AN3249 datasheet(3/7 Pages) STMICROELECTRONICS

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AN3249

M24LR series capacitance for antenna tuning

Figure 2. M24LR equivalent circuit

VOC represents the open circuit voltage available from the antenna. When a sinusoidal

magnetic field (H) flows through the M24LR antenna, the open circuit voltage amplitude

(VOC) can be determined using the following equation.

Equation 1

In this equation, N and S are respectively the number of turns and the surface of the M24LR

antenna, μ is the magnetic permeability of air, H is the magnetic field amplitude, and

ω =2• π • f (f = 13.56 MHz).

RA and LA represent respectively the resistive part and inductive part of the antenna

impedance. RS and CS represent the serial equivalent model of the M24LR impedance.

The voltage amplitude (VAC0-AC1) available on the M24LR coil pads AC0 and AC1 can be

determined using the following equation.

Equation 2

In this equation, ZS is the complex serial impedance of the M24LR. ZS= RS + j • XS and

XS = –1/(CS • ω).

As a image of the M24LR power consumption, RS is a function of VAC0-AC1. The variation of

CS as a function of VAC0-AC1 is shown at low power by the curve in Figure 1.

In the presence of a sinusoidal time varying magnetic field, the voltage VAC0-AC1 available

across the M24LR antenna pads depends on Equations 1, 2 and the variation of CS and RS

as a function of VAC0-AC1.

VOC

•μ h ω

•

VAC0 AC1

–

VOC

ZA ZS

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