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OM62112 Datasheet(PDF) 11 Page - NXP Semiconductors |
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OM62112 Datasheet(HTML) 11 Page - NXP Semiconductors |
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11 / 48 page  2002 Jan 17 11 Philips Semiconductors Product specification 48 × 84 dot matrix LCD driver OM6211 9.11 Temperature compensation Due to the temperature dependency of the liquid crystals viscosity, the LCD controlling voltage (VLCD) must be increased at lower temperatures to maintain optimum contrast. Figure 4 shows VLCD as a function of temperature for a typical high multiplex rate liquid. In the OM6211 the temperature coefficient of VLCD can be selected from 4 values by setting bits TC1 and TC0, see Tables 4 and 8. handbook, full pagewidth MGT848 T VLCD Fig.4 VLCD as a function of liquid crystal temperature (typical values). 9.12 VLCD generator The binary number VOP representing the operating voltage can be set by the serial interface command and can be adjusted (calibrated) by 5 input pins according to the following formula: (1) where: • VPR is an 8-bit unsigned number set by the serial interface command • VOS is a 5-bit two’s complement number set by the 5 input pins VOS4 to VOS0, see Table 9 • VOP is an 8-bit unsigned number used internally for generation of the LCD supply voltage VLCD. To avoid numerical overflow the allowed values of VPR should be limited to the range 32 to 225 (decimal). The corresponding voltage at the reference temperature, Tnom, can be calculated as follows: (2) The generated voltage at VLCD is dependent on the temperature, programmed Temperature Coefficient (TC) and the programmed voltage at the reference temperature (Tnom). (3) Tnom, a and b for each temperature coefficient are given in Table 4. The maximum voltage that can be generated is dependent on the voltage of VDD2 and the display load current. As the programming range for the internally generated VLCD allows values above the maximum allowed VLCD, the user has to ensure while setting the VPR register and selecting the Temperature Compensation, that under all conditions and including all tolerances the VLCD limit of maximum 9 V will never be exceeded. For a particular liquid crystal, the optimum value of VLCD can be calculated for a given multiplex rate. For a Mux rate of 1 : 48, the optimum operating voltage of the liquid crystal can be calculated as follows; (4) where Vth is the threshold voltage of the liquid crystal used. V OP V PR V OS + = V LCD(Tnom) aV OP b × + () = V LCD aV OP + b × () 1TC T T nom – () × + [] × = V LCD 148 + 21 1 48 ---------- – × --------------------------------------- V th × 6.06 V th × = = |
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