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AD8532AR-REEL7 Datasheet(PDF) 11 Page - Analog Devices |
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AD8532AR-REEL7 Datasheet(HTML) 11 Page - Analog Devices |
11 / 20 page AD8531/AD8532/AD8534 Rev. F | Page 11 of 20 THEORY OF OPERATION The AD8531/AD8532/AD8534 are all CMOS, high output current drive, rail-to-rail input/output operational amplifiers. Their high output current drive and stability with heavy capacitive loads make the AD8531/AD8532/AD8534 excellent choices as drive amplifiers for LCD panels. Figure 36 illustrates a simplified equivalent circuit for the AD8531/AD8532/AD8534. Like many rail-to-rail input amplifier configurations, it comprises two differential pairs, one N-channel (M1 to M2) and one P-channel (M3 to M4). These differential pairs are biased by 50 μA current sources, each with a compliance limit of approximately 0.5 V from either supply voltage rail. The differential input voltage is then converted into a pair of differential output currents. These differential output currents are then combined in a compound folded-cascade second gain stage (M5 to M9). The outputs of the second gain stage at M8 and M9 provide the gate voltage drive to the rail-to-rail output stage. Additional signal current recombination for the output stage is achieved using M11 to M14. To achieve rail-to-rail output swings, the AD8531/AD8532/ AD8534 design employs a complementary, common source output stage (M15 to M16). However, the output voltage swing is directly dependent on the load current because the difference between the output voltage and the supply is determined by the AD8531/AD8532/AD8534’s output transistors on channel resistance (see Figure 12 and Figure 13). The output stage also exhibits voltage gain by virtue of the use of common source amplifiers; as a result, the voltage gain of the output stage (thus, the open-loop gain of the device) exhibits a strong dependence on the total load resistance at the output of the AD8531/ AD8532/AD8534. 50µA 100µA 100µA 20µA VB2 M5 M8 M12 M15 M16 M11 OUT M3 M4 M2 M1 IN– IN+ VB3 M6 M7 M10 20µA M13 50µA V+ V– M9 M14 Figure 36. Simplified Equivalent Circuit SHORT-CIRCUIT PROTECTION As a result of the design of the output stage for the maximum load current capability, the AD8531/AD8532/AD8534 do not have any internal short-circuit protection circuitry. Direct connection of the output of the AD8531/AD8532/AD8534 to the positive supply in single-supply applications destroys the device. In applications where some protection is needed, but not at the expense of reduced output voltage headroom, a low value resistor in series with the output, as shown in Figure 37, can be used. The resistor, connected within the feedback loop of the amplifier, has very little effect on the performance of the amplifier other than limiting the maximum available output voltage swing. For single 5 V supply applications, resistors less than 20 Ω are not recommended. 5V RX 20Ω VOUT VIN AD8532 Figure 37. Output Short-Circuit Protection POWER DISSIPATION Although the AD8531/AD8532/AD8534 are capable of providing load currents to 250 mA, the usable output load current drive capability is limited to the maximum power dissipation allowed by the device package used. In any application, the absolute maximum junction temperature for the AD8531/AD8532/AD8534 is 150°C. The maximum junction temperature should never be exceeded because the device could suffer premature failure. Accurately measuring power dissipation of an integrated circuit is not always a straightforward exercise; therefore, Figure 38 is provided as a design aid for either setting a safe output current drive level or selecting a heat sink for the package options available on the AD8531/AD8532/AD8534. TEMPERATURE (°C) 1.5 1.0 0.5 0 0 25 50 75 85 100 TJ MAX = 150°C FREE AIR NO HEAT SINK TSSOP θJA = 240°C/W SC70 θJA = 376°C/W SOIC θJA = 158°C/W MSOP θJA = 210°C/W SOT-23 θJA = 230°C/W Figure 38. Maximum Power Dissipation vs. Ambient Temperature |
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Similar Description - AD8532AR-REEL7 |
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