AN4326 datasheet(7/17 Pages) STMICROELECTRONICS

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AN4326

Choosing the external ESD protection

The break down voltage (VBR) determines the voltage level at which the ESD protection

turns on. To protect the M24LRXXE-R efficiently, the VBR must be as low as possible.

However, during an RF operation, the RF voltage values present between AC0 and GND,

and AC1 and GND, are applied to the external ESD protection. As a consequence, the

external ESD protection also clamps the RF voltage on AC0 and AC1 as soon as it exceeds

VBR. When RF readers use a low modulation index to communicate with tags, the RF

modulation can be canceled, causing communication holes at short distance. Overcoming

this by using a high VBR ESD protection will lead to a weak protection of the M24LRXXE-R.

To offer the best ESD protection to the M24LRXXE-R (8 kV at contact discharge and 15 kV

at air discharge), the external ESD protection voltage levels should be:

•

VBR ≤ 9 V

•

VF ≤ 1 V

The voltage level received on AC0 and AC1 depends on the reader power and antenna, as

well as on the tag antenna. The distance at which the communication stops then depends

on these parameters but also on the modulation index used by the reader. The RF

performance of the M24LRXXE-R tag using an external ESD protection must be validated in

the customer's system.

Since the ESD is affected by the antenna, the ESD robustness must also be validated by

the customer.

Note:

Section 5 and Section 6 show the IEC61000-4-2 ESD test results and the RF read range

results using an NFC phone with a 65 x 45 mm 6-turn antenna.

2.2

Parasitic capacitance of the ESD protection and tag antenna

design

Placing an external ESD protection between the antenna and the M24LRXXE-R causes an

additional parasitic capacitance between the AC0 and AC1 lines.

If Cpar is the parasitic capacitance between AC0 and AC1 due to the external ESD

protection, and CM24LRXXE-R is the M24LRXXE-R built-in tuning capacitance, the total

capacitance used to design the tag antenna inductance becomes:

The inductance of the antenna must then satisfy:

where Ftune is the tuning frequency of the tag.

Note:

See AN2972 for more details on antenna design basics.

The read range of the M24LRXXE-R is given by the distance at which the AC0-AC1 AC

magnitude voltage reaches 2 V, or the AC0-GND (or AC1-GND) voltage reaches 1.8 V. At

this voltage level, the external ESD protection is not active and behaves as a parasitic

capacitance. The maximum read range only depends on the reader (power and antenna)

and the tag antenna (dimension and turns) and the tag antenna tuning frequency.

As a consequence, the total capacitance value, including the external ESD protection, is a

key parameter for antenna tuning and performance. Its value must be evaluated before

designing the antenna.

Ctot C

M24LRXXE-R

Cpar

Lantenna

Ctot 2 π

Ftune

()

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