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LF453 Datasheet(PDF) 6 Page - National Semiconductor (TI) |
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LF453 Datasheet(HTML) 6 Page - National Semiconductor (TI) |
6 / 8 page Application Hints These devices are op amps with an internally trimmed input offset voltage and JFET input devices (BI-FET II) These JFETs have large reverse breakdown voltages from gate to source and drain eliminating the need for clamps across the inputs Therefore large differential input voltages can easily be accommodated without a large increase in input current The maximum differential input voltage is independent of the supply voltages However neither of the input voltages should be allowed to exceed the negative supply as this will cause large currents to flow which can result in a destroyed unit Exceeding the negative common-mode limit with the non-in- verting input or with both inputs will force the output to a high state potentially causing a reversal of phase to the output In neither case does a latch occur since raising the input back within the common-mode range again puts the input stage and thus the amplifier in a normal operating mode Exceeding the positive common-mode limit on a single input will not change the phase of the output however if both inputs exceed the limit the output of the amplifier will be forced to a high state The amplifiers will operate with a common-mode input volt- age equal to the positive supply however the gain band- width and slew rate may be decreased in this condition When the negative common-mode voltage swings to within 3V of the negative supply an increase in input offset voltage may occur Each amplifier is individually biased by a zener reference which allows normal circuit operation on g5V power sup- plies Supply voltages less than these may result in lower gain bandwidth and slew rate The amplifiers will drivea2kX load resistance to g10V over the full temperature range of 0 Cto a70 C If the am- plifier is forced to drive heavier load currents however an increase in input offset voltage may occur on the negative voltage swing and finally reach an active current limit on both positive and negative swings Precautions should be taken to ensure that the power sup- ply for the integrated circuit never becomes reversed in po- larity or that the unit is not inadvertently installed backwards in a socket as an unlimited current surge through the result- ing forward diode within the IC could cause fusing of the internal conductors and result in a destroyed unit As with most amplifiers care should be taken with lead dress component placement and supply decoupling in or- der to ensure stability For example resistors from the out- put to an input should be placed with the body close to the input to minimize ‘‘pick-up’’ and maximize the frequency of the feedback pole by minimizing the capacitance from the input to ground A feedback pole is created when the feedback around any amplifier is resistiveThe parallel resistance and capacitance from the input of the device (usually the inverting input) to AC ground set the frequency of the pole In many instances the frequency of this pole is much greater than the expected 3 dB frequency of the closed loop gain and consequently there is negligible effect on stability margin However if the feedback pole is less than approximately 6 times the ex- pected 3 dB frequency a lead capacitor should be placed from the output to the input of the op amp The value of the added capacitor should be such that the RC time constant of this capacitor and the resistance it parallels is greater than or equal to the original feedback pole time constant The benefit of the SO package results from its very small size It follows however that the die inside the SO package is less protected from external physical forces than a die in a standard DIP would be because there is so much less plastic in the SO Therefore not following certain precau- tions when board mounting the LF453CM can put mechani- cal stress on the die lead frame andor bond wires This can cause shifts in the LF453CM’s parameters even caus- ing them to exceed limits specified in the Electrical Charac- teristics For recommended practices in LF453CM surface mounting refer to Application Note AN450 ‘‘Surface Mount- ing Methods and Their Effect on Product Reliability’’ and to the section titled ‘‘Surface Mount’’ found in any Rev 1 Lin- ear Databook volume 6 |
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