TLE 472x Family Stepper Motor Drivers. Current Control Method and Accuracy

Transcription

1 Applicaion Noe, V 1.0, Augus 2001 ANPS063E TLE 472x Family Sepper Moor Drivers Curren Conrol Mehod and Accuracy by Frank Heinrichs Auomoive Power N e v e r s o p h i n k i n g

2 TLE 472x sepper moor drivers Curren conrol mehod and accuracy 1. Absrac This Applicaion Noe is inended o provide deailed echnical informaion abou he curren conrol mehod ha is used in Infineons sepper moor driver family TLE 472x. Special aenion is paid on accuracy consideraions. 2. Inroducion The TLE 472x sepper moor drivers are inended o be used for bipolar, wo-phase, curren-conrolled sepper moors (see figure 1). 3. Curren Conrol Mehod In general, hree differen mehods are used o conrol he curren in he coil of a sepper moor: 3.1. Peak curren conrol wih fixed offime Wih his conrol mehod, he H-bridge is swiched ino freewheel-condiion as soon as he oupu curren reaches he desired value Iarge. The bridge remains in off-sae for a fixed ime off and is swiched on again auomaically afer off (see figure 2). As a resul, he peak value of he oupu curren equals he desired value. Iarge on off Figure 2: peak curren conrol wih consan off-ime Figure1: TLE 4729 block-diagram 3.2. Peak curren conrol wih fixed frequency Wih his conrol mehod, he H-bridge is swiched ino freewheel-condiion as soon as he oupu curren reaches he desired value Iarge. In conras o he previous mehod, he off-ime is no fixed, bu he bridge is swiched ON afer every ime inerval 1/f, f being he chopper frequency as shown in figure 3. This has he advanage ha he chopper frequency is fixed, i.e. independen of he load characerisics and he supply volage (in oher words: independen of he off-ime off). This conrol mehod is used in he TLE 472x driver family. Applicaion Noe page 1 of 4 V1.0,

3 TLE 472x sepper moor drivers Curren conrol mehod and accuracy Vs Iarge Source Source Sink Load on 1/f + - Rsense Figure 3: peak curren conrol wih fixed frequency Osc Vref 3.3. Mean-value curren conrol (currenmode conrol). The conrol mehods described so far have he disadvanage ha he effecive, mean oupu curren is always smaller han he desired curren. By using suiable filers and comparaors, i is also possible o conrol he curren in such a manner ha he mean value of he oupu curren is equal o he desired value Iarge, as shown in figure 4. Iarge Figure 5: simplified schemaic of curren conrol implemenaion The oscillaor ses he NOR-lach oupu o high wih he oscillaor frequency fosc. By his, he sink-ransisor is urned ON every period 1/f. According o he inpu signals IXX, a reference volage Vref is generaed: IX0 IX1 Symb. min yp max uni H L Vch mv L H Vcs mv H H Vca mv on Figure 4: curren-mode conrol However, for his mehod i is necessary o adap he filering elemens o he load characerisics, so a fully inegraed IC soluion is no possible. For ha reason, curren mode conrol is no implemened in he TLE 472x familly. 1/f 4. Implemenaion As jus described, fixed frequency peak-curren conrol is used in he TLE 472x sepper moor driver family. As simplified schemaic of he implemenaion is shown in figure 5. This reference Volage is compared o he drop across he sense resisor Rsense. As soon as * Rsense equals Vref, he NOR-lach is se o low. This urns OFF he sink ransisor, seing he H-bridge o freewheel (freewheel in source ransisors). This means ha he desired oupu curren is: Iarge = Vref / Rsense 5. Accuracy Consideraions The accuracy of he acual oupu curren value depends on he accuracy of : he inernal comparaor reference volage Vref he exernal sense resisor he curren ripple 5.1. Accuracy of Vref and Rsense The accuracy of Vref is given in he daashee and in above able. For Example, he Sepoin Applicaion Noe page 2 of 4 V1.0,

4 TLE 472x sepper moor drivers Curren conrol mehod and accuracy curren has a nominal value of 450mV and a variaion of 9% and +13%. As he oupu curren depends linearly on Vref, he same accuracy has o be expeced for he oupu curren. The same is rue for he sense resisor. The accuracy of Rsense conribues linearly o he accuracy of Curren ripple As explained earlier, he conrol mehod leads o an average curren smaller ha he desired curren Iarge because he peak curren is limied o Iarge. If DI is he curren ripple, he average curren is,mean = Iarge DI/2 on 1/f DI Iarge,mean Figure 6: Curren ripple, peak- and mean curren To deermine, he curren ripple, we have o calculae he volage drop across he load induciviy (di/d)charge For he rising curren slope (charging he induciviy), we have: Vs = VsauC + UL + R* + Vsal + Rsense*. The parameer we are inerseed in is he curren slew rae: (di/d)c = UL/L = (Vs VsauC Vsal (R + Rsense))/L Les look a an example o carry ou he calculaion: Vs = 12V Rsense = 1.3Ohm = 315mA R = 8Ohm L = 4mH From he daashee, we have (using he values a = 450mA): VsauC = 1V Vsal = 0.3V This leads o : (di/d)c = ( )V / 4mH = 1.95 A/ms (di/d)discharge The same has now o be done for he falling curren slope (discharging of he induciviy, frewheeling): Source Transisor L R Source diode Vs We have Source - UL = R* + VFu + VsauD, (di/d)d = (R* + VFu + VsauD) / L L R Sink For our example, his leads o: (di/d)d = UL/L = - ( )V/4mH = -0.96A/ms on, off and Ripple Rsense Now, we can approximae off and on by making he approximaion of consan di/d: Applicaion Noe page 3 of 4 V1.0,

5 TLE 472x sepper moor drivers Curren conrol mehod and accuracy (di/d)c (di/d)d DI 6. Conclusion The general mehod o conrol he oupu curren ha is used in he TLE 472x sepper moor driver family was explained. An approximaive formula o deermine he curren ripple and hus he difference beween mean oupu curren and desired curren is derived and carried ou for a represenaive example on off on + off = 1/fosc, (di/d)c * on = (di/d)d * off = DI This leads o: on = (1/fosc) / [{(di/d)c / (di/d)d} + 1], off = 1/fosc on, DI = (di/d)c * on For our example wih fosc = 25kHz, his leads o: on = 40µs / [{1.95 / 0.96} + 1] = 13.2µs off = 40µs on = 26.8µs We see ha he ON-ime is much shorer han he OFF-ime. The reason is ha he (higher) supply-volage is used o charge he coil, while during freewheeling only he (smaller) drops accross load resisance and power ransisors conribue o he di/d. This finally gives us he curren ripple: DI = (di/d)c * on = 25.7mA Applicaion Noe page 4 of 4 V1.0,

6 TLE 472x sepper moor drivers Curren conrol mehod and accuracy 7. Disclaimer hp:// Ediion Published by Infineon Technologies AG, S.-Marin-Srasse 53, D München, Germany Infineon Technologies AG 8/d/yy. All Righs Reserved. Published by Infineon Technologies AG Aenion please! The informaion herein is given o describe cerain componens and shall no be considered as warraned characerisics. Terms of delivery and righs o echnical change reserved. We hereby disclaim any and all warranies, including bu no limied o warranies of non-infringemen, regarding circuis, descripions and chars saed herein. Infineon Technologies is an approved CECC manufacurer. Informaion For furher informaion on echnology, delivery erms and condiions and prices please conac your neares Infineon Technologies Office in Germany or our Infineon Technologies Represenaives worldwide (see address lis). Warnings Due o echnical requiremens componens may conain dangerous subsances. For informaion on he ypes in quesion please conac your neares Infineon Technologies Office. Infineon Technologies Componens may only be used in life-suppor devices or sysems wih he express wrien approval of Infineon Technologies, if a failure of such componens can reasonably be expeced o cause he failure of ha life-suppor device or sysem, or o affec he safey or effeciveness of ha device or sysem. Life suppor devices or sysems are inended o be implaned in he human body, or o suppor and/or mainain and susain and/or proec human life. If hey fail, i is reasonable o assume ha he healh of he user or oher persons may be endangered. 8. Infineon goes business Excellence Business excellence means inelligen approaches and clearly defined processes, which are boh consanly under review and ulimaely lead o good operaing resuls. Beer operaing resuls and business excellence mean less idleness and wasefulness for all of us, more professional success, more accurae informaion, a beer overview and, hereby, less frusraion and more saisfacion. Dr. Ulrich Schuhmacher Applicaion Noe page 5 of 4 V1.0,