RF power regulation is a critical factor in the development of NFC devices, as it directly influences performance, reliability, and compliance with industry standards.
There are three main reasons for this:
- If the PN7642 VUP current exceeds the limit given by the product Data sheet, the PN7642 can be damaged.
- If too high RF power is radiated from the antenna, there exists a risk for NFC Cards.
- Too high RF power might lead to exceeding a given RF limit (NFC Forum, ISO, EMVCo).
NXP provides comprehensive documentation on Dynamic Power Control for the PN7642 and PN5190. Designers are expected to adhere to these guidelines, especially when aiming for compliance with standards such as EMVCo.
- PN5190 Dynamic Power Control Quick Calibration and TxShaping Demo
- Automatic DPC Calibration for PN7642 and PN5190
However, if the user's design is intended for infrastructure applications, such as a smart lock. At a minimum, Dynamic Power Control (DPC) should be enabled to serve as a current limiter.
The evaluation can be done with the help of NFC Cockpit.
1// Start DPC Calibration
- "Press" Start DPC Calibration
- "Press" Load protocol
- Make sure that the DPC is "Enabled"
2// Adjust current reduction table
- Set all entries to "0"
- Write into EEPROM
3// Set the "Target" current
- Use approx. the same current as "TxLDO Vales"
- Save to EEPROM
4// Check the power regulation
- Start DPC Calibration
- Place a card or any metal object in the antenna's proximity
- Observe VDDPA and "TxLDO" current
- The current should stay around the given target
- The VDDPA will drop once the antenna is loaded
5// Set a minimum VDDPA in DPC
In the case that the current is still too high, a user can define a minimum VDDPA that is used for the DPC regulation. By default, this value is set to 2.2V. The user can decrease it up to 1.5V.
In that case, NXP also recommends disabling the RDOn control.
Note: The User has to consider the "DPC_TXLDO_MAX_DROPOUT" parameter, which defines the maximum voltage drop on TXLDO. By default, it is set to 3.6 V. That means if the user wants to use the minimum VDDPA 1.5 V, then the maximum TXLDO input shall not exceed 5.1 V. This feature protects the TXLDO from overheating.
Once the evaluation is done, the customer shall program the following EEPROM entries in their application. For more info, see PN7642 Product Data sheet.
DPC_CONFIG (Address: 0x0068) -> example: enabled -> 0x01
DPC_TARGET_CURRENT (Address: 0x0069) -> example: 229 mA -> 0xE5
DPC_TXLDO_MAX_DROPOUT (Addresses: 0x0073 - 0x0074) -> example: 3.6 V -> 0x10,0x0E
DPC_TXLDOVDDPALow (Address: 0x006F) -> example: 1.5 V -> 0x00
DPC_HYSTERESIS_LOADING (Address: 0x006B) -> example: 20 mA -> 0x14
DPC_HYSTERESIS_UNLOADING (Address: 0x006E) -> example: 10 mA - 0x0A
DPC lookup table entries (Addresses: 0x007D - 0x0125) -> example: for current limitation only -> all 0x00
If a user does not want to use a maximum range of VDDPA during DPC (5.7V), e.g., their system uses a 3.3V supply domain. Then, the maximum VDDPA in DPC can be limited by the following EEPROM settings:
TXLDO_VDDPA_MAX_RDR (Address: 0x0007)-> example: 3.0 V -> 0x0F
Note: TXLDO has approx. 0.3V voltage drop. Always set this parameter 0.3V lower.
Once this is done, the user has to check the "TxLDO" current and adjust the target current accordingly. In this case, to approximately 150 mA. If you don´t change it, the DPC starts to limit the power around 229 mA, as has been set in a previous step.
