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LM1253 データシート(PDF) 11 Page - National Semiconductor (TI) |
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LM1253 データシート(HTML) 11 Page - National Semiconductor (TI) |
11 / 57 page Pin Descriptions (Continued) R LIMITH is set to limit the input current into the IC to a maxi- mum value of +1 mA during flyback and −150 µA during nor- mal forward scan. For example if an h flyback with a peak of 100V is used, R LIMITH = 100 kΩ.The internal input imped- ance of pin 14 is low to limit the maximum voltage swing at the input to within the supply rail and ground. The IC inter- face circuit creates a digital signal from this waveform, which is used as the blanking signal, and termed HBLANK. This signal is used by the video amplifier for blanking the video and by the OSD generator as the horizontal sync reference for the PLL. Loss of the horizontal flyback pulse implies that the monitor is not scanning, and therefore no image is being displayed. The HBLANK pulse is still required by the LM2453 CRT driver in order to maintain correct bias conditions in the CRT until the power supplies are switched off, but video will be set at black level to prevent front of screen problems, using the NO_VID line. See also the Loss of Horizontal Flyback sec- tion. Figure 29 shows the external schematic of pin 14, Figure 18 shows the internal schematic. Pin 15 — V Flyback This is an analog signal from the moni- tor vertical scan. The analog waveform is AC coupled and fed to the input of the IC via a current limiting resistor to pre- vent the positive and negative excursions of the signal caus- ing excessive current or voltage swing at the input to the IC. See Figure 10. R LIMITV is set to limit the maximum input voltage swing into the IC to less than the supply rails. The input to the IC is positive edge triggered, and ignores the falling edge. Be- cause of horizontal rate noise on the waveform, the input buffer incorporates hysteresis, triggering at a positive going threshold of V VTH+ and a negative going threshold of VVTH. The input buffer produces a digital signal VSTART which is used to start the VBLANK timer. The positive rising edge of VSTART sets a counter timer, which counts horizontal peri- ods using the HBLANK signal. The timer resets VBLANK when it reaches the value preset in the register VCOUNT (set by the micro-controller over I 2C). While the output VBLANK is active, an AND function prevents any further transitions on the VSTART waveform from retriggering the counter. The positive edge of the VSTART signal is initially transmit- ted through to VBLANK through an OR function, as the timer may take up to one horizontal line period to begin timing the duration of the pulse. The application must ensure that the DS101265-13 FIGURE 9. H Flyback Input Pulse www.national.com 11 |
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同様の説明 - LM1253 |
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