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Keywords: Handheld RFID high-efficiency RFID handsets have a wide range of applications in transportation, access control, logistics, attendance, cargo management, and identification. RFID handheld devices have high requirements on power efficiency, service life, reliability, size, and cost. Therefore, designing a power supply with good stability, high efficiency and low spurs is of great significance for RFID handsets.
1RFID handset hardware structure In the design of RFID system based on embedded system, microprocessor LPC2142 is the main controller, which expands SRAM, Flash, SD card, keyboard, LCD display and sound prompt according to the needs of the system. Data processing, data storage, human-computer interaction, and error alarm prompts, data communication with the host can be performed through the USB interface, the backlight module can provide backlight for the LCD and the keyboard, and the voltage detection module performs battery voltage through the A/D converter of the core processor. The detection, indirectly detecting the remaining battery power, the RF module can send and receive RF signals between the reader and the tag, and the program can be debugged and downloaded through the JTAG interface. The power supply section can provide power for each module in the system that requires power, which is the focus of this paper. The system hardware block diagram is shown in Figure 1.
Figure 1 System hardware block diagram 2 The requirements of the power supply are designed and calculated. The system requires two voltage sources, one is 3.3V, which supplies power for the keyboard, LCD reset circuit, external memory, and RF module. The other is 5V provides power for the system's audible cue circuit and the keyboard and LCD backlight circuits. In order to facilitate carrying, the system is powered by battery. The performance indicators are as follows: (1) power conversion efficiency 80; (2) output current requirement: 3.3V output current 500mA; 5V output current 300mA; (3) fluctuation of two power supply voltages Both are controlled within 5; (4) The battery can be charged via USB input.
3 characteristics of various power chips and selection considerations 3.1 Comparison of various power chip features Table 1 is a comparison of four power chips.
Table 14 Comparison of Power Chips Note: LDO is the abbreviation of LowDropOut, which is a low dropout linear regulator.
3.2 Selection Considerations First, the power chip type must be selected correctly. To determine the input voltage and the required output voltage, determine whether it is boost, buck, or boost/buck. It is important to note that ordinary linear regulators, LDOs, and Buck (or Step-down) DC-DCs can only be stepped down and cannot be boosted. Boost (or Step-up) type DC-DC can only be boosted. Buck.
The reason for emphasizing this is that the manuals of some chips (LDO or step-down DC-DC) give a wide range of input voltages and output voltages, which can easily mislead inexperienced designers. Many of the output voltage ranges in the manual are for the given input voltage range. For a given input voltage, in many cases, the actual output does not reach the given output voltage. This is very important. Deciding on the success or failure of system design should be highly valued.
Secondly, in the power supply design of the handheld device, attention should be paid to the quiescent current of the chip, which has a great influence on the standby time of the system. The quiescent current of the good power chip is at the A level, and the poor chip is at the mA level, which is a thousand times different. The smaller the quiescent current, the less the battery's electrical energy dissipation and the longer the life.
Again, pay attention to the efficiency from the actual load. Power efficiency is closely related to the output current. When the output current is small or large, the efficiency becomes poor. The power chip needs to be selected according to the required current to maximize efficiency.
4 scheme selection and chip selection 4.1 scheme selection scheme 1: 3.3V output adopts LDO, and 5V output adopts charge pump.
Option 2: 3.3V output adopts Buck/Boost type DC-DC, 5V output adopts step-up DC-DC. Since the voltage range of lithium ion battery varies widely, it is 2.5V~4.2V (4.2V is full charge can reach There should be a normal power supply output voltage between the voltages. If a 3.3V output LDO is used, the power supply may not reach the output when the battery voltage drops to 3.4V due to the minimum voltage difference between the input and output. 3.3V voltage is gone. With a charge pump output of 5V, the charge pump is not very efficient when the input and output voltages are close. The second option can maximize power conversion efficiency and extend battery life.
Considering the above comparison, choose the second option.
4.2 Chip selection Through inquiry, it is decided to use TI's two chips TPS63031 and TPS61240 as the voltage conversion chip of 3.3V output and 5V output respectively. The TPS63031 adopts step-up or step-down operation mode in the input voltage range of 2.4-5.5V. Outputs up to 800mA. In the energy-saving mode, when the output current varies between 100 and 500mA, the efficiency is above 80. The TPS61240 is a step-up DC-DC that can operate at 3.5MHz. The output current can reach 450mA. It has a PFM/PWM operation mode. When the load current is about 200mA, it can provide more than 80 efficiencies in the battery voltage range.
Since the microprocessor has high requirements on the 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 regulation accuracy of the output voltage. In order to meet the requirements of differential pressure and reliable operating voltage of the processor, the TPS78320 with lower output voltage than 3.3V can output 3.2V voltage and can output 150mA current. This voltage satisfies the reliable operating power supply voltage range of the microprocessor LPC2142 ( 3.0V ~ 3.6V) and current demand.
In addition, the LDO's quiescent current is only 500nA, which is in line with the energy-saving requirements of battery-powered handheld systems.
5 power circuit design carefully read the chip manual, design and draw the power circuit schematic shown in Figure 2.
U2 and U3 in Figure 2 are DC-DC regulators with 3.3V output and 5V output respectively. U4 is LDO. The 3.3V output of DC-DC is actively filtered by the LDO to provide 3.2V for the microprocessor. The power supply, U1 is Maxim's lithium-ion battery charge management chip MAX1555, which can charge lithium-ion battery through USB.
Capacitors C1, C5, C7, and C3 in the circuit are the input filter capacitors of the chip to improve transient response and suppress noise and ripple. C4, C6, C8, and C2 are the output capacitors of the chip, which are used to keep the circuit stable and filtered. Among them, C1 and C4 should use X7R ceramic capacitors with rated voltage not less than 6.3V. Other capacitors use X5R ceramic capacitors with rated voltage not less than 6.3V. Of course, X7R capacitors are better or better, but the price is more expensive. L1 and L2 should use an inductor with a rated current not less than 2 times the output current and a small DC resistance, which can reduce the loss of the circuit.
In Figure 2, the two Schottky diodes IN1 and IN2 can protect the battery. IN1 is to prevent the USB power supply from reverse-breaking the battery. The function of IN2 is to prevent the battery from forming a self-charging loop with U1. A diode is indispensable. The charger pin/CHG right pull-up resistor R1 is used to indicate the charging state. The /CHG pin is connected to the microprocessor's GPIO pin. When it is in the charging state, the pin outputs a low level; when /CHG becomes When it is high impedance, it means the battery is full.