Datasheet AD8614, AD8644 (Analog Devices) - 9

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AD8614/AD8644 THEORY OF OPERATION
The AD8614/AD8644 are processed using Analog Devices high
OUTPUT SHORT-CIRCUIT PROTECTION
voltage, extra fast complementary bipolar (HV XFCB) process. To achieve a wide bandwidth and high slew rate, the output of This process includes trench-isolated transistors that lower the AD8614/AD8644 is not short-circuit protected. Shorting parasitic capacitance. the output directly to ground or to a supply rail can destroy the Figure 26 shows a simplified schematic of the AD8614/AD8644. device. The typical maximum safe output current is 70 mA. The input stage is rail-to-rail, consisting of two complementary In applications where some output current protection is needed, differential pairs, one NPN pair and one PNP pair. The input but not at the expense of reduced output voltage headroom, a stage is protected against avalanche breakdown by two back-to- low value resistor in series with the output can be used. This is back diodes. Each input has a 1.5 kΩ resistor that limits input shown in Figure 25. The resistor is connected within the current during overvoltage events and furnishes phase reversal feedback loop of the amplifier so that if VOUT is shorted to protection if the inputs are exceeded. The two differential pairs ground and VIN swings up to 18 V, the output current does not are connected to a double-folded cascode. This is the stage in exceed 70 mA. the amplifier with the most gain. The double-folded cascode differentially feeds the output stage circuitry. Two complemen- For 18 V single-supply applications, resistors less than 261 Ω are tary common emitter transistors are used as the output stage. not recommended. This allows the output to swing to within 125 mV from each rail
18V
with a 10 mA load. The gain of the output stage, and thus the open-loop gain of the op amp, depends on the load resistance.
VIN 261Ω AD86x4 V
The AD8614/AD8644 have no built-in short-circuit protection.
OUT
The short-circuit limit is a function of high current roll-off of the output stage transistors and the voltage drop over the 26 0 5- resistor shown on the schematic at the output stage. The voltage 48 06 over this resistor is clamped to one diode during short-circuit Figure 25. Output Short-Circuit Protection voltage events.
VCC – 1.5kΩ 1.5kΩ + VOUT VCC VCC
25 0 5-
VEE
48 06 Figure 26. Simplified Schematic Rev. B | Page 9 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION OUTPUT SHORT-CIRCUIT PROTECTION INPUT OVERVOLTAGE PROTECTION OUTPUT PHASE REVERSAL POWER DISSIPATION UNUSED AMPLIFIERS CAPACITIVE LOAD DRIVE DIRECT ACCESS ARRANGEMENT A ONE-CHIP HEADPHONE/MICROPHONE PREAMPLIFIER SOLUTION OUTLINE DIMENSIONS ORDERING GUIDE