The invention relates to a method for driving a back light emitter of a display apparatus, a circuit applying said method and an apparatus comprising such circuit according to the preambles of claim 1, 5 and 9 respectively. In particular the light emitter is a fluorescent lamp and more in particular the display apparatus comprises several adjacent fluorescent lamps arranged at the back of a display panel of such an apparatus.
A method, a circuit and an apparatus of the above type are known from practice. Usually each lamp is driven by its own driver circuit. Both such driver circuits and the lamps may have different characteristics. Since the human perception of light is sensitive for differences rather than absolute amounts of light energies emitted by such lamps, both the drivers and the lamps must be matched as much as possible to have the lamps emit the same amounts of light energy for identical drive signals. It is difficult to meet the tight specifications of components which are necessary for preventing unwanted perception of different light outputs from different lamps at reasonable costs. It will be even more difficult with AC bursts becoming shorter. This is, for example, the case when the lamps are driven sequentially, that is, in a so-called scanning mode, which is presently applied for liquid crystal display television applications.
It is an object of the invention to solve the drawbacks of the above method and circuit of the types described.
The above object of the invention is achieved by providing a method as described in claim 1.
With said method the amplitude of AC bursts passing through the lamp are of no concern, provided that they are sufficient to provide the light energies wanted during periods of time which may be adjusted according to the method.
The method can be implemented by hardware and/or software, the choice of which may be dependent from frequencies of signals of concern.
The above object of the invention is also achieved by providing a circuit as described in claim 5.
The above object of the invention is also achieved by providing a display apparatus as described in claim 9.
The invention will become more gradually apparent from the following exemplary description in connection with the accompanying drawings. In the drawings:
A today's display apparatus usually comprises a flat screen device, such as a selectively light translucent liquid crystal display (LCD), which is driven by control circuitry of the apparatus.
The diagram shown in
As is knows from practice, a light emitter 4 can be driven continuously or discontinuously. In the latter case alternating current (AC) bursts having a high frequency, such as 50 kHz are generated by an AC pulse generator, such as generator 6 shown in
An AC pulse has typically an on-time of 3 ms and an off-time between two succeeding bursts of about 10 ms. The average light output of a light emitter 4 is dependent on the amplitude of a current through the lamp and of the duration of the AC bursts. Since characteristics of different light emitters 4 and different drivers 8 may be different and may alter in time, it is virtually impossible to obtain and maintain uniform light outputs from different light emitters 4 when using identical AC bursts from an AC burst generator.
According to the invention the prior art circuit comprising an AC pulse generator 6, a driver 8 for driving a light emitter 4 only, is improved by the addition of a control loop, by which at least one entity representing the light being emitted by a light emitter 4 is sensed, the entity is then compared to a reference for providing an error signal, and the error signal is used to adjust the loop such as to minimize said error in time.
Therefore, according to the invention, for each light emitter 4 a current sensor 10 is arranged in series with the driver 8 and the light emitter 4. Current sensor 10 senses a current i flowing through the light emitter 4. The top line of
The inventors conceived that the amount of light emitted during the on-time of a light emitter 4 is determined by the amount of electrical charge that is passed through the light emitter 4 in the first place. It was found that a very good estimation of said amount of electrical charge can be obtained by creating an envelope signal from said current i. Accordingly, the circuit comprises an envelope generator 11 for generating such envelop signal |î|, of which the center line of
The envelope signal |î| is integrated by an integrator 12. Since the envelope signal |î| has some an amplitude and a duration dependent on the current i flowing through the light emitter 4 and since it is integrated in time, the integrated envelope signal provides a representation of the amount of electrical charge Q passed through the lamp and thus provides a representation of the amount of light emitted by the light emitter 4. At the bottom line of
A comparator 14 compares the emitted light representing signal Q and a reference signal REF1 to therewith provide an error signal.
The error signal is supplied to a modulator 16 which is arranged in series between the AC pulse generator 6 and the current sensor 10, such as between the AC pulse generator 6 and the driver 8.
Modulator 16 modulates the duration of the AC bursts which it receives from the AC generator 6 dependent on the error signal supplied by comparator 14. Modulator 16 is arranged to vary the duration of the AC bursts to minimize said error signal.
For the purpose of varying the duration of AC bursts, modulator 16 will take a sample from the error signal after termination of an AC burst, for example at a time t4 shown in
Making and maintaining accurate time settings is difficult, in particular in cases where the plurality of light emitters 4 are not energized at the same time, but alternating the one after the other. This type of operation of light emitters, called scanning mode operation, is known from television applications for avoiding a smeared perception by a watcher.
At sampling times t4 and t9 the integration value Q is larger and smaller than the other respectively. Supposing that a value of the reference signal REF lies between said values of Q, modulator 16 will decrease the duration of a next AC burst after taking a sample of the larger integration value Q at t4.
The diagram of the second embodiment of the circuit according to the invention shown in
The diagram of the second embodiment of the circuit according to the invention shown in
In both cases as illustrated by
The reference signal REF may be fixed or may be varied. In the latter case it can be used to dim the emission of light by all or a specific one of the light emitters 4 on demand from some control circuit.
Dependent on the frequencies, duration of on-time, duration of off-time of AC bursts, the accuracy required for modulating the AC bursts, specifications and costs of analogue/digital circuits and other circuits any combination of the components 10-18 of the circuit according to the invention can be implemented by hardware only or by hardware which operates by the use of software.
Number | Date | Country | Kind |
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04103142.8 | Jul 2004 | EP | regional |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/IB2005/052163 | 6/29/2005 | WO | 00 | 12/19/2006 |