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LM1204 データシート(PDF) 10 Page - National Semiconductor (TI) |
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LM1204 データシート(HTML) 10 Page - National Semiconductor (TI) |
10 / 20 page Applications Hints The LM1204 is a wideband video amplifier system designed specifically for high resolution RGB CRT monitors The de- vice includes circuitry for DC restoration of video signals and also allows contrast and brightness control DC restora- tion is done during the back porch interval of the video sig- nal An internal sync separator generates a back porch clamp signal either from a ‘‘Sync on Green’’ signal applied to the composite video input (pin 18) or from an externally supplied gH Sync signal The LM1204 first looks at the g H Sync input (pin 23) if an external horizontal sync signal is not present then the device syncs off the composite video input The internally generated back porch clamp pulse width is user adjustable A blanking function is also included This allows the user to cutoff the beam current in the CRT’s guns during the blank- ing interval thereby preventing horizontal retrace lines from being visible Normally blanking is done by applying a high voltage pulse at the grid However blanking at the cathode using the LM1204 leads to ease of design and lowered cost Figure 2 shows the block diagram of the green video chan- nel and the control logic The two modes of operation with and without blanking are described below in detail 10 Operation without Blanking For operation without blanking the blank gate input (pin 24) should be connected to a4V This causes the blank com- parator to connect switch S2 to position Y (See Figure 2 ) Furthermore the brightness control input pin (pin 20) should be biased at a potential between 1V (Min) and 38V (Max) it is best to bias this pin at 2V The video signal is AC coupled to the input of the LM1204 as shown for the green channel in Figure 2 During the back porch interval of the video sig- nal (See Figure 3 ) the internally generated back porch clamping pulse goes low causing switches S1A and S1B to be closed The closure of S1A causes gm1 to charge capac- itor C2 to a potential determined by the DC voltage at pin 20 This allows gm1 to set up an average DC bias for the AC coupled video signal at the input of A1 When the back porch clamping pulse is high S1A and S1B are opened With S1A open gm1 is effectively disconnected from C2 C2 now holds the DC bias voltage The transconductance stage gm1 therefore functions as a sample and hold device and holds the input of A1 at the desired DC bias The LM1204 uses black level clamping at the back porch of the video signal to accomplish DC restoration The trans- conductance stage gm2 is enabled during the back porch clamp period to provide a sample and hold function During the back porch clamp period DC feedback from LM1204’s video output is compared with the voltage set by potentiom- eter R9 Depending on A2’s output voltage C6 is either charged or discharged so that the feedback loop consisting of gm2 and A2 is stabilized and the output is clamped to the black level All this occurs during the back porch clamp peri- od During the video portion of the signal gm2 is disabled and C6 holds the fixed black level reference voltage The beginning of each new line on the raster always starts from a fixed reference black level thus restoring the DC compo- nent of each line A2 is a summing amplifier that adds a DC offset component from gm2 to the video signal from the multiplier Adjusting R9 will DC offset the output signals of all three channels thus providing brightness control Individual cutoff adjust- ment for each channel is done by varying the feedback volt- age at each of the R G and B feedback inputs (Pins 38 28 and 33) For example cutoff adjustment for the green chan- nel is done by potentiometer R8 shown in Figure 2 Adjusting the contrast control (potentiometer R3 in Figure 2 ) varies the peak to peak amplitude (includes sync tip if present) of all three video output signals relative to their black reference level The D Gain adjust (pins 9 15 and 13 for R G and B channels respectively) allows the user to individually adjust the AC gain of each channel For example the AC gain of the green channel is adjusted using potenti- ometer R5 as shown in Figure 2 Normally the red channel is set for maximum gain and the gains of the blue and green channels are reduced until white balance is achieved on the CRT monitor’s screen Figure 4 shows the adjustments for operation without blanking 20 Operation with Blanking Much of what was discussed in Section 10 also applies when the LM1204 is used with the blanking function How- ever there are notable differences as described herein For operation with blanking a TTL compatible blanking signal must be applied to the blank gate input (pin 24) During the blanking period the blanking comparator con- nects switch S2 to position X (See Figure 2 ) This causes the LM1204 to level shift the video output signal to the blank level Adjusting R9 will adjust the blank level of all three channels Individual blank level adjustment for each chan- nel is done by varying the feedback voltage at each of the R G and B feedback inputs (pin 38 28 and 33) In Figure 2 this is done by adjusting potentiometer R8 for the green channel During the video portion of the video signal S2 is connected to position Y Brightness control is now accomplished by varying the potential at the brightness control pin (pin 20) Adjusting R6 offsets the video portion of all three output signals relative to the fixed blank level restoring the DC level of the video signal Figure 5 shows the adjustments for operation with blanking 30 Stability Considerations For optimum performance and stable operation a double sided PC board with adequate ground plane is essential Moreover soldering the LM1204 on to the PC board will yield best results Each supply pin (pins 2 4 6 19 31 36 41 and 44) should be bypassed with a 001 mF and a 01 mF capacitor connected as close to the supply pin as is possi- ble When driving the LM1204 from a 75X video source the cable is terminated with 75X to minimize reflections caused by transmission line effects However the input impedance of LM1204 is capacitive and is also affected by the stray capacitance of the PC board Thus the input impedance is a function of frequency This changes the impedance of the cable termination This can introduce overshoot and ringing in LM1204’s pulse response A 100X resistor in series with the blocking capacitor at the video input will minimize over- shoot and ringing (see Figure 8 ) The value of the resistor is empirically determined 100X is a good starting value Since the LM1204 is a wide bandwidth amplifier with high gain at high frequencies the device may oscillate when driv- ing a large capacitiveinductive load To prevent oscillation the amplifier’s gain is rolled off at high frequencies This is accomplished by an RC network comprised of a resistor in 10 |
同様の部品番号 - LM1204 |
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同様の説明 - LM1204 |
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