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LM1279 データシート(PDF) 9 Page - Texas Instruments |
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LM1279 データシート(HTML) 9 Page - Texas Instruments |
9 / 20 page OBSOLETE LM1279 www.ti.com SNOS020B – SEPTEMBER 1998 – REVISED APRIL 2013 (continued) Pin Pin Schematic Description No. Name 11 Sandcastle Input The sandcastle input allows for blanking only, or blanking with DC restoration. Blanking requires a 2V input. Clamping with blanking requires a 4V input. 13 Blue Video Out Video output. For proper black level the output must drive 390 Ω impedance. 15 Green Video Out 18 Red Video Out 16 VCC2 Power supply pin for the output stage. There are no internal connections to VCC1. 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 designing with the LM1279. The human body model ESD susceptibility of these parts is rated at 3 kV(1). 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 diodes be added as shown in Figure 6. 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 impedance 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 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 VCC voltage of the LM1279. 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 the higher resistor value will impair the performance of the LM1279; resulting 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. (1) Human body model, 100 pF discharged through a 1.5 k Ω resistor. Copyright © 1998–2013, Texas Instruments Incorporated Submit Documentation Feedback 9 Product Folder Links: LM1279 |
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同様の説明 - LM1279 |
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