• State Resolved
  • Locked Locked
  • Replies 4 replies
  • Answers 1 answer
  • Subscribers 31 subscribers
  • Views 231 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.

LMT01: LMT01 resistor range

Egil Opsahl
Prodigy 30 points
Part Number: LMT01

What is the usable range of the resistor that converts current to voltage?

Concretely: A 22k resistor would provide logic levels (roughly) compatible with digital GPIO input levels in a 3,3V system.

  • 0
    TI__Genius 9675 points

    Hi Egil,

    You have to take into consideration that the VP - VN during conversion needs to be minimum 2.15V. Using a 22k would not meet this requirement.

    We recommend a 6.81k resistor. Calculations for this are discussed in detail and can be found in section 8.2.1.2 of the LMT01 datasheet. 

    Jalen

  • 0 in reply to Jalen Tate89
    Prodigy 30 points

    Hi Jalen,

    Thank you for your rapid response.

    Before posting my question I did the calculation based on the datasheet, 8.2.1.2 Detailed Design Procedure, and found:

    V_LMT01 = 3.3V - (22k * 1,01) * 39 µA = 2,43V

    So the 2.15 V requirement seems to be met. '

    In the below video from TI a 13,4 k resistor is used. So apparently that value is OK.

    So please allow me to reiterate my question: What is the usable range for the resistor?


  • 0 in reply to Egil Opsahl
    TI__Genius 9675 points

    Egil,

    VP - VN must always satisfy 2.15V. This has to satisfy both the high end lower end of the pulse. You've done the calculation for the lower end, but on the higher end you get 3.3 - (22000 * 140E-06) = .022V. This would not satisfy the condition.

    In the example video, they use a 5V rail to power the LMT01 so VP = 5V in this case. On the high end they get 5V - 1.89V = 3.11V which satisfies the condition. You cannot use a 13.4k resistor for a 3.3V system.

    If you have a concern about reaching the logic levels of your MCU, you will need to use a transistor for level shifting as shown in section 8.3 of the datasheet.

    Jalen

  • 0 in reply to Jalen Tate89
    Prodigy 30 points

    Jalen,

    Thank you for your additional explanation which I believe is not accurate - but enabled me to understand where I got confused.

    Point 1) in datasheet "8.2.1.2 Detailed Design Procedure" is actually the calculation of the maximum value of the resistor in order to respect the minimum VP-VN requirement during data transmission, which is 2.0V (not 2.15).

    Which means that smaller resistor values than calculated here can be used (which is why, in the example calculation, they are selecting downwards from the calculated value). But not larger values - as the 2.0V requirement would not be met.

    Again thank you for your help,

    Kind regards,
    Egil