AN3249 Application note M24LR series internal capacitance considerations for antenna tuning 1 Introduction The datasheet of the M24LR series devices(a) specifies a typical internal tuning capacitance value of 27.5 pF measured at 0.5 VPEAK. While this value is correct in production test conditions, more considerations have to be taken into account for tuning the antenna in an real application environment. This document describes the M24LR series tuning capacitance variation as a function of input voltage, explains the M24LR series tuning capacitance specification and gives a robust tuning capacitance target for the inductive loop antenna design. Table 1: Applicable products Type Applicable products M24LR series(1) Dual Interface EEPROM 1. While the considerations introduced in this document applies to the entire M24LR series, the M24LR64-R has been used hereafter for specific measurements purpose. a. I2C/ISO15693 Dual Interface EEPROM IC December 2013 DocID017760 Rev 2 1/7 www.st.com Tuning capacitance variation as a function of input voltage 2 AN3249 Tuning capacitance variation as a function of input voltage Figure 1 shows an example measurement of the M24LR serial equivalent capacitance as a function of the VAC0-AC1 input RF voltage. The capacitance is measured between 0.2 VPEAK and 2 VPEAK at 13.56 MHz using a network analyzer. Figure 1. M24LR64-R serial equivalent capacitance CS measurement example 069 The dotted line in Figure 1 corresponds to the voltage level (0.5 V) at which the serial capacitance (CS) of the M24LR series is measured in production and specified in the datasheet (27.5 pF). This voltage level has been chosen on the plateau of the capacitance curve to ensure the best measurement repeatability. 3 M24LR series capacitance for antenna tuning 3.1 M24LR series equivalent schematic Figure 2 shows the equivalent circuit of an M24LR mounted on a loop antenna, in the presence of a magnetic field. 2/7 DocID017760 Rev 2 AN3249 M24LR series capacitance for antenna tuning Figure 2. M24LR equivalent circuit 5$ 56 /$ $& 92& &6 9$&$& $& $QWHQQD 0/5 069 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 V OC = N • S • μ • h • ω 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 ZS V AC0 – AC1 = V OC ------------------ZA + ZS 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. DocID017760 Rev 2 3/7 7 M24LR series capacitance for antenna tuning AN3249 Optimizing antenna tuning consists in designing an antenna with the correct impedance for the correct tuning capacitance. Considering variations of CS according to VAC0-AC1, the correct capacitance value used as reference for antenna design must be defined. Please refer to AN2972 “Designing an antenna for the M24LRxx-R and M24LRxxE-R dual interface I2C/RFID devices” for more details on antenna tuning criteria and methodology. 3.2 Correct M24LR series capacitance value for antenna tuning According to Equation 1, VOC depends on the antenna dimensions, number of turns and the magnetic field amplitude flowing into the antenna. As a consequence, for a given antenna, VOC increases when the distance between the M24LR antenna and the reader antenna decreases (because the magnetic field increases). Figure 3 represents the M24LR input voltage variation as a function of VOC for an antenna tuned on a M24LR serial capacitance value based on the curve shown in Figure 1. As explained above, this case corresponds to a decreasing distance between M24LR antenna and reader antenna; the longer the distance from the reader antenna, the closer the value of VOC gets to 0 V . Figure 3. M24LR RF voltage 5HVRQDQFH 9$&$& 492& 92& 069 As VOC increases, a sharp increase of VAC0-AC1 occurs when the serial capacitance of the M24LR reaches a value that satisfies the resonance condition LA • CS • ω2 = 1. At resonance, VAC0-AC1 = Q • VOC where Q = RS/(RA + RS) • 1/(RS • CS • ω). As an example, the Q value for the ANT1 reference design is between 30 and 35. The maximum operating distance from the reader is defined by the maximum distance for which the M24LR input RF voltage VAC0-AC1 remains above 1.9 VPEAK which is the minimum voltage for the M24LR to be properly energized and perform read and write operations. Optimizing an antenna for an operating range consists in designing an antenna allowing VAC0-AC1 ≥ 1.9 VPEAK at the largest distance from reader. 4/7 DocID017760 Rev 2 AN3249 Conclusion To satisfy this condition, antenna inductance must satisfy the resonance criteria LA • CS • ω2 = 1, where CS is the M24LR serial capacitance measured at the minimum operating voltage 1.9 VPEAK (as shown in Figure 1, where CS is close to 29 pF at 1.9 VPEAK). To demonstrate this, Figure 4 represents the M24LR RF Input voltage VAC0-AC1 as a function of the RF voltage VOC available from the antenna for different inductive loop antennas tuned respectively for CS1 < CS2, CS2 = 29 pF and CS3 > CS2. The comparison graphics shown in Figure 4 assume using the same antenna dimensions and number of turns. Figure 4. M24LR RF input voltage different antenna inductance $QWHQQDWXQHGIRU &6 S) 9 9$&$& $QWHQQDWXQHGIRU &6&6 $QWHQQDWXQHGIRU &6!&6 92& 069 As shown in Figure 4, the minimum RF voltage VOC for which VAC0-AC1 is above 1.9 VPEAK (and consequently maximum distance at which the M24LR can operate) is reached for an antenna inductance LA designed for CS = 29 pF. 4 Conclusion Taking into account this information, STMicroelectronics recommends designing antennas for the M24LR series using CS = 29 pF as a reference value, instead of the value specified in the datasheet which corresponds to production test conditions. DocID017760 Rev 2 5/7 7 Revision history 5 AN3249 Revision history Table 2. Document revision history 6/7 Date Revision Changes 23-Jul-2010 1 Initial release. 13-Dec-2013 2 Extended the scope of this application note to the entire M24LR series. 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