Prerequisites:
CLRC663 plus Low Power Card Detection
1// Antenna Size
The stronger the coupling between reader and card, the better the detection range.
Ideally, the NFC reader antenna should have a size and form factor similar to that of the target NFC tag antenna. In practice, the reader antenna is typically designed to be slightly larger—by approximately 10% to 20%—to ensure reliable coupling and performance.
2// Antenna Q-factor
Higher Q-factor typically serves higher detection range. The Q-factor should be selected in accordance with the target communication bit rate.(e.g. Q≈30 for 106 kbit/s).
The antenna Q factor is mainly defined by the mechanical design of the antenna itself, as well as by external damping resistors.
However, for LPCD-insensitive systems, a zero-ohm damping resistor can be used, and the system should then be evaluated through testing.
3// Target tuning
The CLRC663 should typically be used with the asymmetrical tuning as there is no internal power regulation implemented.
However, in some cases, symmetrical tuning may be beneficial, as it can improve detection range and sensitivity. Care must be taken to ensure that, under varying conditions—such as antenna loading by different NFC tags or the presence of nearby metal, the TVDD current does not exceed the specified maximum limits.
In CLRC663 LPCD operation, detection performance improves with higher TX power. However, this also increases current consumption, so an appropriate trade-off must be determined.
As a starting point, an antenna impedance of approximately Z≈ 50 Ω can be used.
4// Receiver setting (external Rx resistors)
Based on the AN11019 (chapter 4.4.1), the peak voltage between RxN and GND (or RxP and GND) shall be in the range of 1.2Vp - 1.7Vp.
Generally, higher Rx voltage levels improve detection range. To increase LPCD sensitivity and range, it can be advantageous to operate close to the upper limit of the allowed Rx voltage.
Please note that excessively high RX voltage may lead to immediate false wake-ups.
5//LPCD Settings
As a starting point, we recommend using the following settings:
These settings provide robust immunity against false wake-ups and are highly efficient in terms of current consumption. The typical detection range for standard NFC tags is approximately 10 mm to 20 mm or even less in an application where the antenna is placed near a metal environment.
Detection performance can be further improved by lowering the threshold settings +0 and -0. If this settings is used, we strongly advice using LPCD_FILTER feature as well. Alternatively, the user can use "high detection range option" + LPCD_FILTER.
After using these settings, we recommend running "Endless LPCD" for a while and checking for any false wake-up rates.
Note: The LPCD filter feature helps prevent false wake-ups. This is especially important when the threshold is set to +0 and -0.
5.1// LPCD RF ON time
If the RF on-time is shorter than approximately 20 µs - 30 µs , the primary detuning effect is dominated by the physical design of the NFC tag. For longer RF on-times, the NFC tag becomes energized, and its electrical parameters begin to contribute significantly to the overall detuning.
Based on this understanding, a longer RF on-time can help increase LPCD detection range. However, this comes at the cost of higher current consumption. Therefore, it is recommended to compensate by adjusting the RF off-time. See an example below.
5.2// LPCD Charge pump
This function allows the TX power to be increased exclusively during the LPCD RF On time.
This can be beneficial if you want to avoid increasing the power in active mode (e.g., by reducing the tuning impedance) but only increase it during LPCD operation.
By activating the LPCD charge pump, detection performance can be improved, but at the cost of increased current consumption.
If this feature is enabled, it is recommended to verify the LPCD RF ping using an oscilloscope. It may be necessary to increase the RF On time to allow the amplitude to properly settle. Especially for TVDD= 3.3V or lower.
6// Summary
| Mode |
Tuning function |
Target impedance |
Q-factor |
Receiver voltage |
LPCD Threshold |
LPCD Filter |
LPCD Charge -pump |
RF On time |
| Standard |
Asymmetrical |
20 Ω - 80 Ω |
10 - 30 |
1.5 Vp |
+1 and -1 |
disable |
disable |
10 us |
| High detection |
Asymmetrical /Symmetrical (1) |
20 Ω - 50 Ω |
>25 (2) |
1.7 Vp |
+0 and -0 |
enable |
enable/disable (3) |
20 us-100 us |
Note (1): Symmetrical tuning can only be applied if the TVDD current does not exceed the maximum allowable value after antenna loading caused by a card or metal object. Please ensure that this condition is met.
Note (2): If high detection performance is required, the external antenna damping resistors are typically replaced with 0 Ω resistors, resulting in a higher antenna Q-factor. In this case, the user must verify that NFC communication continues to operate reliably. Due to the increased Q-factor, this configuration is typically limited to communication speeds of 106 kbit/s
Note (3): Once the charge pump is enabled, the RF On time should be verified and adjusted if necessary.
Please also note that the relationship between TX power and detection distance is not linear or unlimited. Beyond a certain point, enabling the charge pump provides only marginal improvements in detection distance.
Please note that the High Detection mode is generally more susceptible to false wake-ups and results in higher current consumption than the standard mode.
