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AD8565 датащи(PDF) 8 Page - Analog Devices |
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AD8565 датащи(HTML) 8 Page - Analog Devices |
8 / 12 page AD8565/AD8566/AD8567 –8– REV. A APPLICATIONS Theory of Operation The AD856x family is designed to drive large capacitive loads in LCD applications. It has high output current drive, rail-to-rail input/output operation and is powered from a single 16 V supply. It is also intended for other applications where low distortion and high output current drive are needed. Figure 1 illustrates a simplified equivalent circuit for the AD856x. The rail-to-rail bipolar input stage is composed of two PNP differential pairs, Q4–Q5 and Q10–Q11, operating in series with diode protection networks, D1–D2. Diode network D1–D2 serves as protection against large transients for Q4–Q5, to accommodate rail-to-rail input swing. D5–D6 protect Q10–Q11 against zenering. In normal operation, Q10–Q11 are off and their input stage is buffered from the operational amplifier inputs by Q6–D3 and Q8–D4. Operation of the input stage is best understood as a function of applied common-mode voltage: When the inputs of the AD856x are biased midway between the supplies, the differential signal path gain is controlled by resistive loads (Via R9, R10) Q4–Q5. 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–D3 and Q8–D4 networks into cutoff; However, Q4–Q5 remain active, providing input stage gain. Inversely, when common-mode input voltage is increased toward the positive supply, Q4–Q5 are driven into cutoff, Q3 is driven into saturation, and Q4 becomes active, providing bias to the Q10–Q11 differential pair. The point at which Q10–Q11 differential pair becomes active is approximately equal to (VPOS – 1 V). R1 R3 R4 D1 D2 Q4 Q3 BIAS LINE V– D3 D4 Q5 Q4 R5 R6 Q10 Q11 C1 C2 D5 D6 Q8 Q6 R9 R10 FOLDED CASCADE V+ I1 I2 VNEG VPOS Figure 1. AD856x 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–Q5 and Q6–Q8 over the range from (VNEG + 1 V) to (VPOS – 1 V). Outside of this range, input bias current is dominated by the sum of base current of Q10–Q11 for input signals close to VNEG and of Q6–Q8 (Q10–Q11) for signal close to VPOS. From this type of design, the input bias current of AD856x not only exhibits different amplitude, but also exhibits different polarities. Figure 2 provides the characteristics of the input bias current versus common-mode voltage. It is important to keep in mind that the source impedances driving the AD856x inputs are balanced for optimum dc and ac performance. INPUT COMMON-MODE VOLTAGE – V 1,000 –1,000 016 2 4 6 8 101214 800 200 –200 –600 –800 600 400 0 –400 VS = 16V TA = 25 C Figure 2. AD856x Input Bias Current vs. Common-Mode Voltage In order to achieve rail-to-rail output performance, the AD856x design uses a complementary common-source (or gmRL) output. This configuration 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. Also, 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 AD856x. 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 pn junctions will allow current to flow from the input to the supplies. This input current is not inherently damaging to the device as long as it is limited to 5 mA or less. If a condition exists using the AD856x where the input exceeds the supply more than 0.6 V, a series external resistor should be added. The size of the resistor can be calculated by using the maximum overvoltage divided by 5 mA. This resistance should be placed in series with either input exposed to an overvoltage. |
Аналогичный номер детали - AD8565 |
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Аналогичное описание - AD8565 |
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