전자부품 데이터시트 검색엔진 |
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LM2418 데이터시트(PDF) 3 Page - National Semiconductor (TI) |
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LM2418 데이터시트(HTML) 3 Page - National Semiconductor (TI) |
3 / 6 page LM2418Theory of Operation The LM2418 is a high voltage triple CRT driver suitable for VGA display applications The LM2418 features 90V opera- tion and low power dissipation The part is housed in the industry standard 11-lead TO-220 molded power package The heat sink is electrically isolated from the circuitry and may be grounded for ease of manufacturing and RFI shield- ing The circuit diagram of the LM2418 is shown in Figure 1 Q1 and R2 provide a conversion of input voltage to current while Q2 acts as a common base or cascode amplifier stage to drive the load resistor R1 Emitter followers Q3 and Q4 isolate the impedance of R1 from the capacitance of the CRT cathode and make the circuit relatively insensitive to load capacitance The gain of this circuit is bR1R2 and is fixed at b19 The bandwidth of the circuit is set by the collector time constant formed by the load resistor R1 and associated capacitance of Q2 Q3 Q4 and stray layout ca- pacitance Diodes D1 and D2 provide forward bias to the output stage to reduce crossover distortion at low signal levels while R3 provides a DC bias offset to match the out- put level characteristics of the LM1203 RGB Video Amplifier System Proprietary transistor design allows for high band- width with low operating power Figure 2 shows a typical test circuit for evaluation of the LM2418 This circuit is designed to allow testing of the LM2418 in a 50X environment such as a pulse generator and a scope or a network analyzer In this test circuit two resistors in series totaling 495 kX form a wideband low capacitance probe to match the output of the LM2418 to a 50X cable and load Typical AC performance of the circuit is shown in Figure 3 The input signal is AC coupled to the base of Q1 while a DC bias of 12V is applied to the base of Q2 (See Figure 2 ) Thermal Considerations The transfer characteristics of the amplifier are shown in Figures 4 and 5 Power supply current increases as the in- put signal increases and consequently power dissipation also increases The LM2418 cannot be used without heat sinking Figure 5 shows the power dissipated in each channel over the oper- ating voltage range of the device Typical ‘‘average’’ power dissipation with the device output voltage at one half the supply voltage is 18W per channel for a total dissipation of 54W package dissipation Under white screen conditions ie 20V output dissipation increases to 30W per channel or 9W total The LM2418 case temperature must be main- tained below 100 C If the maximum expected ambient tem- perature is 60 C then a maximum heat sink thermal resist- ance can be calculated Rth e (100 C b 60 C) 9W e 44 CW PRECAUTION THE LM2418 IS NOT PROTECTED AGAINST OUTPUT SHORT CIRCUITS The minimum re- sistance the LM2418 can drive is 800X to ground or Va TLK11125 – 7 FIGURE 4 LM2418 DC Characteristics TLK11125 – 9 FIGURE 5 LM2418 Output Swing and Power Characteristics 3 |
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