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LM1208N データシート(PDF) 10 Page - National Semiconductor (TI) |
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LM1208N データシート(HTML) 10 Page - National Semiconductor (TI) |
10 / 23 page Functional Description Figure 6 is a detailed block diagram of the green channel of the LM1208 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 other two video channels are identical to the green channel, only the numbers to the pins unique to each channel are different. The input video is normally terminated into 75 Ω. The termi- nation resistor depends on the impedance of the coax cable being used, 75 Ω being the most common impedance used in video applications. The video signal is AC coupled through a 10 µF capacitor to the input, pin 6. There is no standard for the DC level of a video signal, therefore the signal must be AC coupled to the LM1208. Internal to the LM1208 is a 2.8V reference, giving the input video an offset voltage of 2.8V. This voltage was selected to give the input video enough DC offset to guarantee that the lowest voltage of the video signal at pin 6 is far enough above ground to keep the LM1208 in the active region. The 200 Ω resistor at the input is for ESD protection and for current limiting during any voltage surge that may occur at the input, driving pin 6 above V CC. The input video signal is buffered by −A1. In this circuit descrip- tion an inverting amplifier is shown with a “−” (minus sign) in front of the amplifier designation. The output of −A1 goes to the contrast and drive attenuator sections. The contrast and drive control sections are virtually identical. Both sections take a 0V to 4V input voltage, 4V giving the maximum gain for either the contrast or the drive. This is a high impedance input, allowing for an easy interface to 5V DACs. One may also use 100k potentiometers with no deg- radation in performance. The contrast control section is com- mon to all three channels. It converts the input voltage at pin 12 to a couple of internal DC voltages that control the gain of the contrast attenuator. Referring to the Attenuation vs Con- trast Voltage under typical performance characteristics note that a 4V control voltage results in no attenuation of the video signal. A 0.25V control voltage results in an attenuation of 40 dB. Again note that these internal control voltages are common to all three channels. To minimize crosstalk, these voltages go to pins 1 and 2. Minimizing crosstalk is done by adding the RC network shown in the block diagram ( Figure 6). The 0V to 4V drive control signal comes in on pin 18. Each channel has its own drive section, therefore the crosstalk compensation needed for the contrast control voltages is not required for the drive control, thus no external pins for the drive control. The drive attenuator features a full range gain control over 40 dB. This gives no attenuation of the video signal with a 4V control voltage. A 0.25V control voltage results in an attenuation of 40 dB. The output of the drive attenuator stage goes to A2, the amplifier in the DC restoration section. The video signal goes to the non-inverting input of A2. The inverting side of A2 goes to the output of gm1, the clamp comparator, and the clamp capacitor at pin 8. During the back porch period of the video signal a negative going clamp pulse from pin 14 is applied to the clamp comparator, turning on the comparator. This period is where the black level of the video signal at the output of the LM1208 is compared to the desired black level which is set at pin 19. Figure 7 shows the timing of the clamp pulse relative to the video signal. The clamp capacitor is charged or discharged by gm1, generating the correction voltage needed at the inverting input of A2 to set the video output to the correct DC level. Removing the clamp pulse turns off gm1 with the correction voltage being maintained by the clamp capacitor during active video. Both the clamp pulse and the blank pulse at pin 13 are TTL voltage levels. There are actually two output sections, −A3 and −A4. Both sections have been designed to be identical, except −A4 has more current drive capability. The output transistor shown is part of −A4, but has been shown separately so the user knows the configuration of the output stage. −A3 does not go to the outside world, it is used for feeding back the video signal for DC restoration. Its output goes directly to the inverting input of the clamp comparator via the voltage di- vider formed by the 500 Ω and 4k resistors. −A4 will be close to the same output as −A3 and will temperature track due to the similar design of the two output stages. However, the current at the output of −A4 will be ten times the current at the output of −A3. To balance both outputs, a load resistance of 390 Ω needs to be connected from pin 20, the green video output pin, to ground. Another input to −A4 is the blank pulse. When a negative going blank pulse is applied to pin 13, the output of the LM1208 is driven to less than 0.1V above ground. Using the timing shown in Figure 7 for the blank pulse, the output of the LM1208 will be less than 0.1V during the inactive portion of the video signal. This is a “blacker than black” condition, blanking the CRT at the cath- odes. By using the blank function of the LM1208 no grid blanking is necessary. Note that the DC restoration is done by feeding back the video signal from −A3, but blanking is done at −A4. By using the two output stages, blanking can be done at the CRT cathodes, and at the same time activate the DC restoration loop. V CC1 goes to pins 3, 11, and 25 (see Figure 1 ). These three pins are all internally connected. For proper operation of the LM1208 it is necessary to connect all the V CC1 pins to the input power to the PCB and bypass each pin with a 0.1 µF capacitor. V CC2 is the input power at pins 22 and 23 for the three output stages. This is a separate power input from V CC1, there are no internal connections between the two different power inputs. There must be a connection on the PCB between V CC1 and VCC2. Pins 22 and 23 must be bypassed by a parallel connection of a 10 µF and 0.1 µF capacitors. The ground connections for the LM1208 are at pins 7, 21, and 24. All three ground pins are internally connected, and these pins must also be connected exter- nally to a good ground plane for proper operation of the LM1208. www.national.com 10 |
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同様の説明 - LM1208N |
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