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LM1201M Folha de dados(PDF) 8 Page - National Semiconductor (TI) |
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LM1201M Folha de dados(HTML) 8 Page - National Semiconductor (TI) |
8 / 16 page Circuit Description (Continued) Figure 7 also shows how the contrast control circuit is con- figured Resistors R23 R24 diodes D3 D4 and transistor Q13 are used to establish a low impedance zero TC half supply voltage reference at the base of Q14 The differential amplifier formed by Q15 Q16 and feedback transistor Q17 along with resistors R27 R28 establish a differential base voltage for Q3 and Q4 in Figure 6 When externally adding or subtracting current from the collector of Q16 a new dif- ferential voltage is generated that reflects the change in the ratio of currents in Q15 and Q16 To provide voltage control of the Q16 current resistor R29 is added between the Q16 collector and pin 4 A capacitor should be added from pin 4 to ground to prevent noise from the contrast control pot from entering the IC Figure 8 is a simplified schematic of the clamp gate and clamp comparator section of the LM1201 The clamp gate circuit consists of a PNP input buffer transistor (Q18) a PNP emitter coupled pair referenced on one side to 21V (Q19 Q20) and an output switch (Q21) When the clamp gate input at pin 5 is high (l15V) the Q21 switch is on and shunts the I1 1mA current to ground When pin 5 is low (k13V) the Q21 switch is off and the I1 1mA current source is mirrored or ‘‘turned around’’ by reference diode D5 and Q26 to provide a 1mA current source for the clamp comparator The inputs to the comparator are similar to the clamp gate input except that an NPN emitter coupled pair is used to control the current which will charge or discharge the clamp capacitor at pin 2 PNP transistors are used at the inputs because they offer a number of advantages over NPNs PNPs will operate with base voltages at or near ground and will usually have a greater reverse emitter-base breakdown voltage (BVebo) Because the differential input voltage to the clamp comparator during the video scan peri- od could be greater than the BVebo of NPN transistors resistor R34 with a value one half that of R33 or R35 is connected between the bases of Q23 and Q27 This resis- tor will limit the maximum differential input to Q24 Q25 to approximately 350 mV The clamp comparator common mode range extends from ground to approximately 9V and the maximum differential input voltage is VCC and ground TLH10006 – 8 FIGURE 8 Simplified Schematic of LM1201 Clamp Gate and Clamp Comparator Circuits http www nationalcom 8 |
Nº de peça semelhante - LM1201M |
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Descrição semelhante - LM1201M |
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