.C.

The above scheme will impart DC voltage to the LCD. To correct this, all voltage polarities are inverted on alternate scans. In the timing diagram on the top left, the common plane voltage is shown to reverse polarity every N timing divisions. The resulting net DC voltage over a complete cycle is therefore 0V. In the segment waveform shown at the bottom left, the voltage applied to a given segment is somewhat more complicated because multiple segment drive signals are present. Our segment will only be turned on when the two waveforms are combined, and the resultant total RMS voltage exceeds the threshold voltage of the fluid used.

An alternate and functionally identical scheme is also used wherein the alternate polarity pulse pairs are applied in each scan through the commons. These waveforms are essentially the same as in the above example except that the positive and negative pulses occur during consecutive clock cycles. The resultant DC voltage is still 0V. The display segment still turns on during periods where the total RMS voltage is greater than the threshold voltage of the fluid, which now happens on successive clock cycles.

LEVEL SHIFTING

Because of these constraints, level shifting is applied to the bipolar waveforms discussed above. In the most basic form, a constant voltage of magnitude VS is added to all voltage levels, shifting them to the range 0 to + 2VS, which is compatible with the drivers:

In a more advanced form, level shifting in an amount equal to VD is applied during the intervals when the strobe pulses are positive and in an amount equal to VS when the strobe pulses are negative. This system requires one additional voltage level but provides a real economy of overall supply voltage, reducing it from 2V S to VS + VD. This can be a 20% to 35% reduction depending on N.

In all the above systems the voltage levels required are obtained from a resistive voltage divider. These resistors may be chip external making the system reconfigurable for different N's, if the driver is also reconfigurable. The resistors may also be internal to the chip . The taps off the resistive divider are controlled and switched to the display electrodes by a structure known interchangeably as an analog switch, a bilateral switch or a transmission gate. These act as an interfaces between the normal logic level control signals and the analog levels applied to the display. See our Multiplexing Liquid Crystal Displays app note for a discussion of these divider networks.

The supply voltage required for a given system depends on the LC threshold voltage, N, and the level shifting system used. The expression for the RMS off voltage must be made to match the threshold voltage of the LC fluid. Since the LC threshold voltage has a specific temperature coefficient, the overall supply voltage must be adjusted to track this temperature compensation for wide temperature excursions. The RMS "OFF" voltage in a given system is some constant fraction of the overall supply voltage. Since this fraction must track the LC threshold voltage, the temperature compensation of the threshold voltage must be multiplied by the inverse of the fraction to determine the ultimate temperature compensation of the supply voltage.

Temperature sensing is most easily accomplished with a sensor with a linear characteristic . A small buffer amplifier is generally required to interface the sensor to the output provisions in the display driver. Depending on those provisions, the buffer amp may need to feed the + or - chip power supply or the + or - end of the resistive divider.

THERMAL COMPENSATION

Due to the contrast versus voltage versus temperature characteristics of liquid crystal fluids and the sensitive nature of display drive voltage during multiplex operation, it becomes necessary to compensate LCD drive voltage for applications where the display is subjected to wide temperature excursions. For a typical twisted nematic liquid crystal fluid with a negative temperature coefficient, an under voltage condition with diminished display contrast will result at low temperatures and a "ghosting" or over drive condition will occur at high temperatures if no compensation techniques are employed. That which follows is a discussion of a practical method of providing first order linear compensation, compatible with most available driver circuits. Two examples are presented:

The first factor that needs to be determined in a particular application is the relationship between display RMS "OFF" voltage and display driver circuit supply voltage. Since different drive schemes are utilized, e.g. 3, 4, and 5 voltage level drive arrangements, this is best determined from the manufacturer's data sheet. Also required are the temperature coefficient and threshold voltage of the liquid crystal fluid. The fluid temperature coefficient must be divided by the ratio of RMS "OFF" voltage to driver circuit supply voltage to calculate the ultimate temperature coefficient of the driver circuit supply voltage. In this way the display RMS "OFF" voltage will track its optimum value over the temperature range.

Figure 6 below shows the basic temperature regulation circuit. The heart of the circuit is U1, a National Semiconductor LM335 monolithic temperature sensor, which should be placed in physical contact with the LCD. The LM335 has a basic output voltage coefficient of 110mV/^oC. Resistor R2 supplies operating current to U1, 1 mA nominal. Difference amplifier U2 inverts and scales this coefficient consistent with LCD/driver requirements. Potentiometer R1 provides a means by which the display operating voltage can be set. Note that range limiting resistor R1' and R1" may be added to increase the resolution of R1 and to determine suitable end points. Operational amplifier U2 may be any type capable of supplying sufficient output current to power the display driver circuit(s). An LM307 is a good choice for small systems since its output can swing close to the supply rail, minimizing the magnitude of the positive supply required. R1 must be fed from a regulated supply to maintain a stable output voltage at U2.



FIGURE 6

EXAMPLE #1

Temperature Range:	-10oC to +60oC
Multiplex Mode:	5x7
Liquid Crystal Fluid:	Threshold Voltage: 2.30 Vrms @ 20oC
Temperature Coefficient of Threshold:	-8.0 mV/oC

This drive uses the basic "1/2 select" scheme (4 voltage levels and reference as described earlier). As such, the display RMS "OFF" voltage is .211 times the overall driver supply voltage, for n = 7. The temperature coefficient of the supply voltage can be calculated:

-8.0 mV/oC/.211 = 37.9 mV/oC

The driver supply voltage at the temperature limits can be computed from the display threshold voltages at those limits:

@ -10oC VTHRESH = 2.30 + (-8.0mV/oC *-30oC) = 2.54 Vrms

@ +60oC VTHRESH = 2.30 + ( - 8.0 mV/oC * + 40oC) = 1.98 Vrms

@ -10oC VSUPPLY = 2.54/.211 = 12.0 volts

@ +60oC VSUPPLY = 1.98/.211 = 9.38 volts

Referring to FIGURE 7 below, let R3= 10K, then R4 = 39K to determine the proper temperature coefficient at the output of U2. From the specifications of the LM335 device, it is found that its output voltage is 2.98 +/- .06 volts at 25^oC. Using the basic op amp closed loop gain equation:

V0 = (R4/R3 + 1) VR1 - (R4/R3) VTEMP,

Resolved for R1: VR1 = Vo + (R4/R3)VTEMP/( R4+ 1)/ R3

VR1 should be in the range of 3.95 to 4.05 volts considering the tolerance of U1. Choosing R1 =5K, R1' = 39K, and R1" = 18K allows VR1 to be varied +/- .5 volts, which is more than sufficient range. Since U2 must swing to +9.2 volts output at the low temperature limit, its positive supply is chosen at +12 volts. R2 can now be determined to be ~10K to allow 1 mA nominal current flow into U1. Also shown in FIGURE 8 is a method of level translation from standard 5 volt logic to the temperature controlled voltage of the drivers. U3, an RCA CD40109, low to high voltage converter, translates the incoming control signals. U4A, one-sixth of a CD4060 buffer, allows a high to low voltage logic conversion for the interrupt line feeding back to the main system.

FIGURE 7

Level conversion by returning V_SS to a negative voltage is suggested in the HLCD 0638/0539 data sheet. If the input is assumed to be standard 5 volt logic (HLCD 0538/0539 V_DD terminals tied to +5 volts), V_SS may go as negative as -1.67 volts before input logic level constraints are violated. This net driver circuit supply voltage of 6.67 volts maximum corresponds to a display temperature of 56.7^oC minimum-clearly an unacceptable constraint.

EXAMPLE #2

Temperature Range:	-10oC to +60oC
Multiplex Mode:	3 x 3(6 x 3 for ICM 7233,7234)
Liquid crystal fluid:	Threshold Voltage: 2.30 Vrms @ 20oC
Temperature Coefficient of Threshold:	-8.0 mV/oC

This family of drivers uses the "1/3, select" format of multiplex drive (3 voltage levels and reference). The display RMS "OFF" voltage is .333 times the overall driver internal resistor string voltage. Note that the negative end of the resistor string is brought out separately from the device's V _SS terminal. The temperature coefficient of the display supply voltage can be calculated:

-8.0 mV/oC / .333 = 24mV/oC

The display supply voltage at the temperature limits can be computed from the display threshold voltages at those limits:

@ + 10oC VTHRESH = 2.30 + ( - 8.O mV/oC * -10oC) = 2.38 Vrrns

@ + 55oC VTHRESH = 2.30 * ( 8.0 mV/oC * +35oC) = 2.02 Vrms

Thus: @ + 10oC VDisply supply = 2.38/.333 = 7.15 volts

@ +55oC VDisplay supply = 2.02/.333 = 6.10 volts

Figure 8

FIGURE 8 at left shows a modified form of the basic temperature compensation circuit in which U1 and R2 are reversed in position. This inverts the sense of voltage VTEMP so that the output of U2 goes more positive with increasing temperature to accommodate feeding the negative end of the display drive divider string. Resistor R3 is once again assumed at 10K, this time rendering R4 at 24K to match the required display voltage temperature coefficient. Recalling the resolved op amp closed loop gain equation from the previous example, VR1 is found to be in the range of 1.39 to 1.47 volts, taking into account the LM335 output voltage tolerance. Choosing R1 = 5K, R1' = 15K and R1" = 47K provides more than adequate adjustment range.

Note that at the low temperature design limit the overall display divider supply voltage goes to 5.34 volts The driver data sheet states that the voltage at terminal V_DISP must not go any more negative than 0.3 volts below system ground (V_SS). If this driver is operated from a standard 5 volt logic system, this indicates a small discrepancy (.04 volts) in the display divider supply voltage, raising the minimum acceptable low temperature limit to + 11.7^oC at specification. If lower temperature operation is contemplated using this family of drivers, the more elaborate level shifting scheme utilizing the CD40109 is required to keep the drivers within their specifications.

5. LCD Direct Drive Circuit

The example circuit below shows a typical application of direct drive decoder/driver ICs for numeric displays.

Required signals are 4 bits of BCD data, digit strobe, annunciator bits and square-wave display drive - the 32 HZ input shown on the drawing. The circuit is expandable for displays with more digits or annunciators.

See the IC Link Page for information on obtaining this or many other IC's for direct driving LCD's. Our Applications Note Direct Driving an LCD has additional basic direct drive information.


6. Sample Multiplex Drive Circuit

The example circuit below shows a typical application of multiplex controller/driver ICs for alphanumeric displays. The Hitachi HD44780 easily interfaces with common 4 or 8 bit microprocessors and is fully programmable for font size, duty cycle and display functions such as blinking, clearing and shifting. The character generator is capable of fixed or programmable fonts, including Japanese kana characters. By itself the HD44780 can control a display with either one or two 8-character lines.

This example is typical of Hitachi and compatible devices. See the IC Link Page for information on obtaining these chips.



All LCD drive voltages are generated on-chip with the assistance of a simple resistor divider shown in detail on our page Multiplexing LCD's. These divider networks are somewhat complicated to design, a call to our engineering department at 1-800-786-8710 may be of help during your design efforts.

7. Temperature Compensation (With Schematic)

Due to the contrast versus voltage versus temperature characteristics of liquid crystal fluids, and the sensitive nature of display drive voltage during multiplex operation, it may become necessary to compensate the LCD drive voltage for applications where the display is subjected to wide temperature excursions. For a typical Twisted Nematic Liquid Crystal Fluid with a negative temperature coefficient, an under voltage condition with diminished display contrast will result at low temperatures, and a "ghosting" or overdrive condition will occur at high temperatures if no compensation techniques are employed.

The first factor that needs to be determined in a particular application is the relationship between display RMS "OFF" voltage, and display driver circuit supply voltage. Since different drive schemes (Mux rates) are utilized, this is best determined from the manufacturer's data sheet. Also required are the temperature coefficient and threshold voltage of the liquid crystal fluid. (Assistance in selecting the correct LCD fluid for your application is available from LXD's application engineering department at (440)786-8700).

To calculate the ultimate temperature coefficient of the driver circuit supply voltage, the fluid temperature coefficient must be divided by the ratio of RMS "OFF" voltage to driver circuit supply voltage. In this way, the display RMS "OFF" voltage will track its optimum value over the temperature range.

The figure below shows the basic temperature regulation circuit. The heart of the circuit is U1, a National Semiconductor LM335 monolithic temperature sensor, which should be placed in close proximity to the LCD. The LC335 has a basic output voltage coefficient of 110mV/C. Resistor R2 supplies operating current to U1, 1mA nominal. Difference amplifier U2 inverts and scales this coefficient consistent with LCD driver requirements. Potentiometer R1provides a means by which the display operating voltage can be set.



Note that range limiting resistor R1' and R1" may be added to increase the resolution of R1 and to determine suitable end points. Operational amplifier U2 may be any type capable of supplying sufficient output current to power the display driver circuit(s). An LM307 is a good choice for small systems since its output can swing close to the supply rail, thus minimizing the magnitude of the positive supply required. R1 must be fed from a regulated supply to maintain a stable output voltage at U2.

8. Ghosting in LCD's

The most basic display configuration that will render ghosting is a two segment display in which one segment is energized and the other segment is driven with a signal identical to that which is applied to the display common plane, normally the "off" condition. As drive frequency and/or voltage are increased, the "off" segment starts to turn "on". FIGURE 1 below is a schematic of such a configuration.

Presented below are the electrical models used to characterize the mechanism of ghosting in such an LCD. In this model R_S1 and R _S2 are the segment resistance of segments one and two, and C_S1 and C_S2 are the capacitances of segments one and two. Resistance R_CP represents the effective series resistance in the common plane circuit, consisting of contributions by the common plane electrode and lead-out, the crossover conductors, the common plane lead-out on the segment plane glass, and the substrate to terminal pin interface.

Segments are modeled as capacitors (with a non-linear liquid crystal dielectric), and with series resistances representing the resistivity of the segment electrodes. In the real display there are actually contributions by both electrodes, but for this argument it will suffice to lump them together for each segment.

FIGURE 2 below is simply FIGURE 1 redrawn to make the two-loop nature of the circuit more evident. Loop current i₁ flows through the "on" segment impedances and common plane resistance R_CP. Loop current i₂ flows through the "off" segment impedances and R_CP . From this it can be seen qualitatively that the magnitude of R_CP determines the amount of voltage that reaches the "off" segment and causes ghosting. Note that because the display is always operated below the break frequency of network R_S1 - C_S1, ghosting will be a function of frequency as a result of the changing reactance of C_S1 with frequency. If R_CP were zero, ghosting could never occur. If R_CP were infinite (i.e. common plane open), the "on" and "off" segments are in series, sharing the drive voltage according to the relative magnitude of their impedances. This happens when a crossover connection fails.

In a display consisting of a great number of segments, the electrical model must be expanded to account for contributions of R_CP which may be different for each segment location. In general however, the worst case condition for ghosting will occur when all but one segment are turned "on" and the remaining segment is driven "off". This maximizes the voltage drop across R_CP, which causes the "off" segment to see maximum voltage.



Long Life / High Reliability LCD's

LXD specializes in the manufacture of extremely long life LCD's for those applications in which service life and reliable performance outweigh all other considerations. Our long life LCD's, with temperature ranges as low as -55^oC and as high as 122^oC, are designed for outdoor applications and others where extremes of temperature and humidity are encountered. These include gasoline pumps, marine instruments, oil exploration devices, and many others.

Our processes have been designed to eliminate two major types of display failures. These include "functional" failures that can be traced to defects in materials or process, and failures that are characteristic of a display's end of life behavior.

The incidence rate of both types of defects in normal manufacturing is not high, but since the cost of replacing a display can be twenty times its initial cost, and even higher in some outdoor applications, many users prefer an assured longer service life. The higher purchase price usually results in a lower cost of ownership.

"Functional" field failures, typified by seal failures, shorts, open segments, and ghosting, have been eliminated through manufacturing controls. Each type of defect was studied independently. Conditions or practices that permitted it to occur have been changed or eliminated. Early "end-of-life" failures (loss of alignment, bloating, high current) have also been eliminated through the use of state-of-the-art materials and the development of improved chemical processing and formulations.

LVDS Signal System for TFT Displays

LVDS is a differential signal system that is typically used for high speed data rate transfer. LVDS is not an interface protocol, but rather the hardware mechanism that is used to transmit the data across wires. This system can be used with either serial or parallel or a hybrid interface protocol.

The operation basically has the LCD controller transmitting two logic voltage levels that are compared in the LCD module. LVDS uses the difference in voltage between the two lines to encode information. These two lines are typically twisted pairs. The transmitter sends a small current into one line or the other, depending on the logic level to be sent. The current passes through a resistor which is matched to the impedance of the line at the LCD module, then returns in the opposite direction along the other wire. The LCD module senses the polarity of the voltage across the resistor to determine the logic level.

This type of signal minimizes the amount of radiated electromagnetic noise and power lost to conductor resistance. LVDS is less susceptible to noise than single wire data systems because it uses two wires with opposite current/voltage swings instead of the one wire used in single-ended methods to convey data information. The advantage of the differential approach is that if noise is coupled onto the two wires, the noise appears on both lines equally, and is mostly self-cancelling. The op amp at the receiver/LCD module end basically only looks at the difference between the two signals. Using this differential convention, the signals also tend to radiate less noise than single wire data signals due to the canceling of the induced magnetic fields. Constant current drivers are also not prone to ringing or switching spikes, which further reduces noise.

How LVDS Works



The schematic diagram shows a simple LVDS driver and receiver. It consists of a current source that drives the differential pair lines and an op amp receiver with high input impedance. This causes most of the driver current to flow through the termination resistor, which generates a voltage across the receiver inputs. When the driver switches, it changes the direction of current flow across the resistor, which changes the voltage sense from positive to negative, which creates a logical "one" or "zero".

Glossary of Terms

Active Matrix LCD

Individual pixels of the display are controlled by thin film transistors, which are deposited directly onto the glass substrate. Each pixel is therefore driven in a static mode, resulting in very high contrast and viewing angle.

Annunciator

A word or special symbol which is driven as a single segment

Array Process

Method of manufacturing where displays are arranged in rows and columns on a large laminate and separated after they are filled with LCD fluid.

Backplane

The common electrode connection. There can be multiple backplanes in a multiplexed display.

Common Plane

Same as Backplane.

Contact Ledge

The area along the edge of the parts where electrical connections are made.

Contrast Ratio

The ratio of the luminance between the dark and light areas of the display.

Dual in Line Pins (DIL)

Two rows of pins attached along parallel sides of a display.

Direct Drive

A method of driving a display whereby individual segments are driven from separate edge connections.

Elastomeric Connectors

A thin conductive material used to make connections between an LCD and a PC board.

First Minimum

An LCD construction technique where the cell geometry is optimized for maximum contrast and viewing angle. The geometry is different for each LCD fluid.

Font

The style of a letter or digit.

Ghosting

A condition where segments which are in the "off" condition become slightly visible.

Heat Seal Connector

A thin flexible cable used to connect the LCD to the PC board.

Image Area

The total area bounded by the display characters.

Ink Overlay

The process of applying opaque, colored inks to the display to provide colors or highlight certain areas of annunciators.

Isocontrast Plot

Usually a polar plot showing contrast ratio versus viewing angle. The individual curves represent points of equal contrast.

Liquid Crystal Flui

An organic material which has both liquid and crystalline properties.

Module

An LCD which includes a PCB, driver electronics, bezel, and possibly, a backlight.

Multiplex (Mux)

A method of driving a display where multiple segments are driven from the same edge connection.

Negative Image

A display which has a dark background and lighter active segments, i.e. clear characters on a black background.

Pixel

An individual active segment.

Polarizer

A stretched polymer which transmits light in only one axis. A typical display has polarizers on the front and back.

Positive Image

A display which has a light background and darker active segments, i.e. black characters on a silver background.

Reflective

A viewing mode which uses ambient or other front lighting to provide the illumination for the display.

Segment

An active area within the display which can be turned on and off. This can be a single segment of a 7-segment character, an annunciator, or a pixel in a dot matrix array.

Static Drive

See Direct Drive

Supertwist (STN)

A type of display which uses fluids which "twist" greater than 90^o. An STN display has improved viewing angles and contrast at high multiplex rates.

Transflective

A viewing mode which can use ambient light or backlighting to provide the illumination for the display.

Transmissive

A viewing mode which cannot use any type of front lighting to provide the illumination for the display, it therefore must use a backlight.

Twisted Nematic (TN)

A type of display where the liquid crystal fluid rotates the plane of polarization 90^o.

Viewing Area

The area of a display which is visible through a bezel or cut-out in an instrument.

Viewing Angle

The preferred angle of viewing a display, usually described in comparison to a clock face, i.e. 12 o'clock for above the normal, or 6 o'clock for below the normal.

LCD Module Protocol Interfaces

First Steps in Designing A Custom Display