Design of RFID Handheld System Based on Embedded System

RFID handheld devices are widely used in transportation, access control, logistics, attendance, cargo management, and identity recognition. RFID handheld devices have high requirements for power efficiency, service life, reliability, volume, and cost. Therefore, designing a power supply with good stability, high efficiency, and low stray emission is of great significance for RFID handheld devices.

1 RFID handheld device hardware structure

In the design of the RFID handheld device system based on the embedded system, the microprocessor LPC2142 is used as the main controller. According to the needs of the system, SRAM, Flash, SD card, keyboard, LCD display, and sound prompts are expanded for data processing, data storage, human-computer interaction, and error alarm prompts. Data communication with the host can be carried out through the USB interface. The backlight module can provide backlight for the LCD and keyboard. The voltage detection module detects the battery voltage through the A/D converter of the core processor, thereby indirectly detecting the remaining battery power. The RF module can transmit and receive radio frequency signals between the reader and the tag, and the program can be debugged and downloaded through the JTAG interface. The power supply part can provide power for each module in the system that needs power, which is the focus of this design. The system hardware structure block diagram is shown in Figure 1.

2 Power supply requirements

After design and calculation, the system requires two power supplies, one is 3.3 V, which is used to power the keyboard, LCD reset circuit, external memory, and RF module; the other is 5 V, which is used to power the system's sound prompt circuit and the keyboard and LCD backlight circuit. For portability, the system is powered by batteries, and the performance indicators to be achieved are as follows:

(1) Power conversion efficiency ≥ 80%;
(2) Output current requirements: 3.3 V output current 500 mA; 5 V output current 300 mA;
(3) The fluctuation of the two power supply voltages is controlled within ± 5%;
(4) The battery can be charged through USB input.

3 Characteristics of various power supply chips and selection considerations

3.1 Various power supply chips

3.2 Selection considerations

First, the type of power supply chip must be selected correctly. It is necessary to clarify the input voltage and the required output voltage, and then determine whether to boost, buck, or boost/buck. It is particularly important to note that ordinary linear regulators, LDOs and Buck (or Step-down) DC-DCs can only step down, not step up, and Boost (or Step-up) DC-DCs can only step up, not step down.

The reason for emphasizing this point is that the input voltage range and output voltage range given in the manuals of some chips (LDO or buck DC-DC) are very wide, which can easily mislead inexperienced designers. Many of the output voltage ranges in the manuals are for the given input voltage range. For a specific input voltage, in many cases, the actual output cannot reach the given output voltage. This is very critical and determines the success or failure of the system design, so it should be taken seriously.

Secondly, in the power supply design of handheld devices, attention should be paid to the quiescent current of the chip, which has a great impact on the standby time of the system. The quiescent current of a good power chip is at the μA level, while that of a poor chip is at the mA level, which is a thousand times different. The smaller the quiescent current, the less energy dissipated by the battery and the longer the life.

Thirdly, pay attention to examining the efficiency from the actual load. Power efficiency is closely related to output current. When the output current is very small or very large, the efficiency will become poor. It is necessary to select the power chip according to the required current to maximize efficiency.

4 Solution selection and chip selection

4.1 Solution selection

Solution 1: 3.3 V output uses LDO, and 5V output uses charge pump.

Solution 2: 3.3 V output uses Buck/Boost type DC-DC, and 5V output uses boost type DC-DC.

Since the voltage range of lithium-ion batteries varies widely, there should be a normal power output voltage between 2.5V and 4.2V (4.2V is the voltage that can be reached when fully charged). If a 3.3V output LDO is used, due to the need to meet the minimum voltage difference between input and output, when the battery voltage drops to about 3.4V, the power supply may not be able to reach the output voltage of 3.3V. Using a charge pump to output 5V, the efficiency of the charge pump will not be very high when the input and output voltages are close. The second solution can maximize the power conversion efficiency and extend the battery life.

Considering the above comparison, the second solution is selected.

4. 2 Chip selection

Through inquiry, it is decided to use two chips of TI, TPS63031 and TPS61240, as voltage conversion chips with 3. 3 V output and 5 V output respectively. TPS63031 outputs up to 800 mA current through boost or buck working mode when the input voltage is in the range of 2. 4 to 5. 5 V. In energy-saving mode, when the output current varies between 100 and 500 mA, the efficiency is above 80%. TPS61240 is a boost DC-DC that can work at 3. 5 MHz, with an output current of 450mA. It has PFM/PWM working mode. When the load current is around 200 mA, it can provide more than 80% efficiency within the battery voltage range.

Since the microprocessor has high requirements for power supply ripple, an LDO is added behind the 3.3V output to filter out the large ripple of the DC-DC output and improve the voltage regulation accuracy of the output voltage. In order to meet the requirements of the voltage difference and the reliable working voltage of the processor, the TPS78320 with an output voltage lower than 3.3V is selected. It can output a voltage of 3.2V and a maximum current of 150 mA. This voltage meets the reliable working power supply voltage range and current requirements of the microprocessor LPC2142.

In addition, the quiescent current of the LDO is only 500 nA, which meets the energy saving requirements of the battery-powered handheld system.

5 Debugging

5.1 Debugging steps
After soldering the components on the printed circuit board according to the parameters on the schematic diagram, carefully check whether the value of the components, the soldering direction, and the polarity of the components are soldered correctly. Use a multimeter to carefully check whether there is a cold soldering of the components and whether there is a short circuit phenomenon that should not exist in the components that are close to each other.