Temperature Compensating Driver for a Liquid Crystal Display

This invention relates to liquid crystal displays (LCDs) and more specifically to temperature compensating drivers for LCDs.

LCDs are well known in the art. A typical LCD includes a back plane coupled to a back plane lead and eight segments, each segment being coupled to an associated segment lead. When the voltage at a given segment lead exceeds the voltage at the back plane by a predetermined threshold voltage, the associated segment is dark and therefore visible. Similarly, if the voltage at the back plane exceeds the voltage at the segment lead by the predetermined threshold voltage, the segment is dark and therefore visible. However, if the voltage at the segment lead equals the voltage at the back plane, the segment is transparent and therefore not visible. The eight segments are typically arranged so that any of a number of alphanumeric characters can be displayed by selectively darkening various combinations of segments.

Another type of LCD is the biplexed LCD. A typical biplexed LCD includes sixteen segments and two back planes, each segment being associated with one of the two back planes. Each back plane is coupled to an associated back plane lead. In addition, a typical biplexed LCD display includes eight input leads, each input lead driving two segments. Each segment is either dark or transparent depending upon the difference in the voltage at its associated input lead and its associated back plane.

The above-mentioned threshold voltage is greater at low temperatures than it is at high temperatures. According, it is desirable to drive LCDs with greater voltages at low temperatures in order to ensure that the display segments darken properly. Similarly, it is desirable to drive LCDs with lower voltages at high temperatures to avoid cross talk. (Cross talk occurs when a display segment is darkened by an attempt to darken a different display segment.

A circuit constructed in accordance with the present invention drives an LCD. Of importance,the circuit provides first and second reference voltages for application to the LCD back plane and segments. The difference between the first and second voltages increases when temperature decreases and the difference in voltage decreases when temperature increases. This has the advantage of providing increased voltages to LCDs at low temperature, thereby ensuring that the segments darken properly. In addition, this has the advantage of decreasing the voltage applied to LCDs at high temperature, thereby minimizing undesired cross talk. In one embodiment of my invention, the driver circuit is coupled to a biplexed LCD display.

In one embodiment of my invention, a voltage source (for example, a conventional battery) provides a voltage across two terminals. A set of diodes and a resistor are coupled in series between the two terminals. A first lead is coupled to a node between a first pair of diodes within the set of diodes and a second lead is coupled to a node between a second pair of diodes within the set of diodes. The voltages of these two leads are selectively coupled to the LCD. Of importance, since the difference in voltage at these two nodes is dependent upon the voltage drop across the diodes within the set of diodes, this difference in voltage increases when temperature decreases and decreases when temperature increases. This invention is better understood with reference to the drawing.

FIG. 1 schematically illustrates a circuit for driving an LCD display constructed in accordance with a first embodiment of the present invention.

FIG. 2 schematically illustrates a circuit for driving an LCD display in accordance with a second embodiment of the present invention.

FIG. 1 schematically illustrates a circuit 8 constructed in accordance with one embodiment of my invention for driving an LCD 9. Circuit 8 includes a first lead 10a for receiving an electrical potential VB and a lead 10b coupled to ground. In one embodiment of the invention, voltage VB at lead 10a is approximately 12 volts and is provided by a battery 11. Lead 10a is coupled to a number of resistors R1 through R2. In one embodiment, resistors R1 to R12 are 10K ohm resistors. Each resistor R1 through R12 is coupled to the emitter of an associated PNP transistor Q1 through Q12. The collectors of transistors Q1 through Q12 are each connected to ground. Also as can be seen in FIG. 1, the base of transistor Q1 is coupled to ground, the base of transistor Q2 is coupled to the emitter of transistor Q1, the base of transistor Q3 is coupled to the emitter of transistor Q2 and so on.

It will be appreciated by those skilled in the art that transistors Q1 through Q12 are each configured to act as diodes. Specifically, transistor Q1 provides a diode voltage drop (typically about 0.7 volts) between a node N1 to ground. Transistor Q2 provides a diode voltage drop between a node N2 and node N1. Similarly, transistor Q3 provides a diode voltage drop between a node N3 and node N2. N3 is coupled to the base of transistor Q11 which provides a diode voltage drop between a node N11 and node N3. As can be seen from FIG. 1, the voltage at node N11 is four diode drops or about 2.8 volts above ground. Similarly, the base of transistor Q12 is coupled to the emitter of transistor Q10. Transistor Q12 therefore provides a voltage of about 11 diode voltage drops (typically 7.7 volts) at a node N12. The voltage VHIGH at node N12 is the first voltage level for driving LCD 9 (e.g. voltage VHIGH is selectively applied to the back plane and segments) and the voltage VLOW at node 11 is the second voltage for driving LCD 9 (e.g. voltage VLOW is also selectively applied to the back plane and segments).

It is noted that because voltages VHIGH and VLOW are eleven diode voltage drops and four diode voltage drops above ground, respectively, the difference in voltage VHIGH and VLOW is seven diode voltage drops (about 4.9 volts). Of importance, at low temperatures, the difference between voltages VHIGH and VLOW increases because the voltage drop across each base-emitter junction of transistors Q1 to Q12 increases. Similarly, when the temperature increases, the difference between voltages VHIGH and VLOW decreases. In this way, circuit 8 provides temperature compensated drive voltages for LCD 9.

Circuit 8 includes a set of CMOS inverters 12a to 12j for driving a set of leads 14a to 14j, respectively. Leads 14a to 14j drive the segment leads and back plane leads for LCD 9, which in one embodiment is a biplexed LCD. Of importance, instead of using battery voltage VB and ground to drive leads 14a to 14j, inverters 12a to 12j drive leads 14a through 14j with voltages VHIGH or VLOW, depending on the state of inverter input leads 16a to 16j, respectively. Inverter input leads 16a to 16j are driven by a control logic circuit 18 which can be any of a number of known microprocessors or control circuits.

In one embodiment of my invention, transistors Q1 to Q12, resistors R1 to R12, inverters 12a to 12j and control logic circut 18 are part of a single CMOS integrated circuit. In such an embodiment, control logic circuit 18 within the CMOS integrated circuit uses logic voltage levels of ground and voltage VHIGH.

It is noted that voltages VHIGH and VLOW are generated from transistors Q11 and Q12, the bases of which are coupled to nodes N3 and N10, respectively. Those skilled in the art will appreciate that the difference between voltages VLOW and VHIGH can be increased by connecting the base of transistor Q11 to node N2, node N1 or ground as required, instead of node N3. Alternatively, node N11 can be connected to ground. Similarly, the difference between voltages VHIGH and VLOW can be decreased by connecting the bases of transistors Q11 and Q12 to appropriate transistors Q1 to Q10. For example, FIG. 2 illustrates an embodiment in which the base of transistor Q12 is coupled to the emitter of transistor Q9. In addition, in other embodiments, more than 12 transistors are used, e.g. if it is desired to drive LCD 9 with greater voltages.

The circuit of the present invention is particularly appropriate for automotive applications, as automobile electrical systems must be able to function over larger temperature variations. Further, circuits in an automobile must typically operate off of a single 12-volt battery such as battery 11. In one such embodiment, LCD 9 is a dashboard display.

While the invention has been described with reference to a specific embodiment, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit of the invention. For example, NPN transistors or conventional diodes can be used instead of PNP transistors Q1 to Q12. Accordingly, all such changes come within the present invention.