NXP Semiconductors
AN11740
PN5180 Antenna design
AN11740
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Application note
COMPANY PUBLIC
Rev. 1.1 — 19 June 2018
345311
39 of 62
(1) ITVDDloaded < ITVDDunloaded
Fig 34. Antenna impedance example with Reference PICC loading
The change of the impedance curve indicates a lower ITVDD under loading conditions.
That implies that with the above shown antenna matching the loading with
ReferencePICCs (or typical smart cards) does even decrease the field strength and
ITVDD. This even sometimes helps to meet the maximum EMVCo limits for power
transfer, especially with small antennas.
This loading must be cross checked under normal operating conditions (see 4.2.2.3).
Note:
The loading with the ReferencePICC typically turns the impedance circle in the
smith chart clockwise. At the same time the overall Q-factor drops, so the circle
gets smaller. Due to the tuning of the antenna, the impedance almost stays
resistive, but moves towards a higher impedance. A new resonance (small loop)
at a frequency above 13.56MHz can be seen: this is related to the resonance
frequency of the ReferencePICC itself.
4.2.2.2 “Passive” loading with metal
The other most critical case typically is the loading with some large metal plate. Such
metal plate e.g. might even be an NFC tablet or a large NFC phone.
The Fig 35 shows the unloaded impedance curve (blue) as well as the impedance curve,
when a large smart phone loads the antenna (green), where the battery acts as a
magnetic short cut. In this example the worst-case loading effect occurs in almost 2cm
distance.
