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LM1279 Folha de dados(PDF) 9 Page - National Semiconductor (TI) |
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LM1279 Folha de dados(HTML) 9 Page - National Semiconductor (TI) |
9 / 14 page Applications of the LM1279 (Continued) age surges that may result during the power up and power down modes, or when connecting the monitor to other equip- ment. The monitor designer must include these resistors in his design for good monitor reliability. If additional protection against ESD at the video inputs is necessary, then adding clamp diodes on the IC side of the 30 Ω resistors is recom- mended, one to V CC1 and one to ground (see Figure 6). Sometimes a designer may want to increase the value of the 30 Ω resistors at the video inputs, for additional ESD protec- tion. This is not recommended with the LM1279. C5, C7, and C9 are part of the DC restoration circuit. This circuit is de- pending on a total maximum circuit resistance of about 110 Ω;30Ω input series resistor plus 75Ω for the video termi- nation resistor. Increasing the value of the 30 Ω resistors will exceed the 110 Ω limit. The excellent internal ESD protection and the external clamp diodes (if needed) will provide excel- lent ESD protection. The 30 Ω resistors in series with the OSD inputs are also necessary if the OSD signals are external to the monitor, or if these signals are present any time when the +8V is not fully powered up. Interfacing to the OSD inputs is quite easy since the signal processing necessary to match the OSD sig- nals to the video levels is done internal by the LM1279. There is also no need for an OSD window signal. Any time there is a high TTL signal at any of the three OSD inputs, the LM1279 will automatically switch to the OSD mode. A high TTL OSD signal will give a high video output for that color. The OSD level is fixed, typically 2.3V above the video black level. This will give a fixed brightness to the OSD window, but not at maximum video brightness which could be un- pleasant to the user. Figure 2 and Figure 3 show the timing diagrams of the OSD signals for the LM1279. The recommended load impedance for the LM1279 is 390 Ω. However, some changes in the load impedance can be made. If the load impedance is reduced, the monitor de- signer must confirm that the part is still operating in its proper die temperature range, never exceeding a die temperature of 150˚C. When changing the load impedance, the black level shift is shown in the chart below. The measured V P-P output with under 1% distortion is also listed. Load V P-P Blk. Level Shift 430 Ω 3.62V +15 mV 390 Ω 3.62V 0 mV 330 Ω 3.58V −25 mV 270 Ω 3.51V −45 mV When using a lower load impedance, the LM1279 does go into hard clipping more quickly. This does reduce the head- room of the video output. Board layout is always critical in a high frequency application such as using the LM1279. A poor layout can result in ringing of the video waveform after sudden transitions, or the part could actually oscillate. A good ground plane and proper routing of the +8V are important steps to a good PCB layout. The LM1279 does require very good coupling between V CC1 and V CC2 (pins 2 and 16). This is clearly shown in Figure 7 and Figure 11 with the short and large trace between pins 2 and 16. Both demonstration boards offer the monitor de- signer an excellent example of good ground plane being used with the LM1279. These boards are single sided, yet al- low the LM1279 to operate at its peak performance. The neck board also shows a good example of interfacing to a CRT driver and to the CRT. The video signal path is kept as short as possible between the LM1279 and the CRT driver, and also between the CRT driver and the CRT socket. Actual performance of the LM1279 in the video pre-amp demon- stration board is shown in Figure 8 and Figure 9. References Ott, Henry W. Noise Reduction Techniques in Electronic Systems, John Wiley & Sons, New York, 1976 Zahid Rahim, “Guide to CRT Video Design,” Application Note 861, National Semiconductor Corp., Jan. 1993 www.national.com 9 |
Nº de peça semelhante - LM1279 |
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Descrição semelhante - LM1279 |
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