ST25R3911B Power supply

[RFlod.2]

I have PCB which support 3.3 V or 4 V power supply which I want to connect with ST25R3911B.

 

I want to run 4V power supply as it can provide more current and I assume outputpower of ST25R3911B  will be higher on 4 V than on 3.3 V.

In data sheet of ST25R3911B  it is stated that power supply is 2.4 to 5.5V so this should be fine. BUT when checking the registers 4.0V seems to be an invalid voltage:

 

IO configuration register 2

Bit 7 - sup3 V

0: 5 V supply
1: 3.3 V supply

 

5 V supply, range: 4.1 V to 5.5 V
3.3 V supply, range: 2.4 V to 3.6 V
min. 3.0 V for VHBR

 

So the question is what will happen when using 4.0V for 5V supply range? Will it not work at all?

Also when running PCB on battery voltage this will give voltage bewteen 4.0 to 3.0 V, which seems to be outside specification of ST25R3911B.

Can someone help how to use use ST25R3911B in my application?

 

[Ulysses HERNIOSUS]

Hi RFlod.2,

similar has been discussed in st25r3911b-vdd-range. Similar holds also or ST25R3916(B).

Running ST25R3911B with 3-4V is not outside of the spec of ST25R3911B itself. But for the voltage regulator you will not find a setting which give you a proper drop and thus supply noise suppression.

sup3V should only be set below 3.6V. For ]3.6V;4.1V[  you will need to clear sup3V=0 leading to the the regulator operating without proper voltage drop.

You have also the option to run completely without the regulator / bypassing it.
Even with bypassed regulator l would still recommend in your battery supervision code to adapt the regulator setting  (sup3V, rege/reg/ adjust regulator) to have proper VSP_A voltage and drop.

BR, Ulysses

[RFlod.2]

Thanks for answer.

 

So I need highest possible detection range of RFID tags which option would be best?

1) Run ST25R3911B on 4.0 V in 5 V mode

or

2) Run ST25R3911B on 3.3 V in 3.3 V mode (would mean I need to replace LDO, but is possible)

I would guess output power is higher in 4V power supply than 3.3V ?

 

Also a fast check In RFAL library I could not find where to change supply voltage register..

[Ulysses HERNIOSUS]

Hi,

If you want highest power, then bypassing is probably the best option - not going through the regulator power MOS with its small but existing resistance.

changing supply voltage I would recommend to perform in CHIP_INIT section of the analog configs of RFAL. Currently it clears reg_s bit. Subsequently the init routine will measure VDD, set sup3V accordingly and cleared reg_s causes Adjust Regulators command to be executed.

In case you want to manually set sup3V then you may need to remove this code part and perform settings of regulators within AC. 

Or if you anyhow need to do such steps during run-time, then just perform your own adjustments after initialization - overriding what RFAL proposed.

Ulysses

[KevinB]

Hey RFIod.2,

I also struggled with this recently where I had VDD powered from a battery, and it did not work!  My summary conclusions were...

1. The full VDD range (2.4V to 5.5V) is supported only if the internal regulator is bypassed (as Ulysses mentioned).

2. I chose to use the internal regulator because that seems the more common implementation.  And when using the internal regulator, then VDD must be either in the 3.3V range (2.4V to 3.6V) or the 5V range (4.1V to 5.5V).  These are the levels defined in the IO Configuration Register 2 info, and VDD = 4.0V is not a valid level. I changed my design to power VDD with a 3.3V rail and my issue was resolved.

As for your question about power, I think the radiated power is largely dependent on the antenna current.  As the voltage is reduced, you can keep the antenna current about the same by reducing the DC resistance of the antenna path.