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Moteur de recherche de fiches techniques de composants électroniques |
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Moteur de recherche de fiches techniques de composants électroniques |
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AD8565 Fiches technique(PDF) 9 Page - Analog Devices |
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AD8565 Fiches technique(HTML) 9 Page - Analog Devices |
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9 / 16 page  AD8565/AD8566/AD8567 Rev. G | Page 9 of 16 THEORY OF OPERATION The AD8565/AD8566/AD8567 are designed to drive large capacitive loads in LCD applications. They have high output current drive and rail-to-rail input/output operation and are powered from a single 16 V supply. They are also intended for other applications where low distortion and high output current drive are needed. Figure 28 shows a simplified equivalent circuit for the AD8565/ AD8566/AD8567. The rail-to-rail bipolar input stage is com- posed of two PNP differential pairs, Q4 to Q5 and Q10 to Q11, operating in series with diode protection networks, D1 to D2. Diode network D1 to D2 serves as protection against large transients for Q4 to Q5 to accommodate rail-to-rail input swing. D5 to D6 protect Q10 to Q11 against Zenering. In normal oper- ation, Q10 to Q11 are off, and their input stage is buffered from the operational amplifier inputs by Q6 to D3 and Q8 to D4. Operation of the input stage is best understood as a function of applied common-mode voltage: when the inputs of the AD8565/ AD8566/AD8567 are biased midway between the supplies, the differential signal path gain is controlled by resistive loads Q4 to Q5 (via R9, R10). As the input common-mode level is reduced toward the negative supply (VNEG or GND), the input transistor current sources, I1 and I2, are forced into saturation, thereby forcing the Q6 to D3 and Q8 to D4 networks into cutoff. However, Q4 to Q5 remain active, providing input stage gain. Inversely, when common-mode input voltage is increased toward the positive supply, Q4 to Q5 are driven into cutoff, Q3 is driven into saturation, and Q4 becomes active, providing bias to the Q10 to Q11 differential pair. The point at which the Q10 to Q11 differential pair becomes active is approximately equal to (VPOS − 1 V). R1 R4 R3 D2 D1 Q4 Q3 BIAS LINE V– D4 D3 Q5 Q4 Q10 Q11 C1 C2 D5 D6 Q8 Q6 R10 R9 FOLDED CASCADE V+ I2 I1 VNEG VPOS R5 R6 Figure 28. AD8565/AD8566/AD8567 Equivalent Input Circuit The benefit of this type of input stage is low bias current. The input bias current is the sum of base currents of Q4 to Q5 and Q6 to Q8 over the range from (VNEG + 1 V) to (VPOS − 1 V). Outside this range, the input bias current is dominated by the sum of base currents of Q10 to Q11 for input signals close to VNEG and of Q6 to Q8 (Q10 to Q11) for signals close to VPOS. From this type of design, the input bias current of the AD8565/ AD8566/AD8567 not only exhibits different amplitude but also exhibits different polarities. Figure 29 provides the characteris- tics of the input bias current vs. the common-mode voltage. It is important to keep in mind that the source impedances driving the inputs are balanced for optimum dc and ac performance. INPUT COMMON-MODE VOLTAGE (V) 0 2 1000 –1000 800 200 –200 –600 –800 600 400 0 –400 4 6 8 10 12 14 16 VS = 16V TA = 25°C Figure 29. AD8565/AD8566/AD8567 Input Bias Current vs. Common-Mode Voltage To achieve rail-to-rail output performance, the AD8565/ AD8566/AD8567 design uses a complementary common- source (or gmRL) output. This con-figuration allows output voltages to approach the power supply rails, particularly if the output transistors are allowed to enter the triode region on extremes of signal swing, which are limited by VGS, the transistor sizes, and output load current. In addition, this type of output stage exhibits voltage gain in an open-loop gain configuration. The amount of gain depends on the total load resistance at the output of the AD8565/AD8566/AD8567. INPUT OVERVOLTAGE PROTECTION As with any semiconductor device, whenever the input exceeds either supply voltages, attention needs to be paid to the input overvoltage characteristics. As an overvoltage occurs, the amplifier could be damaged, depending on the voltage level and the magnitude of the fault current. When the input voltage exceeds either supply by more than 0.6 V, internal positive-negative (pn) junctions allow current to flow from the input to the supplies. |
Numéro de pièce similaire - AD8565_15 |
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Description similaire - AD8565_15 |
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