PDF LM2202 Data sheet ( Hoja de datos )

Número de pieza	LM2202
Descripción	230 MHz Video Amplifier System
Fabricantes	National Semiconductor
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No Preview Available ! April 1999 LM2202 230 MHz Video Amplifier System General Description The LM2202 is a very high frequency video amplifier system intended for use in high resolution monochrome or RGB color monitor applications. In addition to the wideband video amplifier the LM2202 contains a gated differential input black level clamp comparator for brightness control, a DC con- trolled attenuator for contrast control and a DC controlled sub contrast attenuator for drive control. The DC control for the contrast attenuator is pinned out separately to provide a more accurate control system for RGB color monitor applica- tions. All DC controls offer a high input impedance and oper- ate over a 0V to 4V range for easy interface to bus controlled alignment systems. The LM2202 operates from a nominal 12V supply but can be operated with supply voltages down to 8V for applications that require reduced IC package power dissipation characteristics. Features n Wideband video amplifier (f−3dB = 230 MHz at VO = 4 VPP) n tr, tf = 1.5 ns at VO = 4 VPP n Externally gated comparator for brightness control n 0V to 4V high input impedance DC contrast control (>40 dB range) n 0V to 4V high input impedance DC drive control (±3 dB range) n Easy to parallel three LM2202s for optimum color tracking in RGB systems n Output stage clamps to 0.65V and provides up to 9V output voltage swing n Output stage directly drives most hybrid or discrete CRT amplifier stages n Replacement for the LM1202 Applications n High resolution CRT monitors n Video switches n Video AGC amplifier n Wideband amplifier with gain and DC offset control Block and Connection Diagram Order Number LM2202N or LM2202M See NS Package Number N20A or M20B © 1999 National Semiconductor Corporation DS012591 DS012591-1 www.national.com 1 page Circuit Description (Continued) provides a 6 dB gain adjustment range. This feature is nec- essary for RGB applications where independent gain adjust- ment 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 volt- age at the minus input of the comparator matches the volt- age 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 op- eration. VIDEO AMPLIFIER SECTION (Input Stage) A simplified schematic of LM2202’s video amplifier input stage is shown in Figure 3. The 5.4V zener diode, Q1, Q6 and R2 bias the base of Q7 at 2.6V. The AC coupled video signal applied to pin 6 is referenced to the 2.6V bias voltage. Transistor Q7 buffers the video signal, VIN, and Q8 converts the voltage to current. The AC collector current through Q8 is IC8 = VIN/R9. Under maximum gain condition, transistors Q9 and Q11 are off and all of IC8 flows through the load resistors R10 and R11. The maximum signal gain at the base of Q13 is, AV1 = −(R10 + R11)/R9 = −2. Signal attenuation is achieved by varying the base drive to the differential pairs Q9, Q10 and Q11, Q12 thereby unbalancing the collector currents through the transistor pairs. Base of Q10 is biased at 5.3V by externally connecting pin 1 to pin 20 through a 100Ω resistor. Pin 2 is connected to pin 3 through a 100Ω re- sistor. Adjusting the contrast voltage at pin 8 produces a con- trol voltage at pin 3 which drives the base of Q9. By varying the voltage at the base of Q9, Q8’s collector current (IC8) is diverted away from the load resistors R10 and R11, thereby providing signal attenuation. Maximum attenuation is achieved when all of IC8 flows through Q9 and no current flows through the load resistors. The differential pair Q11 and Q12 provide drive control. Q12’s base is internally biased at 7.3V. Adjusting the voltage at the drive control input (pin 9) produces a control voltage at the base of Q11. With Q9 off and Q12 off, all of IC8 flows through R10, thus providing a gain of AV1 = −(R10/R9) x VIN = −1. Drive control thus provides a 6 dB attenuation range. FIGURE 2. Block Diagram of the LM2202 Video Amplifier with Contrast and Brightness (Black Level) Control 5 DS012591-8 www.national.com 5 Page Circuit Description (Continued) FIGURE 7. Simplified Schematic of the LM2202 Clamp Gate Circuit DS012591-13 11 www.national.com 11 Page

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