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2.66mH Inductor availability and substitutions (TMS37157)

Bryan Audiffred
Prodigy 100 points
Other Parts Discussed in Thread: TMS37157

As far as I can tell, there isn't a 2.66mH transponder coil in the channel from any vendor.  The recommended ECI part is a special order.  With regard to standard offerings, 2.66uH is non-existent, and 2.38mH is available from several manufacturers.  I'd be delighted to find out otherwise.

Does TI have any comment on changing the transponder coil and compensating for the resonant frequency with a change in the resonant capacitor?

  • 0
    Prodigy 40 points

    I am also having difficulty finding a 2.66uH 134.2kHz antenna.  Any guidance on small form factor antennas would be greatly appreciated.

  • 0 in reply to stephane smith
    Prodigy 40 points

    Also would like to point out that the antenna tolerance is from 2.586mH to 2.734mH.  I'm able to find some 2.7mH inductors at Mouser...

  • 0 in reply to stephane smith
    TI__Mastermind 44150 points

    Our development tool uses a ferrite coil from Neosid.  They should be able to offer other form factor coils as well. 

    http://www.neosid.de/

    If you would like some other options, you may also check with Coilcraft and Toko.   

    http://www.coilcraft.com/

    http://www.toko.com/

  • 0 in reply to Eddie LaCost
    Prodigy 40 points

    As the first poster mentioned, neosid's supplier ECI only has this part on special order.  I was quoted $25/inductor...  Neither coilcraft nor toko carry the 2.66mH coil either.  I made a request to coilcraft to supply 2.66mH rf transponder coils to support the tms37157, we'll see if anything happens.

    At this point, I'm looking at dropping the tms37157. The part is very promising and exactly what I need, but without a reliable 2.66mH coil vendor, I don't see how I can design in the tms37157.  Are there any alternatives?

    I would like to reiterate the first poster's request: can TI please provide an application note on the recommend procedure to compensate different coils with the resonant capacitors?  This clearly violates the spec, but I imagine is possible.

     

     

     

     

  • 0 in reply to stephane smith
    TI__Mastermind 44150 points

    I did not think this was a special order from Neosid.  Can you confirm that you were quoted $25 for the ferrite coil we use on the target board?  The part number is MS32ka/2.66mH.  We will also contact Neosid tomorrow and see how this is offered.  I will let you know what we find out after our discussion.

    In regards to using a different coil inductance, this would violate the spec, but could likely work still.  The resonant frequency is determined by 1/ 2 * pi -sqrt L*C.  As an example, 2.38mH could use a cap value of 560pF to acheive about 137kHz.  This does not take into consideration the other capacitance including trim cap.  Please see the attached app note for a detailed description.  Table 3 should be useful.   

    LF Tag Coil Design Application Note.pdf
  • 0 in reply to Eddie LaCost
    Prodigy 100 points

    I can also confirm a sample price of $25 per coil from ECI (Neosid's US sales representative?) .   I requested a price for 10-20 units and the reply was $500 for 20.  Reels are 3k units. This is a special order part with no distribution through the usual suspects.

    The ECI part number is AS31-0266.  Sales was not forthcoming on why they don't use the Neosid part numbering.  Maybe they ended their relationship with Neosid, and Neosid forgot to update their web site.  

  • 0 in reply to Bryan Audiffred
    Prodigy 40 points

    I was indeed quoted $25/inductor, $500 for 20, or a reel of 3k; exactly like the previous poster.  I was told the price was "credited for serial orders".  

    I also got a response back from coilcraft:  "While we do not offer a 2.66 mH value, you could use a close value and adjust the resonating capacitance value.  For example, with a 2.38 mH coil, the capacitance value for 134.2 kHz would be 591 pF.  For a 2.89 mH, coil it would be 487 pF.  You are welcome to some free samples to try.  Just click on the part numbers and add them to your samples cart."

    I'm currently designing the pcb for the tms37157 and I plan to use a close value from coilcraft.  The land pattern is very similar.

    I'm staying tuned for updates and I'll post my results when available.

     

  • 0 in reply to stephane smith
    TI__Mastermind 44150 points

    I heard back from Neosid on this subject.  They have informed me that they are charging for the sample orders so that they can make sure these are legitimate opportunity requests and not just "nice to have" type requests.  If serial orders are placed, you will be refunded for the higher cost sample order. 

     

  • 0 in reply to Eddie LaCost
    Prodigy 100 points

    That's an entirely reasonable business decision, though it's hard to imagine frivolous requests for such a specific part.  Use of the TMS37157 by nature probably indicates a volume design opportunity. 

    I have also redesigned the circuit to accommodate the more common ~2.38mH value.  560pF is a commonly available 1% NP0/C0G part, and 30pF is around the middle of the tuning capacitance range.  This should provide some tuning room up and down in frequency to accommodate for inductor tolerance and other parasitics.