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ST AN2378 Application note

1. kyy AN2378 Application note Software filter algorithm for open load detection on the L9935 device Introduction This document describes a software algorithm used for detecting an open load condition The algorithm refers to the L9935 two phase stepper motor driver The algorithm identifies the open load failure within the failure class and filters out occurrences of false open load failures Figure 1 L9935 bipolar stepper motor driver device Microcontroller Open load DETECTION filter Open load circuit Open load circuit Open load Software Stepper Motor Open load SS circuit Open load circuit GAPGRI00044 February 2011 D oc ID 12435 Rev 2 1 13 www st com Contents AN2378 Contents 1 When does L9935 open load checking occur 000eeeeee 5 1 1 Full step mode as Aare did oe EG EE aa de DR we DR hs whee mare ee 5 1 2 Half step mode iss SR an Oh eee SEER BARS EER Oe Eb See ES 6 2 Open load detection of L9935 0 cc es 7 3 L9935 open load production test 0c eee eee ees 9 4 Open load filter algorithm ss eks sa RR RR RR RR ee 10 Appendix A Further information see kaas RE RE RR RR RR RR RR RR RE 11 A 1 Reference documents 0200 0c eee eee eee eee eens 11 REVISION history is sai ke ks eee be tena cad eN ee ORs ew oa we eee n 12 2 13 Doc ID 12435 Rev 2 ky AN2378 List of tables List of tables Table 1 P
2. command indicates an open load error Secondly an error counter should be used For example all open load failure occurrences should be counted It should be possible to increase the functionality of the counter by resetting the counter in case of detecting a possible open load situation without open load failure Doc ID 12435 Rev 2 ky AN2378 Further information Appendix A Further information A 1 Reference documents 19935 Two phase stepper motor driver Datasheet Doc ID 5198 ky Doc ID 12435 Rev 2 11 13 Revision history AN2378 Revision history 12 13 Table 2 Document revision history Date Revision Changes 19 Jun 2006 1 Initial release Editorial and formatting changes throughout 28 Feb 2011 2 Added Figure 5 Open load measurement concept with simplified Vmes curve if Vmes lt 0 V TLSI is switched off Added Section 3 L9935 open load production test Doc ID 12435 Rev 2 AN2378 Please Read Carefully Information in this document is provided solely in connection with ST products STMicroelectronics NV and its subsidiaries ST reserve the right to make changes corrections modifications or improvements to this document and the products and services described herein at any time without notice All ST products are sold pursuant to ST s terms and conditions of sale Purchasers are solely responsible for the choice selection and use of the ST products and services descr
3. shown in Figure 4 During this polarity reversal the flyback pulse is used for the open load detection The process described above is also valid for motor coil B a The symbol lspunt in this application note is equivalent to the symbol Ip in the device datasheet Doc ID 12435 Rev 2 5 13 When does L9935 open load checking occur AN2378 1 2 6 13 Half step mode Figure 3 Output current of L9935 in half step mode GAPGRI00046 dhar 61 lspunt Plus 100 lspuns regulated with chopping at low side and slow decay mode flyback using the high side path 2har Changing from 61 spunt tO 61 lshunt iS done in two steps with a zero ampere current step in between The curve to obtain 0 A in the motor coil A is used to detect the open load because in this step the current is free wheeling This step to 0 A in the motor coil uses fast decay as shown in Figure 4 The process described above is also valid for motor coil B Doc ID 12435 Rev 2 AN2378 Open load detection of L9935 2 Open load detection of L9935 The open load detection is done during flyback using fast decay The fast decay path is shown in Figure 4 the coil energy stored in Ly is fed back from GND to Vaat Figure 4 Open load detection during free wheeling GAPGRI00047 If the voltage drop at Rehunt Plus Rps on Of TLs does not reach the reference voltage Vef an open load failure is detected A failu
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6. o test with the parameters defined in Table 7 Devices that pass this test do not generate an open load error if the load current 16 us after going in flyback remains higher than 90 mA Table 1 Production test conditions Rsnun includes max 0 2 ohm contact resistance Parameters ST screening test Unit Veatt 14 V Rehunt 1 2 ohm Rps on 0 4 ohm Road 82 ohm LLoad 5 mH lt ous 0 17 A lt 16us 0 089 A VoUT 6us 0 143 V 1 Open load case the current is not regulated that is Vgat R lt Irec and ignoring back EMF gives the voltage over the comparator as expressed in Equation 3 Equation 3 Ros on Rshunt R Vines t Vat conte SEE 1 2 ol An open load is detected if the magnitude of the flyback voltage 16 us after going in flyback is less than 60 mV Equation 4 V 16s lt 60mV mes Doc ID 12435 Rev 2 9 13 Open load filter algorithm AN2378 4 10 13 Open load filter algorithm To ensure recognition of a true open load failure the following algorithm is recommended First of all it is helpful to separate the open load failure from other failures In this case the failure code has to be related to the sent data In case of switching the output current to 0 A or inverting the current the failure code has to be monitored As specified in the datasheet if the error codes 11 01 11 occur the combination of these three error codes plus the sent
7. re is flagged only if the failure lasts longer than 16 us Figure 5 Open load measurement concept with simplified V mes curve if V mes lt 0 V Tis is switched off Vmes open oa 60mV 16us GAPGRI00048 Doc ID 12435 Rev 2 7 13 Open load detection of L9935 AN2378 The flyback pulse is dependent on different parameters Equation 1 di Vi Veate R 1 EMF i T eN E R Repu t Ry t 2 Ros on Equation 2 dt di L gt 16us Voan R 1 EMF e Decreasing motor current leads to an increase of sporadic open load failures Increasing motor resistance Ry temperature increase production spread new motor generation leads to an increase of sporadic open load failures e Increasing motor speed gt increases back EMF electromagnetic force increase of sporadic open load failures Incase of resonance effects back EMF could be similar to Vp 4 decrease of motor current I increase of sporadic open load failures In case of current regulation mode increase of Vea decrease of flyback time gt increase of sporadic open load failures Therefore it is not possible to directly test against a certain open load current threshold without also defining application dependent parameters 8 13 Doc ID 12435 Rev 2 ky AN2378 L9935 open load production test 3 L9935 open load production test For testing the open load functionality STMicroelectronics uses a special go no g
8. roduction test conditions Rsnunt includes max 0 2 ohm contact resistance 9 Table 2 Document revision history ie EE EE ee teas 12 OT Doc ID 12435 Rev 2 3 13 List of figures AN2378 List of figures Figure 1 L9935 bipolar stepper motor driver device EE EE SE EE SE eee 1 Figure 2 Output current of L9935 in full step mode ie ee ke de 5 Figure 3 Output current of L9935 in half step mode nananana aaaea 6 Figure 4 Open load detection during free wheeling EE EE eee Ee Ee Ee de ke 7 Figure 5 Open load measurement concept with simplified Vmes curve if Vmes lt O V Tig is switched OM sore De a ey pon ie antes hon iene Spe N EE Ss etna g 7 4 13 Doc ID 12435 Rev 2 ky AN2378 When does L9935 open load checking occur 1 1 When does L9935 open load checking occur The flyback pulse indicates the availability of an inductive load The flyback pulse is checked after reversing the polarity of the motor current full step mode namely 61 Ishunt or 100 Ispunt or after changing the current from 61 Ighynt tO 0 Ighunt half step mode please refer to L9935 datasheet see A 1 Reference documents Full step mode Figure 2 Output current of L9935 in full step mode GAPGRI00045 Tiu 61 lsnunt OF 100 lsnuns regulated with chopping at low side and slow decay mode flyback using the high side path 2 reversing the polarity of the motor coil A using fast decay as

ST AN2378 Application note

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