Edit Date: August xxxxxxx. TRF7970A EVM Modification for Wireless Power in NFC Tag Emulation Mode ABSTRACT

Transcription

1 PRELIMINARY EVM Wireless Power TRF7970A EVM Modification for Wireless Power in NFC Tag Emulation Mode Kostas Aslanidis, Ali Ertugrul, Christian Buchberger ABSTRACT The TRF7970A EVM is development platforms based on the TRF7970A NFC/RFID Transceiver IC. Under normal conditions, the development platforms operate in RFID and NFC modes as Transceiver respectively Initiator. One of the NFC specified operating modes is the NFC Tag Emulation, where the device behaves like a Tag. For operation, the board is powered over the USB interface. For Tag Emulation mode, it can be desired that the board operate in a standalone mode without a connection to an external power source. In such applications wireless power supply could be a very big advantage. The scope of this document is a short guide how to modify the board for wireless powering. Copyright 2013, Texas Instruments Incorporated.

2 Table of Contents Page 1 Scope TRF7970A EVM Wireless Powering Modification for TRF 7970A EVM Connecting the Diodes on TRF7970A EVM Tag Emulation FW for MSP430F Performance Measurements Annex 1: Setup Parameters... 7 Summary of Figures Page Figure 1: TRF7970A EVM Modification Schematic... 4 Figure 2: TRF7970A EVM Board Modification... 5 Figure 5: Voltage Measurements Scope Copyright 2013, Texas Instruments Incorporated. 2

3 The TRF7970A EVM is powered normally over the USB port. The scope of this document is to show a concept which allows the evaluation board to operate without power supply over the USB. In that case the power is derived from the RF field using the onboard or an external antenna and some minor modification on the TRF7970A EVM. The HW modification is the addition of two diodes. To simplify the implementation and the tests, a FW is provided supporting only the NFC Tag Emulation mode. This FW version allows the EVM to operate in a standalone mode. The wireless operation will be an advantage for applications, where the NFC Tag Emulation Mode is required. The scope of this document is a short guide how to modify the circuit of the EVM boards, to allow a wireless powering operation. 2 TRF7970A EVM The TRF7970A EVM is a reference design intended to be used as development platforms for applications where the TRF7970A NFC Transceiver IC is used. Normally it acts as RFID/NFC readers/initiators, but do also support the NFC Tag Emulation Mode. For this Tag Emulation mode it can be desired that the boards are powered wirelessly to eliminate the need of a battery or the connection to an external power source. The modification are simple and low cost. Copyright 2013, Texas Instruments Incorporated. 3

4 2.1 Wireless Powering Modification for TRF 7970A EVM To use the onboard PCB antenna as the power source, the antenna has to be tapped between R6 and C21 (at the Coil, see Figure 1), by adding a diode. For better performance, it is recommended to use a diode with a low forward voltage drop. The voltage drop should be as low as 0.4V@10mA and suggests the use of Schottky Diodes (e.g. BAT43). To prevent overvoltage at pin Vin, that could harm the circuitry, a Zener diode with a breakdown voltage of 5.1V has to be added between Vin and Ground. No additional secondary antenna or power management is required. That makes the modification very cost efficient. Figure 1 shows the simplified circuit diagram for the modifications. The EVM board design files can be found under: Figure 1: TRF7970A EVM Modification Schematic The Coil represents the onboard antenna. C23, C24, R6, C21 and C22 form the antenna matching circuit. Any other external antenna can be used instead. The connection of the Diode must be always directly at the antenna. The antenna shall be re-mached after the impmlementation of the modifications. To improve the performance, the resistor R6 can be changed to 6kOhm. The corresponding matching component values for R6=1kOhm and 6.8kOhm can be found in Connecting the Diodes on TRF7970A EVM The Schottky Diode is connected between the Coil and Vin and the Zener Diode between Vin and Ground. The connection to ground can be chosen freely, the easiest would be the SMA connector pinholes. See Figure 2. Copyright 2013, Texas Instruments Incorporated. 4

5 Figure 2: TRF7970A EVM Board Modification 2.3 Tag Emulation FW for MSP430F2370 A FW version for the MSP430F32370 supporting the NFC Tag Emulation Mode, is available on the TRF7970AEVM product folder. Copyright 2013, Texas Instruments Incorporated. 5

6 2.4 Performance Measurements Figure 3 shows the operating voltage when the board is powered wirelessly. The tests have been executed using following Readers: - TRF7970AEVM with the on-board antenna - TRF7970AEVM with a 2.5x2.5 cm antenna - TRF7970A_PA, a 1W Power Amplifier based on the TRF7970A with an external 5x5cm antenna. 6 5 Voltage over Distance Measurement Reader: TRF 7970A EVM Reader: TRF 7970A EVM with 2.5x2.5 cm antenna Reader: TRF 7970A 1W with 5x5 Antenna 4 Voltage [V] Distance [cm] Figure 3: Voltage Measurements The Reading distance: - TRF7970AEVM with the on-board antenna 3cm - TRF7970AEVM with a 2.5x2.5 cm antenna 2cm - TRF7970A_PA, a 1W PA5x5cm antenna. 5cm Copyright 2013, Texas Instruments Incorporated. 6

7 3 Annex 1: Setup Parameters The components and the parameters used in the tests are listed below. A BAT43 Schottky diodes with forward voltages of 250mV@10mA A Zener diode with a breakdown voltage of 5.1V Copyright 2013, Texas Instruments Incorporated. 7