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LDC1000 LC TANK current

Peter Verheij
Prodigy 140 points

How can I calculate the LC TANK current ?

Actually I need to know the maximum current draw with a given  LC TANK + target material + distance.

My application is on a tight power budget , but conditions on the minimal and maximal Rp can also be accurately determined

  • 0
    TI__Expert 6550 points
    Hello Peter,

    The current LDC drives into the LC tank is: Vpp/Rs, where Vpp is the Voltage Amplitude you set in the GUI, and Rs is the series resistance of your LC tank. Please note that Rs is measured by the impedance analyzer in DC. At operating frequency, you can approximate AC Rs to DC Rs since skin effect wont take much affect at those frequencies. For power budget, you need the current consumption of the IC itself + the current LDC drives into the LC tank. The current in the LC tank is Q(quality factor)*the current driven by the LDC.

    Best Regards,
    Natallia Holubeva
  • 0 in reply to Natallia Holubeva1
    Prodigy 140 points

    My coil measures 6.5Ω DC resistance. and has 34μH : it oscillates at 1.9MHz with a 200pF capacitor.

    When I calculate Rs back from the formula in the FAQ I get values in the 10 ....13Ω range depending on temperature.

    4V/10Ω=400mA ! this can hardly be the case !

    In order to get an answer a little mre close to reality, I took the EVM module and I cut the Vdd trace and bridged it with a BAT54 shottky diode.

    Now Vdd is 4.7 V instead of 5V but at least the whole thing starts up. When everything is up and running i put a multimeter in current measuring mode across the

    BAT54 diode.Total current consumption is 2.57mA.

    Now this gives me an idea of what to expect, but it is not a worst-case value that i need.

    About skin effect : skin depth is 66.2/√f [mm] and comes to 0.046mm at 2MHz, My PCB traces are 0.1mm , about twice the skin depth, so it starts to play a role.

  • 0 in reply to Peter Verheij
    TI__Expert 6550 points

    Hello Peter,

    The minimum Rs LDC1000 can work with is 798 Ohms as described in the datasheet. Please, refer to page 10 of the datasheet. What is the quality factor of your sensor? The following link will also be useful for you: http://e2e.ti.com/blogs_/b/analogwire/archive/2014/11/21/inductive-sensing-sensor-frequency-constraints

    Please, let me know if you have any question.

    Best Regards,

    Natallia Holubeva

  • 0 in reply to Natallia Holubeva1
    Prodigy 140 points

    The problem in this discussion is apearently that you mention Rs while you mean to say Rp,  i'll reconsider things.

    The minimum valu for Rp is progammed to 9235

    Q factor is about 32

    The link to the blog give valuable information, thank you

  • 0 in reply to Peter Verheij
    TI__Expert 6550 points
    Hello Peter,

    I am sorry - I meant Rp in my first post. The Q looks good.

    Best Regards,
    Natallia Holubeva