Unlimited Capacitive Load Drive Op Amp Takes …

1 LM8261,LM8272 Unlimited Capacitive load Drive Op Amp Takes Guesswork Out Of DesignLiterature Number: SNOA808 Application BriefUnlimited Capacitive load Drive Op Amp Takes Guesswork Out Of DesignApplication Brief 108 The DUT (device under test) will operate under closedloop DC and open loop AC conditions. Therefore, themeasured results will be a true representation of loopgain including the effect of CL. The resultant T/Rmeasurement (magnitude and phase) will aid indetermining the PM for a given CL. One such plotdone for LM8272, Unlimited capacitance load Drive op amp, is shown in Fig. 2:Whether or not an op amp circuit is capable of driving a Capacitive load successfully, dependson several factors: Op amp internal architecture ( ROUT,phase margin, compensation, etc.) Closed loop gain and output loading load capacitance valueDriving a capacitor also entails the op amp soutput current capability since changing thevoltage across a capacitor requires an adequatesupply of current from the op amp.

2 Thisarticle will present a lab method to measureamplifier stability under closed loop addition, a new op amp architecture willbe presented that would ease this class of ap-plications by using an internal mechanism toimprove Loop Phase Margin MeasurementOne of the internal op amp parameters whicheffects cap load Drive performance is ROUT,output impedance. In fact, an ideal op ampwith zero output impedance will be able to Drive any capacitance with no phase margin (PM)degradation. However, in reality, for almost allcases, ROUT cannot be ignored. By using anetwork analyzer (HP4195A or equivalent) and the circuit shown in Fig. 1, it is possible to measure closed loop performance undercapacitive loading:NationalSemiconductorThe Sight & Sound of InformationHooman : Circuit To Measure Phase Margin Under Capacitive LoadingThis plot has been corrected for 26 dB gain lossthrough RSand the RHS axis is made to read phasemargin directly. In fact, with the LM8272, the PMstays positive for any and all capacitors, as can beseen from Fig.

3 3plot:DUTRt50 ohmRTRs1 KohmRmeasure50 ohmHP4195A or equivalentHP4195A or equivalent+_CLRf100 KohmC110 F50403020100-10-20100K1M10M100 MGain400 pF10 pFLoop Gain (dB)f (Hz)PM (Deg.)100806040200-20-407060504030201001 01001K10K100K1 MPM (Deg.)CL (pF) : LM8272 Phase Margin Vs. Capacitive : LM8272 Loop Gain Vs. # 11/21/01 10:08 AM Page 1 National Semiconductor Corporation, 2001. National Semiconductor andare registered trademarks of National Semiconductor Corporation. All rights reserved. It can be shown that the PM degradation forLM8272 with the chosen capacitor values is lessthan what would be expected if the op amp openloop parameters ( dominant pole frequency)stayed fixed. The LM8272 has specifically beendesigned such that a heavy Capacitive load willinternally shift the dominant pole frequencyhigher. This feature is intended to keep the phaseshift around the loop to less than 180 under anycapacitive load . The LM8272 architecture isexplained further , it is important to remember that as inmost op amps, the addition of a series isolationresistor between the output and the load improvesthe settling and overshoot ArchitectureTo understand how LM8272 achieves unlimitedcapacitive load Drive capability, its internal blockdiagram is shown in Fig.

4 4:Visit The National Edge, our onlinetechnical journal for an archive of Application Briefs and otherinteresting Semiconductor2900 Semiconductor Box 58090 Santa Clara, CA 950521-800-272-9959 Visit our web site more information,send email Sight & Sound of Informationthe output transistors thus lowering the effectiveinternal Miller capacitance. The internal polefrequency increases at the same time a low frequencypole is created at the op amp output due to the largeload capacitor. In this fashion, the internal dominantpole compensation, which works by reducing theloop gain to less than 0 dB when the phase shiftaround the feedback loop is more than 180 , varieswith the amount of Capacitive load and becomes lessdominant when the load capacitor has increasedenough. Hence, the op amp is very stable even athigh values of load capacitance resulting in theuncharacteristic feature of stability under allcapacitive conclusion, readily available op amps have alwayssuffered from inability to Drive Capacitive loads andinstabilities associated with that.

5 A new op ampdesign, LM8272 (Dual) and LM8261 (Single) havemostly alleviated this problem to the extent thatthese devices can even be used as voltage buffers withheavy capacitors sitting right at their +500R9D1D2D3D4500R10 INPUT_O1O2R1R2Q11Q12V+R4R3Q3Q4Q13V+ +_A2 OUTPUTQ10 Ccomp10 Ccomp9 VBIAS2R5R6O5O6V+Output IQControl and Current LimitI1Q9+ : LM8272 Simplified SchematicThe output stage is comprised of complementaryNPN and PNP common-emitter stages to permitvoltage swing to within a Vce (sat)of either supplyrail. Q9 supplies the sourcing and Q10 suppliesthe sinking current load . Output current limitingis achieved by limiting the Vceof Q9 and frequency compensation circuit includesMiller capacitors from collector to base of eachoutput transistor (Ccomp9and Ccomp10). At lightcapacitive loads, the high frequency gain of theoutput transistors is high, and the Miller effectincreases the effective value of the capacitorsthereby stabilizing the op amp. Large capacitiveloads greatly decrease the high frequency gain # 11/21/01 10.

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