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Datenblatt-Suchmaschine für elektronische Bauteile |
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Datenblatt-Suchmaschine für elektronische Bauteile |
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LM1207A Datenblatt(PDF) 11 Page - Texas Instruments |
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LM1207A Datenblatt(HTML) 11 Page - Texas Instruments |
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11 / 26 page  ESD and Arc-Over Protection The ESD cells of the LM1205A/LM1207A are improved over the original LM1205/LM1207 parts. The monitor designer must still use good PC board layout techniques when de- signing with the LM1205A/LM1207A. The human body model ESD susceptibility of these parts is rated at 3 kV (Note 4) . This is a 50% improvement over the original parts. How- ever, many monitor manufacturers are now testing their monitors to the level 4 of the IEC 801-2 specification. This re- quires 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 just as important with these new parts as with the original LM1205/LM1207 parts. The advan- tage given by the LM1205A/LM1207A will provide better pro- tection against ESD and arc-over, but these improved parts are not a substitute for good PCB layout. Figure 5 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 5. The 1N4148 diode has a maximum capacitance of 4 pF. This would have little effect on the response of the video system due to the low imped- ances 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 5. It is shown as a dashed line, indicating an alternative 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 LM1205A/LM1207A. The zener diode must be located 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 this may also have an effect on the system bandwidth. The new ESD cells of the LM1205A/LM1207A also in- creases its tolerance against arc-over. Once again the moni- tor designer must be careful in his PCB layout for good ESD 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 vulnerabil- ity is mainly through the ground traces on the PCB. For proper operation all ground connections associated with the video pre-amp, including the grounds to the bypass capaci- tors, 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 of an effective ground plane, yet using only a single sided PCB layout. 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 LM1205A/LM1207A may become vulnerable to arc-overs. Improper grounding is by far the most common cause of a video pre-amp failure during arc-over. DS012866-30 FIGURE 5. Recommended Video Input ESD Protection www.national.com 10 |
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Ähnliche Beschreibung - LM1207A |
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