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LM1279 データシート(PDF) 7 Page - National Semiconductor (TI) |
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LM1279 データシート(HTML) 7 Page - National Semiconductor (TI) |
7 / 14 page Pin Descriptions (Continued) Pin No. Pin Name Schematic Description 16 V CC2 Power supply pin for the output stage. There are no internal connections to V CC1. ESD and Arc-Over Protection The ESD cells of the LM1279 are improved over the ESD cells used in typical video pre-amps. The monitor designer must still use good PC board layout techniques when de- signing with the LM1279. The human body model ESD sus- ceptibility of these parts is rated at 3 kV (Note 4). However, many monitor manufacturers are now testing their monitors to the level 4 of the IEC 801-2 specification. This requires the inputs to the monitor to survive an 8 kV discharge. If the monitor designer expects to survive such levels he MUST provide external ESD protection to the video pre-amp inputs! PC board layout is very important with LM1279 as with other video pre-amps. The LM1279 provides excellent protection against ESD and arc-over, but the LM1279 is not a substitute for good PCB layout. Figure 6 shows the recommended input protection for a video pre-amp. The two diodes at the video pre-amp input and after the 30 Ω series resistor offers the best protection against ESD. When this protection is combined with a good PCB layout, the video pre-amp will easily survive the IEC 801-2 level 4 (8 kV ESD) testing commonly done by monitor manufacturers. If the protection diodes are moved to the video input side of the 30 Ω resistor, then the ESD protection will be less effective. There is also the risk of damaging the diodes since there is no resistor for current limiting. In such a design a heavier duty diode, such as the FDH400, should be used. It is strongly recommended that the protection di- odes be added as shown in Figure 6. The 1N4148 diode has a maximum capacitance of 4 pF. This would have little affect on the response of the video system due to the low imped- ance of the input video. Many monitor designers prefer to use a single zener diode instead of the recommended two diodes at the video pre- amp input. The required location of the zener diode is shown in Figure 6. It is shown as a dashed line, indicating an alter- native to the two diode solution. The zener diode does give the savings of one component, but now the protection is less effective. To minimize capacitance, the zener diode needs to have a zener voltage of 24V or higher. This is well above the V CC voltage of the LM1279. The zener diode must be lo- cated at the video input for protection against a low voltage surge. The 30 Ω resistor is needed to limit the current of such a voltage surge, protecting the video pre-amp. Protection against ESD by using a zener diode is about as effective as having the two diodes at the video input (same location as the zener diode). A higher series resistor may be necessary for protection against the zener voltage, but the higher resis- tor value will impair the performance of the LM1279; result- ing in a lower bandwidth and a less stable black level. For maximum reliability the monitor designer should not consider the zener diode solution for ESD protection of the LM1279. The ESD cells of the LM1279 also gives good tolerance against arc-over. Once again the monitor designer must be careful in his PCB layout for good arc-over protection. In the video chain only the outputs of the CRT driver are directly exposed to the voltages that may occur during arc-over. A good PCB layout is the best protection for the video pre-amp against arc-over. The pre-amp vulnerability is mainly through the ground traces on the PCB. For proper operation all ground connections associated with the video pre-amp, in- cluding the grounds to the bypass capacitors, must have short returns to the ground pins. A significant ground plane should be used to connect all the pre-amp grounds. Figure 16, the demo board layout, is an excellent example on an ef- fective ground plane, yet using only a single sided PCB lay- out. Here is a check list to make sure a PC board layout has good grounding: • All associated grounds with the video pre-amp are con- nected together through a large ground plane. • CRT driver ground is connected to the video pre-amp ground at one point. • CRT and arc protection grounds are connected directly to chassis, or the main ground. There is no arc-over current flow from these grounds through the pre-amp or CRT driver grounds. If any one of the above suggestions are not followed, then the LM1279 may become vulnerable to arc-over. Improper grounding is by far the most common cause of a video pre- amp failure during arc-over. Functional Description Figure 1 on the front page shows the block diagram of the LM1279 along with the pinout of the IC. Each channel re- ceives a video input signal at its input amplifier (-A1). The output of the input amplifier goes to the contrast attenuator stage. For easy interfacing to 5V DACs all controls inputs, in- cluding the contrast control, use a 0V to 4V range. The con- trast control has no attenuation with an input of 4V, and has full attenuation (over −40 dB) with a 0V input. All three chan- nels will accurately track the contrast control setting at pin 10. Each channel will have the same amount of attenuation for a given input voltage typically to within ±0.3 dB. All chan- nels will track because the contrast control is the first stage of attenuation and the internal control voltage generated from the input voltage is common to all three channels. The output of the contrast attenuator goes to the drive at- tenuator. This stage has a 12 dB control range. This stage is used for color balance, so the adjustment range has been limited to 12 dB for a more accurate color balance. Each DS100091-12 FIGURE 6. Recommended Video Input ESD Protection www.national.com 7 |
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