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AD8591ARTZ-REEL датащи(PDF) 11 Page - Analog Devices |
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AD8591ARTZ-REEL датащи(HTML) 11 Page - Analog Devices |
11 / 16 page AD8591/AD8592/AD8594 Rev. B | Page 11 of 16 THEORY OF OPERATION The AD859x amplifiers are CMOS, high output drive, rail-to- rail input and output single-supply amplifiers designed for low cost and high output current drive. The parts include a power saving shutdown function that makes the AD8591/AD8592/ AD8594 op amps ideal for portable multimedia and telecommunications applications. Figure 33 shows the simplified schematic for the AD8591/AD8592/ AD8594 amplifiers. Two input differential pairs, consisting of an n-channel pair (M1, M2) and a p-channel pair (M3, M4), provide a rail-to-rail input common-mode range. The outputs of the input differential pairs are combined in a compound folded- cascode stage that drives the input to a second differential pair gain stage. The outputs of the second gain stage provide the gate voltage drive to the rail-to-rail output stage. The rail-to-rail output stage consists of M15 and M16, which are configured in a complementary common source configuration. As with any rail-to-rail output amplifier, the gain of the output stage, and thus the open-loop gain of the amplifier, is dependent on the load resistance. In addition, the maximum output voltage swing is directly proportional to the load current. The difference between the maximum output voltage to the supply rails, known as the dropout voltage, is determined by the on-channel resistance of the AD8591/AD8592/AD8594 output transistors. The output dropout voltage is given in Figure 5 and Figure 6. 50µA 100µA 100µA 20µA VB2 M5 M8 M12 M15 M16 M11 OUT M3 M4 M1 IN– IN+ VB3 M6 M7 M10 20µA M13 50µA V+ V– M9 M14 M2 * * * * M337 SD INV * * M340 *ALL CURRENT SOURCES GO TO 0µA IN SHUTDOWN MODE. INV M31 M30 Figure 33. Simplified Schematic INPUT VOLTAGE PROTECTION Although not shown in the simplified schematic, ESD protection diodes are connected from each input to each power supply rail. These diodes are normally reverse-biased, but turn on if either input voltage exceeds either supply rail by more than 0.6 V. If this condition occurs, limit the input current to less than ±5 mA. This is done by placing a resistor in series with the input(s). The minimum resistor value should be mA 5 , MAX IN IN V R ≥ (1) OUTPUT PHASE REVERSAL The AD8591/AD8592/AD8594 are immune to output voltage phase reversal with an input voltage within the supply voltages of the device. However, if either of the inputs of the device exceeds 0.6 V outside of the supply rails, the output could exhibit phase reversal. This is due to the ESD protection diodes becoming forward-biased, thus causing the polarity of the input terminals of the device to switch. The technique recommended in the Input Voltage Protection section should be applied in applications where the possibility of input voltages exceeding the supply voltages exists. OUTPUT SHORT-CIRCUIT PROTECTION To achieve high output current drive and rail-to-rail performance, the outputs of the AD859x family do not have internal short- circuit protection circuitry. Although these amplifiers are designed to sink or source as much as 250 mA of output current, shorting the output directly to the positive supply could damage or destroy the device. To protect the output stage, limit the maximum output current to ±250 mA. By placing a resistor in series with the output of the amplifier, as shown in Figure 34, the output current can be limited. The minimum value for RX is mA 250 SY X V R ≥ (2) For a 5 V single-supply application, RX should be at least 20 Ω. Because RX is inside the feedback loop, VOUT is not affected. The trade-off in using RX is a slight reduction in output voltage swing under heavy output current loads. RX also increases the effective output impedance of the amplifier to RO + RX, where RO is the output impedance of the device. RX 20Ω VOUT AD8592 +5V VIN Figure 34. Output Short-Circuit Protection POWER DISSIPATION Although the AD859x amplifiers are able to provide load currents of up to 250 mA, proper attention should be given to not exceeding the maximum junction temperature for the device. The junction temperature equation is TJ = PDISS × θJA + TA (3) where: TJ is the AD859x junction temperature. PDISS is the AD859x power dissipation. θJA is the AD859x junction-to-ambient thermal resistance of the package. TA is the ambient temperature of the circuit. |
Аналогичный номер детали - AD8591ARTZ-REEL |
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Аналогичное описание - AD8591ARTZ-REEL |
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