Power requirements for NFC (RFID) chip

Question

I'm new to electronics so please bear with me. I'm trying to work out what kind of battery would be required to use with the following chip:

http://www.nxp.com/products/interface_and_connectivity/nfc_contactless_reader_ics/MFRC52202HN1.html#overview

The chip will be used in a portable device that needs to run for 10 hours and 200 NFC tag scans before the battery needs to be replaced/recharged. The chip will be set to constantly scan for NFC tags.

I've read that a NFC tag scan would normally use around 15mA but I can't work out how much power a chip like this uses while actively "scanning" for tags and how that translates to battery usage?

Are there any tutorials that would help me get my head around this?

Answer 1

If you read the datasheet, it gives information on the typical and maximum current consumption for the IC. It also gives power down consumption and other details. I'd also check the other documentation (app notes, etc) to see if there is more information/advice on power characteristics.

On page 3 note 7, it says typical current consumption is below 100mA. 60mA is given as the typical consumption for the transmitter supply, digital and analog ~7mA each, and the pin supply up to 40mA. With this info you can get a pretty good idea of what capacity battery you need for 10 hours operation.

Assuming maximum consumption, and less than 100% efficiency (e.g. regulator used) we can do some calculations. We'll assume 80% efficiency and 100mA continuous draw:

100mA * 10h * (1/0.8) = 1250mAh

This is probably very, very conservative, but gives you a figure which will certainly be plenty. However the only way to get very accurate figures is to run some tests yourself, since consumption will depend on how many IO pins you are using, other circuit activity, regulator efficiency, temperature, etc, etc.

I would probably look at using a 3.7V Li-Ion cell, since it's a good voltage, they are conveniently sized and there are plenty of options around 1000mAh (eBay, Sparkfun, Digikey, etc) There are also cheap charging ICs available (Microchip do some good ones) Of course it's up to you, anything that provides the required capacity would work, but you have to consider what is most suitable for your project (size, weight, cost, etc)

Answer 2

You need to distinguish worst case and average power consumption. It occurs to me you are more interested in the latter.

Average power consumption

Here you have to understand how RFID and specifically 14443 A work. In a nutshell, the reader keeps its antenna resonating at 13.56 MHz, and when a token is in the field, the latter gets power. Token has an antenna tuned to the same frequency ±manufacturing error. Thus both antennas resonate together.

Furthermore reader sends out commends to the token by modulating the field, while token responds by by modulating losses in the established resonating circuit.

In the simple case, protocol requires only a few messages back and forth.

This should give you the baseline, that is the minimum work required to identify a token.

Real energy use will be much higher than baseline:

• most of the time there is no token and energy is lost in reader antenna and surrounding air. although only a little at a time, it will amount to a lot over hours of operation.
• some tags are worse than others, losing a lot more energy per read
• if several tags are present in the field, reader goes through anticollision sequence, totally oversimplifying the subject, think of reader trying to talk to every card it may see in sequence over and over
• if your environment contains any conductive parts, energy is lost as Eddy current
• your reader and microcontroller need energy to run too

In practice you need to build a prototype, go out in the real world and test your system. You will be surprised how much your runtime will depend on simple factors most notably temperature (cold kiss batteries) and nearby objects (your body or metal of your car will eat up reader field). Smaller factors are humidity, how slow tokens are brought in and removed, and many others.

However not all is lost, there are energy efficient readers out there, here are some tricks I've seen in the field:

• reader is not constantly on, the antenna is switched on long enough to detect and card, then switched off for a "short" period of time
  • read every second -- requires that card is brought next to reader and kept there for a while
  • read 4~10 times a second -- almost imperceivable to average user, yet massive power saving
• extra input to detect that a token is likely nearby
  • light sensor, when there's a sudden ambient light change, typically it got darker, start scanning for tokens and give up after several seconds
  • antenna integrated into or around a button, user has to push the button with their token to have it read. microprocessor and reader ic can be put to deep sleep until button is pressed. IIRC manufacturer claimed a year or several years of operation on a 9V lithium battery, several reads per day and some electromechanical actuation.

Worst case power consumption

While already covered by Oli's answer, perhaps you want to distinguish absolute worst power consumption (antenna internally shorted) from reasonable, external worst case, such as operation in rain, next to metal or sources of interference.