How can I adjust the output power of the OM5577

[mdylewsky]

     Hi,

Actually, this is a two-part question:

1) I am curious what the output power of the OM5577 is. It doesn't seem to be listed in any datasheets. I'm trying to work with very small NFC tags, so I need a certain level of power.

2) I'm looking to adjust the output power of the OM5577. Whatever it's at now, it's not high enough, because it's having trouble reading these NFC tags. Is there any way in software or hardware to adjust the power? I've found nothing of substance in any datasheets or application notes.

Thanks,

Matt

[Jorge_Gonzalez]

Dear Matt Dylewsky,

Thank you for using the NFC community.

I will replicate here the information provided in your support case, for other customers interested.

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The field strength is mostly determined by the antenna design and the matching circuit. You can check the application notes AN11564 and AN11565 about this subject:

http://cache.nxp.com/documents/application_note/AN11564.pdf

http://cache.nxp.com/documents/application_note/AN11565.pdf

There is a way to control TX pins impedance via the CWGsN setting:

- Default value for Tx conductance is F which means lower resistance/impedance at Tx and max current.

The value can be set from 1 (min power/current) to F (max power/current) and this can be manually changed looking at the register CLIF_ANA_TX_AMPLITUDE_REG, bits 28 to 31. [See the attached picture].

You can refer to the PN7120 User Manual and the application note AN11564 about register configuration.

- The max RF Output power is set by Antenna Tuning once the GSN value in reader mode is set (default to max value = F). Different antenna Z tunings are possible (Transmitter Load). More the antenna tuning is at low impedance and more current will be on the antenna.

The ITVDD current must not exceed the limiter value 150 - 180 mA.

With a recommended Antenna Tuning @50-70 ohm the ITVDD is 60-100 mA and having TVDD = 3.1 V we can calculate a RF Power = 310 mW = .31 W.

At the limiting threshold current = 150-180 mA the max RF output power is 0.5 W.

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Regards!

Jorge Gonzalez

[jlcheung]

Hi,

I have a question about the match both antenna and drivers.

With a recommended Antenna Tuning @50-70 ohm,  the value of Tx conductance can be set from 1 (min power/current) to F (max power/current) .

They didn't need to match？ the Antenna impedance isn't equal to the Tx impedance(conductance), is't ?

[Jorge_Gonzalez]

Hi JL Cheung,

TX impedance is the combined impedance of EMC filter + matching circuit + antenna. It is not strictly required that the TX impedance matches the TX pins resistance/impedance, although it might be convenient according to maximum power transfer theorem.

You need to decide on TX impedance (transmitter load) according to your requirements:

- Lower TX impedance equals higher antenna current (max 180 mA for PN7120) and higher distance.

- Higher TX impedance equals lower antenna current sacrificing output power, hence shorter distance.

Regards!

Jorge Gonzalez