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LM1203A データシート(PDF) 7 Page - National Semiconductor (TI) |
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LM1203A データシート(HTML) 7 Page - National Semiconductor (TI) |
7 / 18 page Applications Information Figure 4 shows the block diagram of a typical analog RGB color monitor The RGB monitor is used with CADCAM work stations PC’s arcade games and in a wide range of other applications that benefit from the use of color display terminals The RGB color monitor characteristics may differ in such ways as sweep rates screen size CRT color trio spacing (dot pitch) or in video amplifier bandwidths but will still be generally configured as shown in Figure 4 Separate horizontal and vertical sync signals may be required or they may be contained in the green video input signal The video input signals are usually supplied by coax cable which is terminated in 75X at the monitor input and internally AC coupled to the video amplifiers These input signals are ap- proximately 1V peak to peak in amplitude and at the input of the high voltage video section approximately 6V peak to peak At the cathode of the CRT the video signals can be as high as 60V peak to peak One important requirement of the three video amplifiers is that they match and track each other over the contrast and brightness control range The Figure 4 block labeled ‘‘VIDEO AMPLIFICATION WITH GAIN AND DC CONTROL’’ describes the function of the LM1203A which contains the three matched video amplifi- ers contrast control and brightness control Circuit Description Figure 5 is a block diagram of one of the video amplifiers along with the contrast and brightness controls The con- trast control is a DC-operated attenuator which varies the AC gain of all three amplifiers simultaneously while not intro- ducing any signal distortions or tracking errors The bright- ness control function requires a ‘‘sample and hold’’ circuit (black level clamp) which holds the DC bias of the video amplifiers and CRT cathodes constant during the black level reference portion of the video waveform The clamp com- parator when gated on during this reference period will charge or discharge the clamp capacitor until the plus input of the clamp comparator matches that of the minus input voltage which was set by the brightness control VIDEO AMPLIFIER SECTION Figure 6 is a simplified schematic of one of the three video amplifiers along with the recommended external compo- nents The IC pin numbers are circled and all external com- ponents are shown outside the dashed line The video input is applied to pin 6 via a 10 mF coupling capacitor DC bias for the video input is through the 10k resistor connected to the 28V reference at pin 11 The low frequency roll-off of the amplifier is set by these two components Transistor Q1 buffers the video signal to the base of Q2 Q2’s collector current is then directed to the VCC1 supply directly or through the 2k load resistor depending upon the differential DC voltage at the bases of Q3 and Q4 This differential DC voltage is generated by the contrast control circuit which is described in the following sections A 001 mF decoupling capacitor in series with a 30X resistor is required between pins 2 and 3 to ensure high frequency isolation between the three video amplifiers which share these common connec- tions The video signal is buffered by Q5 and Q6 and DC level shifted by the voltage drop across R5 The magnitude of the current through R5 is determined by the voltage at pin 8 The voltage at pin 8 is set by the clamp comparator out- put current which charges or discharges the clamp hold ca- pacitor during the black level period of the video waveform Transistors Q9 and Q10 are Darlington connected to ensure a minimum discharge of the clamp hold capacitor during the time that the clamp capacitor is gated off Q7 Q8 and R6 form a current mirror which sets a voltage at the base of Q11 Q11 buffers the video signal to the base of Q12 which provides additional signal gain The ‘‘Drive’’ pin allows the user to trim the Q12 gain of each amplifier to correct for gain differences in the CRT and high voltage cathode driver gain stages A small capacitor (several pico-Farads) from the ‘‘Drive’’ pin to ground will cause high frequency peaking and slightly improve the amplifier’s bandwidth TLH11441 – 8 FIGURE 4 Typical RGB Color Monitor Block Diagram http www nationalcom 7 |
同様の部品番号 - LM1203A |
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同様の説明 - LM1203A |
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