Design of an integrated voltage regulator circuit for radio frequency cards

1. Introduction

In recent years, with the development of wireless communication technology and microelectronics technology, contactless IC card (RF card) technology has flourished and has been rapidly popularized and promoted in many fields, such as public transportation automatic ticketing systems, resident ID cards, phone cards, bank cards, etc. Passive power supply technology is one of the key technologies of RF cards, and is currently mainly solved through the principle of electromagnetic induction and integrated voltage stabilization circuits. When the RF card enters the reader's magnetic field, it obtains energy from the magnetic field through electromagnetic induction, that is, an AC current is induced at both ends of the card's coil, and a DC voltage can be obtained after rectification and voltage stabilization. This article discusses a self-feedback switching voltage stabilization circuit designed specifically for RF cards using a 0.35um CMOS process.

2. Structural design and working principle of voltage stabilization circuit

Integrated voltage regulator circuit is also called integrated voltage regulator. When the input voltage or output current changes within a certain range, its output voltage remains unchanged. It has been widely used in various electronic devices to replace the voltage regulator assembled by discrete devices.

2.1 Circuit structure design

The integrated voltage stabilizing circuit mainly includes the following parts: a reference source circuit, a voltage regulating circuit and a power switch circuit.

The reference source circuit is composed of a bandgap reference source composed of a two-stage CMOS differential amplifier circuit and a transistor circuit. Its structure is shown in Figure 1.

Active resistor P0 and polycrystalline resistor R7 form a bias circuit to provide bias current for the circuit. The two inputs of the secondary differential amplifier are connected to the Q1 terminal and the Q2 terminal. According to the reference source principle, only when the input offset voltage of the amplifier circuit is small and is not affected by temperature, the output of the reference source can maintain good performance. According to the function of the amplifier and the bandgap reference source principle, it can be obtained:

I1R6=I2R4 (1)

From equation (1), we can see that the input offset voltage of the amplifier in the circuit is close to zero. Therefore, the voltage value of the REF point after stabilization is as follows:

VREF=VQ1+VR6=VQ1+R6I1= VQ1+R4I2 (2)

Because the base and collector of the PNP transistor are connected, the VQ1 value is equivalent to the forward voltage drop VBE value of the BE junction diode in the transistor, and VBE is generally 0.6~0.8V.

The temperature coefficient of the BE junction diode in the transistor is negative, while the temperature coefficient of the resistor is positive. In equation (2), VQ1 and VR6 can compensate each other as the temperature changes, so the output VREF of the reference source is insensitive to temperature changes.

The voltage regulation circuit is the core part of the voltage stabilization circuit, including two-stage CMOS differential amplifier circuit COMP and voltage regulation and feedback circuit, as shown in Figure 2.

The inputs of the two differential amplifiers are obtained by voltage-dividing resistors. After comparison and amplification, MA1 and MB1 are obtained through feedback regulation and current-limiting protection circuits to control the opening and closing of the switch tube in the power switch circuit.

The power switch circuit consists of a storage capacitor, a rectifier composed of an NMOS tube, and a switch circuit, as shown in Figure 3. P1 and P2 are directly connected to the two ends of the coil L0, and alternating current is induced on P1 and P2 through electromagnetic coupling. After rectification, a DC voltage VDD is generated at the end of the storage capacitor C0. After the N2 tube is turned on, the voltage regulating capacitor C5 forms a discharge loop so that the current on P1 and P2 starts to charge C5 and stops charging C0, so that the voltage at both ends of C0 remains stable, that is, a stable power supply voltage is provided for the load circuit.

When the RFID card enters the magnetic field of the reader, an AC induced current is generated on P1 and P2 after electromagnetic coupling of the coil, which is converted into a DC current through the rectifier, and the energy storage capacitor C0 and the voltage regulating capacitor C5 are charged at the same time. The capacitance of C5 is very small, and the current passing through the rectifier can instantly fill it. Since the N2 tube is cut off on both sides of C5 and there is no discharge circuit, the current on P1 and P2 will only charge the capacitor C0, and the power supply voltage VDD is generated at both ends of C0. VDD continues to increase with the charging process of the capacitor. The active resistor and diode in the rectifier make the voltage amplitude at both ends of P1 and P2 increase, causing the potential of point a to rise accordingly; at the same time, the output of the voltage sampling circuit also increases with the increase of VDD. When the VDD voltage value reaches V0 (see Figure 4), the sampled output voltage is greater than the reference voltage VREF. At this time, the voltage values ​​of the outputs MA1 and MB1 in the voltage regulating circuit can turn on the two tubes N1 and N2 successively. Because the source end of the N2 tube is grounded, the voltage on a begins to decrease after the N2 tube is turned on, causing P1 and P2 to charge C5 again. Since the N2 tube is always in the on state, C5 also starts to discharge at the same time. After that, C5 and N2 tubes are always in a state of charging and discharging at the same time, and the voltage at point a oscillates within a certain range. The charging and discharging of C5 keeps the voltage peaks on P1 and P2 at a certain potential through feedback, and the capacitor C0 is no longer charged, so the voltage difference across C0 remains stable. The VDD obtained at this time is the working voltage we need. When the RF card is working normally, the voltage on the energy storage capacitor C0 will drop due to the consumption of the load circuit. When the VDD value is less than the V0 value, the N2 tube will be cut off, and the C5 capacitor will not have a discharge loop. After P1 and P2 fully charge C5, they will continue to charge C0 to increase the voltage difference across C0, that is, VDD rises. In this way, a self-feedback voltage regulator is formed in the circuit.

3. Simulation results

In the normal working environment of the RFID card, the coupling coefficient between the card and the reader is very small, generally around 0.1~0.35, and the reader signal voltage is generally 12V. In the simulation verification, a 12V, 13.56MHz test excitation is added to obtain the induced current on the inductor L0. Using a 0.35um SPICE model, the coupling coefficient is set to 0.25, and the VDD stable voltage is 3.35V. The Hspice simulation results are shown in Figure 4:

4. Conclusion

Through the above design and simulation analysis, it can be seen that this voltage-stabilizing circuit can obtain a stable voltage in a short time and can adjust automatically; the multi-target wafer test results are basically consistent with the simulation results and meet the design requirements, so it has good practicality and reference value.