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AD8594AR Datasheet(PDF) 8 Page - Analog Devices |
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AD8594AR Datasheet(HTML) 8 Page - Analog Devices |
8 / 15 page AD8591/AD8592/AD8594 –8– REV. A AD8591/AD8592/AD8594 APPLICATION SECTION Theory of Operation The AD859x family of amplifiers are all 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 making the AD8591/AD8592/AD8594 op amps ideal for portable multimedia and telecom applications. Figure 29 shows the simplified schematic for an AD8591/AD8592/ AD8594 amplifier. 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, which drives the input to a second differential pair gain stage. The outputs of the second gain stage provide the gate volt- age 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. Also, the maximum output voltage swing is directly proportional to the load current. The difference be- tween the maximum output voltage to the supply rails, known as the dropout voltage, is determined by the AD8591/AD8592/ AD8594 output transistors’ on-channel resistance. The output dropout voltage is given in Figure 1 and Figure 2. 50 A 100 A 100 A 20 A VB2 M5 M8 M12 M15 M16 M11 OUT M3 M4 M1 IN– IN+ VB3 M6 M7 M10 20 A M13 50 A V+ V– M9 M14 M2 * * ** M337 SD INV * * M340 *NOTE: ALL CURRENT SOURCES GO TO 0 A IN SHUTDOWN MODE INV M31 M30 Figure 29. AD8591/AD8592/AD8594 Simplified Schematic Input Voltage Protection Although not shown on the simplified schematic, ESD protec- tion diodes are connected from each input to each power supply rail. These diodes are normally reverse biased, but will turn on if either input voltage exceeds either supply rail by more than +0.6 V. Should this condition occur, the input current should be limited to less than ±5 mA. This can be done by placing a resistor in series with the input(s). The minimum resistor value should be: R V mA IN IN MAX ≥ , 5 (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 device’s inputs exceeds +0.6 V outside of the supply rails, the output could exhibit phase reversal. This is due to the ESD protection diodes be- coming forward biased, thus causing the polarity of the input terminals of the device to switch. The technique recommended in the Input Overvoltage 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 cir- cuit 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, the maximum output current should be limited to ±250 mA. By placing a resistor in series with the output of the amplifier as shown in Figure 30, the output current can be limited. The minimum value for RX can be found from Equation 2. R V mA X SY ≥ 250 (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 tradeoff in using RX is a slight reduction in output voltage swing under heavy output current loads. RX will also increase the effec- tive output impedance of the amplifier to RO + RX, where RO is the output impedance of the device. +5V RX 20 VOUT VIN AD8592 Figure 30. Output Short Circuit Protection Power Dissipation Although the AD859x family of 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 equation for finding the junction temperature is given as: TP T DISS A A JJ =× + θ (3) Where TJ = AD859x junction temperature PDISS = AD859x power dissipation θ JA = AD859x junction-to-ambient thermal resistance of the package; and TA = The ambient temperature of the circuit |
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