A large-amplitude current of 13.56 MHz flows through an NFC inductor. For this reason, different points must be considered when selecting an NFC inductor as compared to a typical matching inductor. This section explains the characteristics of an NFC circuit and the key points for selecting an inductor.
Previously, it was common to use an IC with a low output during transmission as the NFC control IC. However, as NFC antennas continue to grow smaller in order to be mounted in mobile devices, the NFC communication performance decreases with the smaller current amplitude. Therefore, the number of ICs with a high output during transmission is increasing in order to achieve high NFC communication performance. When selecting an NFC inductor, it is important that the inductance does not change even when there is a high current amplitude.
Figure 3 shows the dependency of the inductance on the current amplitude for the LQW18CNR16J00 (winding type), LQM18JNR16J00 (multilayer type: new), and the LQB18NNR22J10 (multilayer type: old). During NFC communications, an alternating current with a current amplitude of approximately 100 to 700 mA (peak to-peak) flows through the inductor. The inductance of the LQB18N roughly doubles at a current amplitude of 400 mA (peak to-peak), but the LQW18C and the LQM18J achieve an inductance that does not change even when the current amplitude exceeds 1 mA (peak to-peak).
Not only does the LQM18J maintain a constant inductance while conducting a current, it also exhibits favorable NFC communication performance. As shown in Figure 4, the NFC Forum (industry organization which formulates NFC standards and common specifications) measured the communication performance at the measuring point indicated by the red dot during their performance tests. The results of the LQW18C and LQM18J communication performance measurements are shown in Figure 5. The 4.1 V line is the passing standard for this test, and higher voltages indicate favorable NFC communication performance. The LQM18J and the LQW18C are able to achieve equivalent communication performance.
Figure 3 Comparison of the inductance change rate vs. the current amplitude for the LQW18C, LQM18J, and LQB18N
Figure 4 Measuring point for the NFC Forum compliant communication performance measurement
Figure 5 Results of the LQW18C and LQM18J communication performance measurements