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WEBENCH® Tools: webench coil designer

sean platnauer27
Prodigy 10 points
Other Parts Discussed in Thread: LDC0851

Tool/software: WEBENCH® Design Tools

first is the current the current in the coil

second the copper thickness reduction seems to increase the currant is this correct , the dc resistance increases and the currant increases. 

can you please explain why

thanks Sean

  • 0
    TI__Intellectual 995 points
    Hi Sean,

    The current reported by tool is the current which flows through the coil. This is the current signal which is driven into the coil. The voltage is fixed (e.g. 1.8V for LDC0851, 4V for LDC1000 etc.), as long as the sensor inductance is above the minimum value.

    So a higher dc resistance from a thinner copper can result in a larger current. This has the effect of increasing the device current consumption.

    Regards,
    Pradeep
  • 0
    TI__Intellectual 1490 points

    Hi Sean,

    The current depends on the resonant impedance of the LC tank (impedance at the resonant frequency) Irms = Vrms / Zresonance. So as the series resistance of the coil increases (with copper thickness reduction), due to increased damping, the resonant impedance actually decreases and hence the current through the LC tank increases (for a fixed rms input voltage). For example with LDC0851 and default coil selection, we get Resonance Impedance = 14408.49 Ohm. If we assume Vp-p of the LC driver = 3.3V (max supply voltage), Vrms = Vp-p/(2*sqrt(2) ) = 1.167V, so Irms = Vrms/Zresonance = 1.167/14408.49 = 0.08mA (same as you see in the tool).

    To illustrate the resonant impedance variation with series resistance, below example shows the plot of impedance of parallel LC tank with varying series resistance of the Inductor (L = 5.35uH, C = 68pF, R = 4.54 Ohm, 5.46 Ohm, 7.7 Ohm). You can observe, as resistance increases the resonant impedance decreases. (Note: This is just an ideal example which shows the effect of damping on resonant impedance - the values may not match exactly with the coil designer tool as this example not include additional parasitic effects like self resonance of coil).

    Regards,

    Srikanth Pam

    Online Design Tools