• State TI Thinks Resolved
  • Locked Locked
  • Replies 3 replies
  • Answers 1 answer
  • Subscribers 40 subscribers
  • Views 253 views
  • Users 0 members are here
Support feedback
Options
Options
Similar topics

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

WEBENCH® Tools/LDC1612: Coil sensor's DC Resistance, Rp & IDrive

Dinesh Kumar2
Prodigy 120 points
Part Number: LDC1612

Tool/software: WEBENCH® Design Tools

Hello!!

Good Day!!!

I am using LDC1612 IC in one of my application. (attached the coil sensor specifications which am using, designed with the help of TI's Webench Coil designer tool).

As I am unable to measure the AC parallel resistance, RP of the coil am using, please suggest any method or ways to relate/derive the required drive current with the help of DC resistance of the coil, as it is easier for me to measure. (As in the data sheet, the drive current register codes are listed down against the Rp of the sensor. It will be much helpful, if I get the look up table for the drive current registers against the Coil sensor's DC resistance or any theoretical calculations to derive the Rp using the DC resistance of the coil.

Expecting the best solution. Thanks in advance.

 

  • 0
    TI__Genius 17440 points

    Hello Dinesh,

    Just so you know, the "Resonance impedance" in the Webench tool is the same as Rp. You should be able to use that value (~6kohms) as a good starting point for your drive current. You may need to fine tune it a little bit. If so, all you need is an oscilloscope. You can use this application note as a guide. You can also use an oscilloscope to measure the Rp of your LC tank. 

    Best Regards,

  • 0 in reply to Kristin Jones93
    Prodigy 120 points

    Thanks Kristin,

    But in the data sheet, the resonance impedance is given in maximum and minimum ranges to set the drive current (please refer the image shared in my earlier post), whereas in the coil designer webench designer tool the value is definite. 

    Say for example, I am getting 6.190.867 Ohms as resonance impedance (with DC resistance 6.9 ohms) at temperature 35 C. And am getting 4081.444 Ohms as resonance impedance (with DC resistance 8.053 Ohms) at temperature 80 C.

    My question is, does setting the same drive current for these both designed coils is advisable as the coil values may get drift if the drive current is not fine tuned. It will be much helpful, if I get a detailed explanation from you on this. 

    1. All the coil PCBs we are manufacturing can not be identical practically, hence requires individual drive currents for each and every coils manufactured (otherwise it is drifting). setting drive current individually is also not practically possible.

    2. Or it is possible to manufacture coil PCBs so identical?, so that we could have a default drive current.

    3. Should I consider the coil PCB as failure if the conversion value gets drifted in the results?

    Expecting your guidance to get a clear understanding.

    Thanks in Advance.

    Regards,

    Dinesh

  • 0 in reply to Dinesh Kumar2
    TI__Genius 17440 points

    Hi Dinesh,

    The drive currents in table 42 are recommended settings for the full range of Rp values in each row. For your example, both temperatures could likely use the same drive current setting (18). Though the 80C Rp value is a little outside of this range, it may be okay. The better way to check this is actually using an oscilloscope to monitor the sensor oscillation amplitude. This application note should help with that method. Table 42 is a great resource for initially picking a drive current setting, but it's best to double-check the setting using an oscilloscope. 

    Overall, it's important to keep the sensor oscillation amplitude between 1.2V - 1.8V for the whole temperature range and target movement range. For some applications it may be necessary to use more than one drive current setting to accomplish this.

    1. Can you clarify what you mean by drift? What device behavior are you seeing? Typically we've seen quite consistent PCB coils. I also recommend double-checking the sensor capacitors you are using; we recommend using NP0/COG grade capacitors which are extremely stable across temperature. In addition, these are the methods we recommend to test the coils' Rp values: http://www.ti.com/lit/an/snoa936/snoa936.pdf

    2. Yes, this is definitely possible. We use a single default drive current setting for the LDC1612EVM, which has PCB coils.

    3. Not necessarily. If the sensor oscillations look stable, then it may be an issue with the mechanical setup or other environmental conditions.

    Best Regards,