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LM1201M データシート(PDF) 7 Page - National Semiconductor (TI) |
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LM1201M データシート(HTML) 7 Page - National Semiconductor (TI) |
7 / 16 page Circuit Description (Continued) The ‘‘Drive’’ pin will allow the user to set the maximum gain of the amplifier based on the range of input video signal levels and the CRT stage gain if it is fixed or limited When using three LM1201 devices for high resolution RGB appli- cations the ‘‘Drive’’ pin allows the user to trim the gain of each channel to correct for differences in the three CRT cathodes A small capacitor (12 pF) in shunt with a 51X drive resistor at this pin will extend the high frequency gain of the video amplifier by compensating for some of the inter- nal high frequency roll off The 51X resistor will set the sys- tem gain to approximately 8 or 18 dB The video signal at the collector of Q9 is buffered and level shifted down by Q10 and Q11 to the base of the output emitter follower Q12 Between the emitter of Q12 and the video output pin is a 50X resistor which is included to prevent spurious oscilla- tions when driving capacitive loads An external emitter re- sistor must be added between the video output pin and ground The value of this resistor should not be less than 330X otherwise package power limitations may be exceed- ed when worst case (high supply max supply current max temp) calculations are made If negative going pulse slewing is a problem because of high capacitive loads (l10 pF) a more efficient method of emitter pull down would be to con- nect a suitable resistor to a negative supply voltage This has the effect of a current source pull down when the minus supply voltage is b12V and the emitter current is approxi- mately 10 mA The system gain will also increase slightly because less signal will be lost across the internal 50X re- sistor Precautions must be taken to prevent the video output pin from going below ground since IC substrate cur- rents may cause erratic operation The collector current from the video output transistor is returned to the power supply at VCC3 pin 10 When making power dissipation cal- culations note that the datasheet specifies only the VCC1 and VCC2 supply currents at 12V The IC power dissipation contribution of VCC3 is dependent upon the video output emitter pull down load In normal operation the minimum black level voltage that can be set at the video output pin is approximately 2V at maximum contrast setting In applications that require a low- er black level voltage a resistor (approximately 16 kX) can be added from pin 3 to ground This has the effect of raising the DC voltage at the collector of Q4 which will extend the range of the black level clamp by allowing Q5 to remain active In applications that require video amplifier shutdown due to fault conditions detected by monitor protection cir- cuits pin 3 and the wiper arms of the contrast and bright- ness controls can be grounded without harming the IC This assumes some series resistance between the top of the control potentiometers and VCC Figure 7 shows the internal construction of the pin 3 26V reference circuit which is used to provide temperature and supply voltage tracking compensation for the video amplifier input The value of the external DC biasing resistors should not be larger than 10 kX when using more than one LM1201 (eg in RGB systems) because minor differences in input bias currents on the individual video amplifiers may cause offsets in gain TLH10006 – 7 FIGURE 7 LM1201 Video Input Voltage Reference and Contrast Control Circuits http www nationalcom 7 |
同様の部品番号 - LM1201M |
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同様の説明 - LM1201M |
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