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ICM7231 Datasheet(PDF) 11 Page - Intersil Corporation |
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ICM7231 Datasheet(HTML) 11 Page - Intersil Corporation |
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11 / 19 page  29 Figure 8 shows the curve of contrast versus applied RMS volt- age for a liquid crystal material tailored for VPEAK = 3.1V, a typical value for 1/3 multiplexed displays in calculators. Note that the RMS OFF voltage VPEAK/3 ≈ 1V is just below the “threshold” voltage where contrast begins to increase. This places the RMS ON voltage at 2.1V, which provides about 85% contrast when viewed straight on. All members of the ICM7231 and ICM7232 family use an internal resistor string of three equal value resistors to generate the volt- ages used to drive the display. One end of the string is con- nected on the chip to VDD and the other end (user input) is available at pin 2 (VDISP) on each chip. This allows the display voltage input (VDISP) to be optimized for the particular liquid crys- tal material used. Remember that VPEAK = VDD - VDISP and should be three times the threshold voltage of the liquid crystal material used. Also it is very important that pin 2 never be driven below VSS. This can cause device latchup and destruction of the chip. Temperature Effects and Temperature Compensation The performance of the LCD material is affected by tempera- ture in two ways. The response time of the display to changes of applied RMS voltage gets longer as the display tempera- ture drops. At very low temperatures (-20oC) some displays may take several seconds to change a new character after the new information appears at the outputs. However, for most applications above 0oC this will not be a problem with avail- able multiplexed LCD materials, and for low-temperature applications, high-speed liquid crystal materials are available. At high temperature, the effect to consider deals with plastic materials used to make the polarizer. Some polarizers become soft at high temperatures and per- manently lose their polarizing ability, thereby seriously degrading display contrast. Some displays also use sealing materials unsuitable for high temperature use. Thus, when specifying displays the following must be kept in mind: liquid crystal material, polarizer, and seal materials. A more important effect of temperature is the variation of threshold voltage. For typical liquid crystal materials suitable for multiplexing, the peak voltage has a temperature coefficient of -7 to -14mV/oC. This means that as temperature rises, the thresh- old voltage goes down. Assuming a fixed value for VP, when the threshold voltage drops below VPEAK/3 OFF segments begin to be visible. Figure 9 shows the temperature dependence of peak voltage for the same liquid crystal material of Figure 8. For applications where the display temperature does not vary widely, VPEAK may be set at a fixed voltage chosen to make the RMS OFF voltage, VPEAK/3, just below the threshold voltage at the highest temperature expected. This will prevent OFF segments turning ON at high temperature (this at the cost of reduced contrast for ON segments at low temperatures). For applications where the display temperature may vary to wider extremes, the display voltage VDISP (and thus VPEAK) may require temperature compensation to maintain sufficient contrast without OFF segments becoming visible. Display Voltage and Temperature Compensation These circuits allow control of the display peak voltage by bringing the bottom of the voltage divider resistor string out at pin 2. The simplest means for generating a display voltage suitable to a particular display is to connect a potentiometer from pin 2 to VSS as shown in Figure 10. A potentiometer with a maximum value of 200k Ω should give sufficient range of adjustment to suit most displays. This method for generating display voltage should be used only in applications where the temperature of the chip and display won’t vary more than ±5oC (±9oF), as the resistors on the chip have a positive tem- perature coefficient, which will tend to increase the display peak voltage with an increase in temperature. The display voltage also depends on the power supply voltage, leading to tighter tolerances for wider temperature ranges. APPLIED VOLTAGE (VRMS) 100 90 80 70 60 50 40 30 20 10 0 TA = 25 oC θ = -10o θ = -30o θ = 0 VON = 2.1V 0 1234 θ = +10o VOFF = 1.1VRMS 0- 0+ FIGURE 8. CONTRAST vs APPLIED RMS VOLTAGE AMBIENT TEMPERATURE (oC) 6 5 4 3 2 1 0 -10 0 10 20 30 40 50 PEAK VOLTAGE FOR 90% CONTRAST (ON) PEAK VOLTAGE FOR 10% CONTRAST (OFF) FIGURE 9. TEMPERATURE DEPENDENCE OF LC THRESHOLD ICM7231, ICM7232 |
Similar Part No. - ICM7231_02 |
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Similar Description - ICM7231_02 |
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