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LM1212N データシート(PDF) 5 Page - National Semiconductor (TI) |
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LM1212N データシート(HTML) 5 Page - National Semiconductor (TI) |
5 / 18 page Circuit Description Figure 2 shows a block diagram of the LM1212 video ampli- fier along with contrast and brightness (black level) control Contrast control is a DC-operated attenuator which varies the AC gain of the amplifier Signal attenuation (contrast) is achieved by varying the base drive to a differential pair and thereby unbalancing the current through the differential pair As shown in Figure 2 a 53V bias voltage is internally con- nected to the positive input of the attenuator and to Pin 1 Pin 3 provides a control voltage for the negative input (pin 2) of the attenuator The voltage at pin 3 varies as the voltage at the contrast control input (pin 8) varies thus providing signal attenuation The gain is maximum (0 dB attenuation) if the voltage at pin 8 is 4V and is minimum (maximum atten- uation) if the voltage at pin 8 is 0V The 0V to 4V DC-operat- ed drive control at pin 9 providesa6dB gain adjustment range This feature is necessary for RGB applications where independent gain adjustment of each channel is required The brightness or black level clamping requires a ‘‘sample and hold’’ circuit which holds the DC bias of the video ampli- fier constant during the black level reference portion of the video waveform Black level clamping often referred to as DC restoration is accomplished by applying a back porch clamp signal to the clamp gate input pin (pin 14) The clamp comparator is enabled when the clamp signal goes low dur- ing the black level reference period (see Figure 2 ) When the clamp comparator is enabled the clamp capacitor con- nected to pin 12 is either charged or discharged until the voltage at the minus input of the comparator matches the voltage set at the plus input of the comparator During the video portion of the signal the clamp comparator is disabled and the clamp capacitor holds the proper DC bias In a DC coupled cathode drive application picture brightness func- tion can be achieved by varying the voltage at the compara- tor’s plus input Note that the back porch clamp pulse width (tW in Figure 2 ) must be greater than 100 ns for proper operation OSD blanking is activated through an active high TTL signal at pin 20 When pin 20 has a high logic level the video output at pin 17 drops to k 100 mV During this time an external signal can be applied to the output driving the vid- eo level to the desired level for the OSD window This OSD signal will back bias the emitter of the internal output tran- sistor Q23 (see Figure 4 ) The maximum voltage to this transistor under this condition is 5V This signal must also supply enough current to the pull-down transistor Q24 which is about 100 mA The OSD blanking can only be done during the active video period If activated during the clamp pulse period the OSD blanking will interfere with the DC restoration VIDEO AMPLIFIER SECTION (Input Stage) A simplified schematic of LM1212’s video amplifier input stage is shown in Figure 3 The 54V zener diode Q1 Q6 and R2 bias the base of Q7 at 26V The AC coupled video signal applied to pin 6 is referenced to the 26V bias voltage TLH12354 – 8 FIGURE 2 Block Diagram of the LM1212 Video Amplifier with Contrast and Brightness (Black Level) Control http www nationalcom 5 |
同様の部品番号 - LM1212N |
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同様の説明 - LM1212N |
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