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AD8614ARTZ-REEL7 Arkusz danych(PDF) 10 Page - Analog Devices |
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AD8614ARTZ-REEL7 Arkusz danych(HTML) 10 Page - Analog Devices |
10 / 16 page AD8614/AD8644 Rev. B | Page 10 of 16 INPUT OVERVOLTAGE PROTECTION As with any semiconductor device, whenever the condition exists for the input to exceed either supply voltage, attention needs to be paid to the input overvoltage characteristic. As an overvoltage occurs, the amplifier can 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 pin junctions energize, allowing current to flow from the input to the supplies. Observing Figure 26, the AD8614/AD8644 have 1.5 kΩ resistors in series with each input, which helps to limit the current. This input current is not inherently damaging to the device as long as it is limited to 5 mA or less. If the voltage is large enough to cause more than 5 mA of current to flow, an external series resistor should be added. The size of this resistor is calculated by dividing the maximum overvoltage by 5 mA and subtracting the internal 1.5 kΩ resistor. For example, if the input voltage could reach 100 V, the external resistor should be (100 V ÷ 5 mA) – 1.5 kΩ = 18.5 kΩ. This resistance should be placed in series with either or both inputs if they are subjected to the overvoltages. OUTPUT PHASE REVERSAL The AD8614/AD8644 are immune to phase reversal as long as the input voltage is limited to within the supply rails. Although the device’s output does not change phase, large currents due to input overvoltage can result, damaging the device. In applica- tions where the possibility of an input voltage exceeding the supply voltage exists, overvoltage protection should be used, as described in the previous section. POWER DISSIPATION The maximum power that can be safely dissipated by the AD8614/AD8644 is limited by the associated rise in junction temperature. The maximum safe junction temperature is 150°C, and should not be exceeded or device performance could suffer. If this maximum is momentarily exceeded, proper circuit operation is restored as soon as the die temperature is reduced. Leaving the device in an overheated condition for an extended period can result in permanent damage to the device. To calculate the internal junction temperature of the AD8614/AD8644, the following formula can be used: TJ = PDISS × θJA + TA where: TJ is the AD8614/AD8644 junction temperature. PDISS is the AD8614/AD8644 power dissipation. θJA is the AD8614/AD8644 junction-to-ambient package thermal resistance. TA is the ambient temperature of the circuit. The power dissipated by the device can be calculated as: PDISS = ILOAD × (VS – VOUT) where: ILOAD is the AD8614/AD8644 output load current. VS is the AD8614/AD8644 supply voltage. VOUT is the AD8614/AD8644 output voltage. Figure 27 provides a convenient way to determine if the device is being overheated. The maximum safe power dissipation can be found graphically, based on the package type and the ambient temperature around the package. By using the previous equation, it is a simple matter to see if PDISS exceeds the device’s power derating curve. To ensure proper operation, it is important to observe the recommended derating curves shown in Figure 27. 1.5 0 1.0 0.5 AMBIENT TEMPERATURE (°C) –35 –15 5 254565 85 14-LEAD SOIC PACKAGE θJA = 120°C/W 14-LEAD TSSOP PACKAGE θJA = 180°C/W 5-LEAD SOT-23 PACKAGE θJA = 230°C/W Figure 27. Maximum Power Dissipation vs. Temperature (5-Lead and 14-Lead Package Types) UNUSED AMPLIFIERS It is recommended that any unused amplifiers in the quad package be configured as a unity-gain follower with a 1 kΩ feedback resistor connected from the inverting input to the output, and the noninverting input tied to the ground plane. |
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