NXP Semiconductors
AN11740
PN5180 Antenna design
AN11740
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Application note
COMPANY PUBLIC
Rev. 1.1 — 19 June 2018
345311
30 of 62
EMVco specifications as well as the ITVDD limit of the PN5180 (250mA), if the antenna
impedance is low as such (i.e. in the typically required range of 20 Ω).
The PN5180 provides a special function to dynamically control the ITVDD, which is
called DPC. The DPC dynamically controls the ITVDD, and therefore the field strength.
So, with the help of DPC the advantages of the symmetrical antenna tuning can be
combined with a low antenna impedance without exceeding the specification limits.
Especially for EMVCo design with small antennas the symmetrical tuning shows its
advantages, if carefully designed. Details of the DPC and the symmetrical antenna tuning
can be found in [14].
For symmetrical antenna tuning the following EMC filter values are chosen:
L0 = L0A = L0B = 470nH
C0 = C0A = C0B = 220pF + 33pF
f
Cutoffsym
= 14.6MHz
Note:
This EMC filter with a lower cut off frequency is chosen to improve the transfer
function.
Note:
Without proper DPC calibration the loading and detuning might exceed the
ITVDD limit, if the symmetrical tuning is used. This might destroy the NFC reader
IC.
4.2.1.4 Calculate the matching components
The next step is to calculate the values of the matching circuit. The input for the Excel
sheet as shown in Fig 25 needs to be:
“Measured” values:
La = L = 1533nH (measured antenna coil inductance)
Ca = C
pa
= 0.1pF (estimated parallel capacitance of the antenna coil)
Ra = R
Coil
= 3Ω (measured antenna coil resistance)
Preset values:
Q = 25 (defined Q-factor, see section 4.2.1.2)
Rmatch = 20Ω (defined target impedance, see section 4.2.1.2)
L0 = L0A = L0B = 470nH (EMC filter inductance, see section 4.2.1.3)
C0 = C0A = C0B = 56pF + 68pF (EMC filter capacitance, see section 4.2.1.3)
Calculated Values (see Fig 25):
Rq = R
Q
=
1.1Ω
