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STN LCD Drive and Contrast Voltage
The basic principle of operation of an LCD subpixel is that a drive voltage is applied between the electrodes disrupts the natural spiral of the nematic liquid crystal molecules. When the spiral is disrupted, the subpixel stops transmitting light and appears dark. The drive voltage is switched onto the electrodes by driver ICs, which are operated by signals running at normal logic levels.
On Kyocera STN LCDs, this drive voltage must always be positive. It must be strong enough to disrupt the subpixel, but not so strong that it disrupts adjacent subpixels. In the normal-temperature KCS057 series, it is between 24.1 and 27.5 volts. In the normal temperature KCS038 series it is between 29.1 and 32.0 volts. The exact drive voltage depends on temperature and wide temperature models have wider ranges.
It is supplied as a positive DC voltage, but circuitry on the LCD rapidly reverses its connections to the electrodes so that a DC field is not present long enough to permanently bias the liquid crystal material. The KCS038 series must be supplied with a randomizing frequency between 300 and 800 Hz, which the LCD uses to drive these field reversals. Larger LCDs are provided with a randomizing circuit to generate this frequency internally. The KCS038 series must actually be provided with three drive voltages and three grounds, biased slightly apart. Larger LCDs are provided with a bias voltage circuit which generates the three voltages from the provided drive voltage.
In this discussion comments about a KCS series LCD apply equally to KHS and KRS. Check section 5 of the full specification for exact voltage ranges. In the KCS038 series, the three biased drive voltages are named V0, V1 and V2 and the corresponding biased returns are named V3, V4 and V5. In the KCS057 series, the LCD drive voltage is named VEE and its return is through the common ground named VSS.
The liquid crystal material itself always requires this rather high LCD drive voltage. On most newer LCD models, Kyocera provides a DC-DC converter, which generates the required drive voltage from the supplied logic voltage. These models will not have voltages named VEE, V0, V1, etc present on the interface. Instead, they will have a voltage named VCONT.
The STN LCD drive voltage must be adjustable. Section 5 of the specification gives the range within which VEE, V5-V0 or VCONT must be adjustable. Vop is the name of the exact voltage level on the input voltage which produces the maximum contrast ratio (CR). Section 6.2 of the specifications show this definition of Vop and note that the x-axis definition changes among these LCDs. The sole purpose of VCONT is convey this adjustment, so it draws virtually no current. This is discussed further on the power consumption page.
The VCONT voltages are lower than the logic voltage, so it is easy for a customer to simply connect a 10 kOhm potentiometer across the logic voltage and connect VCONT to the wiper. 50 kOhm and higher is too difficult to adjust because the sweet spot is so small. The maximum and minimum values for VCONT given in specification section 5 merely indicate a range which might be required, not safety limits. VCONT voltage values between the logic voltage and ground will not harm the LCD. The use of this adjustment is discussed further on the temperature consideration page.
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