DVI Applications by forrests


									                                                                                                                   # 10

DVI Applications

The Digital Visual Interface specification from the Digital          When both sets of channels are used, odd pixel data are
Display Working Group (DDWG) has standardized the                    encoded on Channels 0- 2, and even pixel data are
means for clocking pixel data discretely to the monitor or           encoded on Channels 3-5.
flat panel display.
                                                                     This creates a potential difficulty for protection device
CRT displays provide an essentially continuous row of                partitioning from the 26-pin DVI connector.
pixels that are scanned by a constant velocity electron
beam, thereby tracing as many pixels onto a screen as                The Single Channel TMDS configuration requires 4
the bandwidth of the RGB signals will allow.                         differential signals (Clock + Channels 0-2), or
                                                                     8 lines to be protected from transients and ESD.
Plasma and LCD displays, however, have discrete pixel
grids that must match exactly with the X-Y image                     The Dual Channel TMDS configuration requires an
driven from the video graphics chip. Many of these                   additional 6 lines of protection for Channels 3-5.
discrete grid displays include Digital-to-Analog frame
conversion logic to synchronize standard analog VGA
RGB signals to the LCD / Plasma X-Y grid.

Even a very small timing difference between the graphics
chip and the frame recapture logic can seriously
deteriorate and alias the image quality, as the pixilated
image content is interpolated to “fit” the fixed X-Y grid.

                                                                                  FIG 2 DVI layout using 4 PLR0502

                                                                     Using discrete protection devices of any type can cause
                                                                     imbalances in differential loading, since parasitic load can
                                                                     vary substantially, sometimes by even more than their
                                                                     tolerances for specified characteristics.

                                                                     Figure 2 shows a sample layout for a Single Channel DVI
                                                                     connector with four PLR0502’s.
                               FIG 1
                                                                     By utilizing one PLR0502 dual channel device on each
The DVI interface provides a standard method to transfer             differential pair, the designer can guarantee the close load
encoded digital pixel data synchronized to a master clock            matching between each (+) and (-) signal, since the dual
that can be used in LCD and Plasma displays to eliminate             devices are fabricated on the same silicon die.
this aliasing and image degradation.
                                                                     An additional method is to use two PLR0504 as shown in
The encoding scheme uses “Transition Minimized                       Fig 3 to decrease board space usage. The quad die is
Differential Signaling,” or TMDS, across 4 or 7 differential         fabricated on the same silicon thus providing excellent load
pairs, see Figure 1. DVI adds the additional protection              matching.
challenge of limiting digital clock/data skew, above and
beyond the inherent bandwidth and loading issues of
high frequency VGA signals. Data Channels 3 through 5
are optional, and may be populated in some applications,
and omitted in other lower cost designs.

2929 South Fair Lane • Tempe, Arizona • 85282
Tel: 602-431-8101 • Fax: 602-431-2288 • Web: www.protekdevices.com
                                                                                                                  Page 1 of 2
                                                                                        # 10


                                                                     DFN-16 PLR0508

                     FIG.3 using 2 PLR0504

Alternatively, to provide the maximum channel-to-channel
matching, a PLR0508 can be used as shown in Figure 4.

                                                                     SOT-543 PLR0502

                                                                     SOT-563 PLR0504
            FIG.4 One PLR0508 protecting all lines

Finally, a combination of PLR0508, PLR0504, and/or
PLR0502 packages can be used to protect a Dual
Channel DVI port.

The combination low clamping voltages, ultra low
capacitance, low leakage current, low parasitic load and
close port-to-port matching makes the PLR series ideal
for DVI port protection.

2929 South Fair Lane • Tempe, Arizona • 85282
Tel: 602-431-8101 • Fax: 602-431-2288 • Web: www.protekdevices.com
                                                                                       Page 2 of 2

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