In thisconclitionthe control circuitmay be content tooperatestability at one half of the oscillator fre-quency,as shown in Figure13.Asin normal op-eration,the output is turned off whenthe current reaches the desired peakvalue anddecays until the oscillator sets the flip-flop and the curren t again starts to increase. However since t1 is longer thant2thecurrenthasnot yetreac hedthe peak valuebefo re the secondoscillatorpulse oc-curs.The second oscillatorpulsethenhasno ef-fectandcurrentcontinues to increa se until theset peak value is reached and theflip-flopis resetby
the comparator.The currentcontrolcircuit is com-pletelycontent to keep operating in thiscondition.
In factthe circuit may operateon oneoftwo sta-ble conditions depending on the random time when the peak current isfirstreach edrelative to the oscillatorperiod.
The ea siest,andrecomm ended,solutionisto ap-plythechoppingsignalto only oneofthephase inputs, as imple mented with the L297, In the phase chopping mode,or theL6506.
Anot hersolutionthatworks,insomecases, is to fix a largeminimum duty cycle, in the range of 30%,by applyinganexternalcloc k signal to the syncinputof theL297 or L6506.Inthis configura-tionthecircuitmust outputatleastthe rninimrm dutycycleduring eachclock period.Thisforces thepointwherethe peak currentis detectedto be late rineach cycle andthechoppingfreq uency to lock on the fundamental.Themain disadvantage ofthis approachisthat it sets ahigherminirru m currentthatcanbe controlled .The curre ntin the motoralso tends to oversh ootduring the firstfew chopp ing cyclessincethe actualpeak current is not be sensed during theminimum duty cycle.
EFFECTSOF BACKEMF
Asmentioned earlier,thebackEMF in a stepper motor tends to increase the duty cycle of the choppingdrive circuitssinceitopposes curre nt in-creased and aids current deca y. In extrem e, cases where the power supply volta ge is low comparedtothepeak backEMF ofthe motor,the dutycyclerequir edwhen usingthe phase chop-ping may exceed50%and theproblemwith the Figure13:When the outp utdutycycleexceeds50%thechopping circuit may sinchronize of a
sub-harmonicoftheoscillatorfrequ enc y.
l
Expected Molor
c..,....
tActualMotorcurrent
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73
CNC Robotics
APPUCATIONNOTE
stability of the operating frequency discussed abovecan occur.At this point the constant fre-quency chopping techniquebecomes impractical toimplementand a choppingtechniquethatuses constant off timefreq uencymodulationlike imple-mented in the PBL37 17A,TEA3717.TEA3718, andL6219ismoreuseful.
WhyWon'tthemotor move
Manyfirsttimeusersofchoppingcontroldrives firstfind thatthe motor doesnot movewhen the circuitIsenabled.Simplyput the motor isnot gen -eratingsufficienttorqueto turn.Providedthat the motoriscapableof producing therequired torque atthesetspeed,the problemusuallylies inthe current controlcircuit. Asdiscu ssed in the pre-vious section thecurrentsensingcircuit can be fooled.Inextreme casesthe noise issolargethat theactualcurrent throu gh themotor is essentially zeroand the motor is producingno torque. An-other symptom of this isthat the current being drawnfromthepower supplyis verylow.
Avoid Destroyingth e Driver
Manyusershavefirstask whythedevicefailed in the application.Inalmostevery casethe failure was caused byelectr icaloverstresstothe device.
specifically voltages or currentsthatareoutside of the devicerating s.Whenevera driverfails,a careful eval uationof the operatingconditio ns in the application is in order.
Themost common failure encounteredisthe re-sultof voltagetransientsgeneratedbythe induc-tanceinthe motor.A correctlydesignedapp lica-tion will keep the peak voltage on the power supply,acrossthecollecto r toemitter of the out-putdevicesand,formonolithicdrivers,from one output to the otherwithin themaximumrating of the device.Aproperdesignincludespowers up-ply filteringandclampdiodes and/orsnubber net-workson theoutput[6].
Selectingthe correctclamp diodes for the appli-catio nisessential.Theproper diode is matched
to the speedof the switchingdevice and main-tains a
VF
thatlimitsthe peak voltage within the allowablelimits. When the diodes are not inte-gratedthey must beprovid edexternally.The di-odes should have switch ing characteristics that arethesameor betterthan the switching timeof the output transistors . Usuallydiodesthathavea reverse recovery timeoflessthan150nsare suf-ficientwhenusedwith bipolaroutputdeviceslike the L298N,The 1N4001 seriesofdevices,for ex-ample,Is not a good selection because it is a slow diode.Althou ghitoccurs lessfreque ntly,excesscurrent canalsodestroy the device.In mostapplications the excess currentisthe resultofshortcircuitsin the load.If theapplicationispron tohaveshorted loads the designer may consider implementing someexternalshort circuitprotection(7].
Shootthroughcurrent,the current thatflowsfrom supp lyto ground due tothe simLJtaneous conduc-tion ofuppe r and lowertransistors in the bridge output. is another concern. The design of the L298N,l293 and L6202 all include circuitry spe-cifically to prevent this phenomena. The user should notmistake the reversereco very current ofthediodes ortheparasiticstruct uresinthe out-putstage as shoot throughcurrent.
SELECTEDREFERENCES
Ill Sax,Herbert.,"StepperMotorDriving"(AN235) [2]"Constant Current ChopperDriveUps Stepper-Motor Performanc e"(AN468)
[3]Hopkins,Thomas ."Unsing theL6506for Cur-rentContro lof Stepping Motor s" (AN469) [4]"The l297Steper Motor Controller"(AN470) [5]Leenouts,Albert.The Artandpractjce
of
Step MotorControl.Ventura CA:Intertec Communica-tionsInc.(805) 658-0933.1987[6]Hopkins.Thomas ."Controlling Voltage Tran-sisntsin Full Bridge Drivers" (AN280)
[7]Scrocchi G.and FusaroliG."ShortCircuit Pro-taction on L6203". (AN279)
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Chapter 2 / Electr onics
APPLICATIONNOTE
Informationfurnishedis believed tobeaccurate and reliable.Howeve r,SGS-THOMSONMicroelectro nics assumesnoresponsibility lor the consequences ofUSlIof such inro rmalion ,nor loranyinfringementof patents orotnarri~hlSalthird parties which mayresuttfromitsuse.No Ii·
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THOMSON Micfoolecltonicsproduct s arEinolaulhofiledfor use ascritical components inlilasupportdevicesorsystemswithout express writteni1WfOVaJ01SGS-TH~Microelectronics.
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