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LF255D 데이터시트(PDF) 5 Page - STMicroelectronics |
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LF255D 데이터시트(HTML) 5 Page - STMicroelectronics |
5 / 14 page The LF155, LF156, LF157 series are op amps with J- FETinput transistors. TheseJFETs havelarge reverse breakdown voltagesfromgatetosource or drain elimi- natingtheneed of clamps across the inputs.Therefore large differential input voltages can easily be accom- modatedwithoutalarge increaseof inputcurrents. The maximum differential input voltage is independent of the supply voltage. However, neitherof thenegativein- put voltagesshouldbe allowed to exceedthe negative supply as this will cause large currents to flow which can result in a destroyed unit. Exceeding the negative common-modelimit on either inputwill causeareversal of thephasetotheoutputandforce the amplifier output to the correspondinghigh or lowstate. Exceedingthe negativecommon-mode limit on bothinputs will force the amplifier outputto a highstate.In neithercasedoes a latch occur since raising the input back within the common-mode range again puts the input stage and thustheamplifier in a normal operatingmode. Exceed- ingthepositive common-modelimit on asingle input will not changethephase of the output however, if bothin- putsexceedthe limit, theoutput of theamplifier will be forcedto ahighstate.Theseamplifiers will operatewith the common-mode input voltage equal to the positive supply. In fact, the common-modevoltagecanex- ceedthepositivesupplyby approximately 100mV inde- pendentof supply volt-age and over thefull operat- ingtemperaturerange.The positive suplly can there- forebe used as a referenceonaninput as, forexample, in a supply current monitor and/or limiter. Precautions- shouldbe taken to ensurethat thepowersupply for the integrated circuit never becomes re-versed in polarity or that the unit is not inadvertentlyin-stalledbackwards in a socket as an unilimited current surge throughthe resulting forward diode within the IC couldcausefusin- goftheinternalconductorsandresultin a destroyedunit. Because these amplifiers are JFET rather than MOS- FET input op amps they do not require special han- dling. Allof thebiascurrentsintheseamplifiersareset by FET current sources. The drain currents for the amplifiers are therefore essentially independent of supply volt- ages. As with most amplifiers, care should betakenwith lead dress, components placement and supply decoupling in order to ensure stability. For example, resistors from the output to an input should be placed with the body close to theinput to minimiz ”pickup” and maximize the frequency of the feedback pole by minimizing the ca- pacitancefrom the input to ground. A feedback pole is createdwhen the feedbackaround any amplifier is resistive. The parallel resistance and capacitancefromthe input of thedevice(usually the in- vertinginput)toacgroundsetthefrequencyofthepole.In many instances the frequency of this pole is much greaterthanthe expected3 dBfrequencyof the closed loopgain and consequentlythereisnegligible effect on stability margin. However, if the feedback pole is less than approximately six time the expected 3 dB fre- quencyaleadcapacitor should be placed from the out- put to the input of the op amp. The value of that 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 con- stant. APPLICATION HINTS LF155 - LF156 - LF157 5/14 |
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