GB2288671A

GB2288671A - Liquid crystal display circuit

Info

Publication number: GB2288671A
Application number: GB9408041A
Authority: GB
Inventors: Keith Charles Jordan
Original assignee: Delco Electronics Overseas Corp
Current assignee: Delco Electronics Overseas Corp
Priority date: 1994-04-22
Filing date: 1994-04-22
Publication date: 1995-10-25
Anticipated expiration: 2014-04-22
Also published as: GB9408041D0; GB2288671B; DE19514762A1

Description

2288671 LIQUID CRYSTAL DISPLAY CIRCUIT AND REGULATED POWER SUPPLY THEREFOR

The present invention relates to a liquid crystal display circuit and to a power supply therefor.

As is known in the art, the contrast level of a liquid crystal display (LCD) changes with temperature due to the effect of temperature on the crystals of the display. In order to adjust for the change in contrast, it is necessary with reducing temperature to increase the voltage at which the LCD is driven and with increasing temperature to reduce the LCD driving voltage. Such temperature compensation is particularly suitable for automotive environments, where ambient temperatures can vary significantly.

is In one type of prior art system, a dedicated

LCD voltage supply circuit is provided with an input which specifies the voltage at which the LCD is to be driven. The level of the signal into this input is made temperature dependent by means of a resistor network which includes a thermistor.

In an alternative prior art system, the voltage across a thermistor is converted to a pulse width modulated signal, the pulse width of which varies with temperature. The pulse width modulated signal is then used as a profile for the LCD driving voltage.

A problem with these prior art systems is that they are expensive, in part because of the use of a dedicated LCD drive voltage and in part due to the use of expensive components.

The present invention seeks to provide an improved power supply for a liquid crystal display.

According to an aspect of the present invention, there is provided a liquid crystal display 2 circuit comprising a liquid crystal display including a drive input; drive means operative to drive the liquid crystal display; control means for controlling the drive means; and a regulated power supply including a supply output coupled to power supply inputs of the control means and the drive means and to the drive input of the liquid crystal display, the supply output in use varying in dependence upon temperature.

By making the whole of the supply voltage of the system temperature dependent, it is not necessary to provide a separate dedicated supply to the liquid crystal display. It is possible to limit the variation in the supply voltage to between around 5 to 4.5 volts (for example, between around -300C to +85OC) and to be around 5.0 volts at normal working temperatures (for example, around 2SOC), thereby ensuring that the remainder of the circuitry of the system is not affected by the changes in the supply voltage.

If necessary, the variation in the system supply voltage can be scaled substantially to match the voltage variation required for the liquid crystal display.

Preferably, the regulated power supply includes a feedback input operative to control the supply output, temperature dependent means being coupled to the feedback input so as in use to change the voltage at the feedback input with changing temperature. The regulator automatically adjusts its output voltage so that its feedback input voltage is equal to an internal reference voltage, for example 1.23 V.

In the preferred embodiment, the temperature dependent means includes one or more diodes coupled so 3 as to conduct during operation of the circuit. Diodes are very cheap components and temperature dependant, making them suitable for providing low cost temperature compensation.

Alternatively, the temperature means may include a thermistor.

Preferably, the temperature dependent means is disposed within a series circuit including one or more resistances, the series circuit being coupled between the supply output, the feedback input and ground.

In the preferred embodiment, calibration means are provided for calibrating the temperature dependence of the supply output. The use of calibration means avoids the need to change the values of the circuit components each time a different liquid crystal display or other component is chosen, thereby making the circuit more adaptable to new applications. The design for a particular application involves determining the temperature dependent slope of the display voltage. Also, as a result of the choice of the resistance of the display, the nominal display voltage as seen at the display can be determined.

The calibration means preferably includes a plurality of resistances connected to, or a series circuit coupled between, the supply output, the feedback input and ground and operative to contribute to the voltage level at the feedback input; the control means being operative to determine the voltage level at the feedback input at a predetermined temperature and to couple the resistances selectively high or low so as to cause the voltage at the feedback input to be equal to a reference bandgap voltage.

The use of such resistors enables the circuit to be automatically calibrated whenever a 4 different liquid crystal display is fitted to the circuit, effectively avoiding the need for any separate laborious or time consuming calibration stage for the liquid crystal display. 5 Alternatively, the calibration means may include a variable resistance in the or a series circuit coupled between the supply output, the feedback input and ground. According to another aspect of the present invention, there is provided a regulated power supply for supplying a drive voltage for a liquid crystal display comprising a supply output; a feedback input operative to control the supply output; and temperature dependent means coupled to the feedback input so as in use to change with changing temperature the voltage at the feedback input and thereby the voltage of the supply output.

Such a regulated power supply can provide a direct temperature dependent drive voltage for a liquid crystal display without the need for any additional signal processing.

Preferably, the temperature dependent means includes one or more diodes coupled so as to conduct during operation of the power supply. Alternatively, the temperature means may include a thermistor.

An embodiment of the present invention is described-below, by way of illustration only, with reference to the accompanying drawing, in which:

Figure 1 is a circuit diagram of the principal components of an embodiment of power supply for a liquid crystal display; and Figure 2 is a circuit diagram of the principal components of an embodiment of liquid crystal display circuit.

Referring to Figure 1, the embodiment of power supply shown is intended to provide both a driving voltage V= for a liquid crystal display (LCD) and the driving voltage Vcc for the remainder of the LCD drive circuitry (shown in Figure 2 in connection with a second embodiment of the invention). In addition, the power supply can provide a plurality of other voltage levels Vn to V, which may be required for liquid crystal display bias voltages.

The power supply includes a voltage regulator 10 of conventional form, for example of the type known as LP2951, which includes a voltage input Vin for receiving an unregulated supply voltage such as the voltage from a vehicle battery. The regulator 10 also includes a regulated voltage output Vout and a feedback input Fb for receiving a controlling voltage used in controlling the output voltage Vout.

Coupled across the output Vout and feedback input Fb Of the regulator 10 is a first resistor RI, which is connected in series with a thermistor RV1 having a negative temperature coefficient. The thermistor RV1 is coupled to the feedback input Fb Of the regulator 10 and ground, while a second resistor R2 is coupled in parallel across it. A plurality of resistors R3, R4 and R5n-RS, are coupled in series between the output Vout of the regulator 10 and ground and collectively provide the system drive voltage Vcc (equivalent to the output Vout of the regulator 10), the LCD drive voltage VLCD 30 (equivalent to Vout[R4+{R5n to R51}1/[R3+R4+{R5n to R51}] and a plurality of additional voltage levels Vn to V, provided appropriate points in the resistor series R3, R4 and R5n to R51. When the temperature of the thermistor RV1 rises, for example as a result of a rise in ambient 6 temperature, its resistance drops causing a drop in the voltage level at the feedback input Fb Of the regulator 10. In order to keep the voltage at the feedback input Fb equal to a reference bandgap voltage (normally between 1.2 to 1.3 volts), the regulator 10 increases its output Vout to compensate for the drop at input Fb. The rise in the output voltage Vout results in a rise in the voltage at the feedback input Fb until it returns to the reference bandgap voltage. Similarly, when the temperature of the thermistor RV1 drops, for example as a result of a drop in ambient temperature, its resistance rises causing a rise in the voltage level at the feedback input Fb Of the regulator 10, which reduces its output Vout to reduce the voltage at input Fb until it returns to the reference bandgap voltage. Thus, with rising ambient temperature, the voltage output Vout drops, while with lowering ambient temperature, the voltage output Vout rises. 20 The values of the resistors R3, R4 and R5n to R51 are chosen to provide the required LCD bias voltages. The resistors are chosen at a predetermined temperature. The temperature dependent circuitry then causes the LCD drive voltage (VLCD) to vary in a manner as to compensate for the change in contrast of the LCD as a result of the varying temperature whilst ensuring at the same time that the change in the circuit supply voltage Vcc does not drop below a level which would cause malfunction of any of the circuit components or rise above a level which could cause component damage. Preferably, the circuit supply voltage is limited to between around 5.5 to 4.5 volts (for example, between around -300C to +85OC) and is chosen to be around 5.0 volts at normal working temperatures (for example, around 25OC), while the LCD 7 drive voltage V= is varied between around 4.2 and 4.9 volts required for the LCD chosen in this example.

Referring to Figure 2, the embodiment of liquid crystal display circuit shown provides a single supply voltage Vc& both for the circuit power supply and for the LCD drive voltage. Furthermore, in place of the thermistor RV1, a number of diodes DI-D4 are coupled in series between the output Vout and the feedback input Fb Of the voltage regulator 10.

Between the set of diodes D1D4 and the input Fb, there is provided a resistor R6, while between the input Fb and ground there is provided a resistor R7.

The values of the resistors R6 and R7 are chosen such that under average operating temperature is (in this example 2SOC), taking into account the total forward voltage drop across the four diodes D1-D4 (1.8 volts in this example), the voltage at the input Fb is at the reference bandgap, voltage (in this example somewhere between 1.2 to 1.3 volts). In this example, resistor R6 has a resistance of 4.32 kQ, while the resistor R7 has a resistance of 3. 01 kQ for the chosen diodes.

A decoupling capacitor Cl is connected between Vout and ground for high frequency stability, while a smoothing capacitor C2 is coupled between the output Vout and ground for supplying short term high current levels and for suppressing noise. In this example, capacitor Cl has a capacitance of 10OnF, while capacitor C2 has a capacitance of 220gF.

A microprocessor 12 controls the circuit and in particular LCD driver 14, of conventional type, and three calibration resistors R8, R9 and Rl, which are coupled between the resistors R6 and R7 and to the input Fb Of the regulator 10. The LCD driver 14 drives an LCD 16 which is chosen for each particular 8 application. The supply voltage Vcc, is connected to the power supply inputs of the microprocessor 12 and of the LCD driver 14 and to the drive input of the LCD 16.

The forward voltage drop of each diode D1-D4 has, in this example, a negative temperature coefficient of -2mV/0C. Therefore, the four diodes together have a temperature coefficient of -8mV/OC, required to provide sufficient compensation for changes in contrast of the chosen LCD 16. Thus, with rising temperature, the total voltage drop across the diodes D1-D4 drops to cause the voltage at the input Fb to rise as a result of the rise in voltage across the resistors R6 and R7. The rise in the voltage at input Fb causes the regulator 10 to reduce the output voltage Vout and thereby to provide a lower voltage VcC, for driving the LCD 16 and for powering the microprocessor 12 and LCD driver 14.

Similarly, with reducing temperature, the total voltage drop across the diodes D1-D4 increases to cause the voltage at the input Fb to drop as a result of the drop in voltage across the resistors R6 and R7. The drop in the voltage at input Fb causes the regulator 10 to increase the output voltage Vout and thereby to provide an increasing voltage Vc& for the LCD 16 driving the LCD 16 and for powering the microprocessor 12 and LCD driver 14.

Should the chosen LCD require a different driving voltage to the voltage level required to power the other circuit components, an arrangement of series connected resistors similar to the resistors R3, R4 and R5n to R51 of the embodiment of Figure 1 could be provided. In practice, only R3 would alter for a different LCD as the bias voltages would always be proportional to V=.

9 The calibration resistors RS-R10 are used to calibrate the regulator 10, the network of diodes D1-D4 and resistors R6 and R7 so that at normal operating temperatures the voltage at the input Pb is equal to the reference bandgap voltage.

In this example, the resistors have resistances ordered in a binary progression; in particular resistor R8 has a value of 270 kQ, resistor R9 a value of 130 kQ and resistor R10 a value of 68kM The resistors are then coupled either high (to Vc& or to ground so as to contribute to current fed to resistor R7 in a positive or negative way, and thereby to calibrate the voltage level at the input Fb Of the regulator 10.

is In practice, once the LCD 16 is selected, it is connected to the circuit in the manner shown in Figure 2. Diodes having a combined temperature coefficient equivalent to the variation in drive voltage required to maintain the normal contrast level of the LCD 16 are then coupled between the output Vout of the regulator 10 and the resistor R6. The circuit is then powered at the normal operating temperature and the voltage Vc& (or alternatively the voltage at the input Fb) is measured by production test equipment. On the basis of the measured voltage, the test equipment determines the amount of adjustment required to bring the voltage Fb to the reference bandgap voltage and therefrom which resistor or resistors R8-R10 should be coupled high or to ground to bring the voltage at the input Fb to the reference bandgap voltage. The result of this determination is then stored in computer memory, for example in non-volatile memory, so that on subsequent operation of the circuit the resistors R8-Rlo are automatically coupled as determined during calibration.

is The three resistors R8-RIO enable the circuit to be automatically calibrated during production, effectively avoiding the need for any physical calibration, for example cut links, select on test resistors, variable resistors and the like.

In an another embodiment, in place of the three calibration resistors R8RlO, a single resistor is added, which is selected during initial calibration of the circuit and then fixed to the relevant connections. Alternatively, resistor R7 may be replaced by a variable resistor, which adjusted during initial calibration.

Although the microprocessor 12 and LCD driver 14 are shown as separate units, they may be part of a common unit, for example the integrated circuit known as 68HCO5L4.

It will be apparent that the temperature dependent device could be connected between the feedback input FB and ground, in which case it would have a positive temperature coefficient. This alternative is particularly suitable for use with temperature dependent resistors.

In the example described above, the supply voltage is varied between substantially 4.5 to 5.5 V. Since there is a tendency to reduce system supply voltages, for example 3.3 V seems to be an aim for the future, the range of 4.5 to 5.5 volts is given only in relation to existing supply voltages. The actual voltage range will vary around the system voltage used.

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Claims

1. A liquid crystal display circuit comprising a liquid crystal display including a drive input; drive means operative to drive the liquid crystal display; control means for controlling the drive means; and a regulated power supply including a supply output coupled to power supply inputs of the control means and the drive means and to the drive input of the liquid crystal display, the supply output in use varying in dependence upon temperature.

2. A liquid crystal display circuit according to claim 1, wherein the regulated power supply includes a feedback input operative to control the supply output, temperature dependent means being coupled to the feedback input so as in use to change the voltage at the feedback input with changing temperature.

3. A liquid crystal display circuit according to claim 2, wherein the temperature dependent means includes one or more diodes coupled so as to conduct during operation of the circuit.

4. A liquid crystal display circuit according to claim 2, wherein the temperature means includes a thermistor. 25

5. A liquid crystal display circuit according to claim 2, 3 or 4, wherein the temperature dependent means is disposed within a series circuit including one or more resistances, the series circuit being coupled between the supply output, the feedback input and ground.

6. A liquid crystal display circuit according to any one of claims 2 to 5, comprising calibration means for calibrating the temperature dependence of the supply output.

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7. A liquid crystal display circuit according to claim 6, wherein the calibration means includes a plurality of resistances connected to the supply output or a series circuit coupled between the supply output, the feedback input and ground and operative to contribute to the voltage level at the feedback input; the control means being operative to determine the voltage level at the feedback input at a predetermined temperature and to couple the resistances selectively high or low so as to cause the voltage at the feedback input to be equal to a reference bandgap voltage.

8. A liquid crystal display circuit according to claim 6, wherein the calibration means includes a variable resistance in the or a series circuit coupled between the supply output, the feedback input and ground.

9. A regulated power supply for supplying a drive voltage for a liquid crystal display comprising a supply output; a feedback input operative to control the supply output; and temperature dependent means coupled to the feedback input so as in use to change with changing temperature the voltage at the feedback input and thereby the voltage of the supply output.

10. A regulated power supply according to claim 9, wherein the temperature dependent means includes one or more diodes coupled so as to conduct during operation of the power supply.

11. A regulated power supply according to claim 9, wherein the temperature means includes a thermistor.

12. A regulated power supply according to claim 9, 10 or 11, wherein the temperature dependent means is disposed within a series circuit including one or more resistances, the series circuit being 13 coupled between the supply output, the feedback input and ground.

13. A regulated power supply according to any one of claims 9 to 12, comprising calibration means for calibrating the voltage of the supply output.

14. A regulated power supply according to claim 13, wherein the calibration means includes a plurality of resistances connected to the or a series circuit coupled between the supply output, the feedback input and ground and operative to contribute to the voltage level at the feedback input; the regulated power supply comprising control means operative to determine the voltage level at the feedback input at a predetermined temperature and to couple the resistances selectively high or low so as to cause the voltage at the feedback input to be equal to a reference bandgap voltage.

15. A regulated power supply according to claim 13, wherein the calibration means includes a variable resistance in the or a series circuit coupled between the supply output, the feedback input and ground.

16. A liquid crystal display circuit substantially as hereinbefore described with reference to and as illustrated in Figure 2 of the accompanying drawings.

17. A regulated power supply substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.