Patent Application
20100171773
The invention relates to projection devices equipped with semiconductor light sources. Microdisplay applications for front and rear projection can primarily be mentioned here.
Recently, powerful semiconductor light sources such as high-power light emitting diodes have increasingly been used in applications which had previously been reserved for high-pressure discharge lamps. Precisely in the field of projection, the semiconductor light sources are not driven continuously, but rather operated in pulsed fashion in order to meet the requirements there. In that case, very short pulses are employed, which in return have a very high power density.
A plurality of colors, usually at least 3 colors, are used in display applications. The drive circuit is divided into video electronics, which condition the input signal, and driver electronics, which are often split into a digital and an analogue part connected via a digital/analogue converter. In accordance with the input signal, the video electronics pass instructions to the driver electronics to switch on a specific color. In the digital part of the driver electronics, a process is thereupon initiated which, after a specific time, results in the outputting of a brightness value for the desired color. Said brightness value and the timing are held in a table generated by the video electronics. In response to the request of the video electronics, the so-called strobe, the digital driver electronics read out said table and generate a desired value signal on the basis of the values read out. The analogue driver part thereupon switches on the corresponding colored semiconductor light source with the desired brightness. Since the colors are projected successively in display applications, the semiconductor light sources have to be switched off again after the pulse that has been output in order that the next color can be projected. This requires a further cycle in the digital driver electronics. A zero value has to be input into the buffer memory of the analogue/digital converter and said value then has to be output as analogue value, whereupon the analogue part switches off the semiconductor light source again.
Therefore, a certain time elapses between the strobe signal and the actual switching on or off of the semiconductor light source. In the case of switching on the semiconductor light sources this does not pose a problem since this delay time is known to the video electronics and concommitently taken into account in the timing. In the case of switch-off, however, this can pose a problem since the minimum switched-on duration is dependent on the processing speed for the driver electronics. For this reason, the driver electronics must have a fast logic and fast analogue/digital converter since otherwise they cannot achieve the required processing speed. However, fast analogue/digital converters and a fast logic (e.g. a highly clocked microcontroller) are very cost-intensive, which has a disadvantageous effect on the pricing of the end product.
Therefore, it is an object of the invention to propose a drive method and a circuit arrangement which no longer have the disadvantages mentioned above and manage with a slower and therefore more cost-effective logic.
This object is achieved by means of the features of patent claim 1 and of method claim 7. Particularly advantageous embodiments of the invention are described in the dependent claims.
The invention is based on the fact that the semiconductor light sources is always switched off after the pulsed light emission. This makes it possible to input a new value into the analogue/digital converter only once and to switch the semiconductor light source on and off by means of a switch. This has the advantage that slow components can be used without restricting the performance of the overall arrangement.
The block diagram of a circuit arrangement according to the prior art is shown in
The block 21 represents the read-out of the table. This begins at the instant T1, as soon as the timing signal is logic 1. As soon the values from the table have been determined, they are written to the buffer memory of the digital/analogue converter (block 23). In block 25, as soon as the buffer memory has been completely updated, the digital/analogue converter is put into action, with the result that an analogue signal corresponding to the digital value is present at the analogue output. The driver circuit requires the time tLOG1 for all this. As soon as the analogue signal is output, the analogue part of the driver circuit switches on the LED modules 5. This process requires the time tDACset1.
At the instant T3, the timing signal becomes logic zero again. At this instant, the entire procedure is run through a further time in order to set the analogue value to zero again and thus to switch off the LED module. The time between the instants T1 and T2 is equal to the timing signal duration tSTB. The processing time tLOG1 is crucially responsible for the minimum pulse duration of the entire arrangement, because if the duration of the processing time tLOG1 comes close to the minimum timing signal duration tSTB.min, the timing can no longer be complied with. The minimum pulse length and thus the minimum timing signal duration tSTB.min are therefore directly dependent on the processing speed of the driver circuit.
This is the starting point for the invention. According to the invention, the analogue/digital converter is used only for setting the brightness, with the result that, for each color, said converter now only has to run through the conversion procedure once instead of twice. A switch is arranged between the analogue output downstream of the control logic 3 and the LED module, which switch either switches the analogue value of the analogue/digital converter through the LED module or switches the analogue input of the LED module to ground, which corresponds to a logic zero. The signals and actions of a drive method according to the present invention are shown in the lower third of
The signal 51 describes the output of the digital/analogue converter. The latter is switched over as soon as the digital processing is concluded. This takes the time tLOG2, which is longer than the time tLOG1. As soon as the analogue/digital converter has reliably switched over, the switch SX of the corresponding color is switched on (signal 53). The power driver thus receives the analogue signal 51 from the digital/analogue converter and switches on the LEDs (signals 55, 57).
After the time tSTB, that is to say the pulse duration, the switch is switched to ground again, whereupon the LEDs are switched off. In this case, the analogue/digital converter remains at its original value, that is to say that the analogue/digital converter signal is not brought to zero. For the next cycle, the analogue/digital converter is merely set to the new brightness.
By virtue of the fact that the analogue/digital converter only operates for setting the brightness, the processing time is significantly less critical. The analogue/digital converter, after all only operates before the pulse, that is to say that the delay time of the converter is unimportant for the minimum pulse length. The invention, on the contrary, has the major advantage of being able to realize very short minimum pulse lengths, since practically no processing has to take place during the pulse, rather the majority of the processing can take place during the pulse intermissions. Since the longer processing time before the pulse can easily be compensated for by the video electronics, the drive method according to the invention has no disadvantages whatsoever even in the case of components operating more slowly.
The time available to the driver electronics to accomplish other things as can be seen from block 27, is likewise lengthened by comparison with the prior art. A further advantage is that this time can occur during the pulse and during the pulse intermissions.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/EP07/55792 | 6/13/2007 | WO | 00 | 12/4/2009 |
