At present, the reference voltage source is widely used in integrated circuits such as high-precision comparators, A/D, D/A converters, and dynamic random access memories. The reference voltage source is an important unit module in the integrated circuit.
Its reference voltage accuracy, temperature stability and noise immunity directly affect the performance of the chip and even the entire system. Especially in D/A, A/D data conversion systems, the performance of the reference source is closely related to the quantization accuracy of the quantizer. With the increasing accuracy of D/A and A/D, the design of an accurate and stable reference source becomes the key. Therefore, designing a high-performance reference is of great significance.
1 Analysis circuit design and principle
1.1 Analysis of traditional bandgap reference
In the conventional bandgap voltage reference structure, a stable output Vref with a constant temperature is obtained at the output by a linear combination of VT having a positive temperature coefficient and a voltage VBE having a negative temperature coefficient. Figure 1 is a conventional bandgap reference. However, in practical applications, compensating for high-order voltage components that are not compensated in Vref is the key to the design. The higher temperature coefficient is mainly derived from the temperature characteristics of the bipolar transistor.
After finishing, I got:
According to the above formula, the first-order coefficient term can be easily eliminated by adjusting the circuit under most processes. However, since the value of the process parameter r and the coefficient δ introduced by the resistor are not well offset, the high-order voltage component still exists. That is, the C 2 term cannot be eliminated, resulting in a temperature coefficient that cannot be sufficiently low.
1.2 Improved high-order compensation bandgap reference source
In order to obtain a bandgap reference source with a sufficiently low temperature coefficient, the higher order temperature coefficient needs to be further compensated, and the compensation method is as shown in the circuit structure shown in FIG. On the basis of the traditional circuit, the compensation circuit structure is added: since the gain of the operational amplifier A 3 is large, the operational amplifier forces the terminal voltages of Q 2 and R 4 to be equal, then I 4 =V BE , Q 2 /R 4 , current mirror Let the current flowing through transistor Q 3 :
Thus a difference Tln T term is generated between the V BEs of Q 2 , Q 3 . This difference term is introduced into I R1 through the op amp gm 1 , gm 2 to correct the high order terms in V BE , Q1 .
In Figure 2, the input is connected to V 1 , V 2 and V 2 , V 3 of the four-input op amp, the output ends of which are connected together, so they have the same gain A1, the parameters are exactly the same, that is, the output impedance is the same:
For tubes Q 1 , Q 2 , they are identical, so their terminal voltages are only related to the current flowing through their collectors.
make (The constants B 1 , B 2 are controlled by the resistance value, the temperature coefficient and the tube V BE voltage; gm 1 , gm 2 is the transconductance at the input of the op amp A 1 , which is determined by the input-to-tube aspect ratio and the static operating point. In the actual design, the high-order term is adjusted by adjusting gm 1 , gm 2 , and R 4 is adjusted to eliminate the first-order term. Finally, repeated optimization can obtain a good temperature coefficient.
1.3 overall circuit analysis
The circuit structure proposed here is shown in Figure 3. The system consists of four modules: power-saving and bias circuits, op amps, reference voltage output modules, and high-order curvature compensation. The working principle of the reference core structure and the high-order curvature compensation circuit is highlighted in the improved bandgap reference previously analyzed. The power control switch VC1 shown on the left of Figure 3, when VC1 is low (0), M6 is on, M4 is off, then M7 gate potential is high, M7 is off, then M7 branch current is 0, current Mirror M10, M11 mirror M7 branch current, resulting in the differential amplifier's tail current is 0, the differential amplifier does not work, the whole circuit is not working, in the power-saving state; when VCl is high (3.3 V), M6 is off When M4 is turned on, the bias circuit composed of M1 to M6 provides a suitable bias voltage for the M7 gate. The bias of the Cascode structure (M8, M9, M10, M11) is achieved by voltage self-biasing. At the same time M10, M11 replicates the M7 branch current, M12, M13 voltage self-bias, providing a bias voltage for the tail current source. The bias circuit provides all of the bias voltages used in the stage-folding cascode op amp circuit. In practical circuits, to match, the length of the tube in the bias circuit should be the same as the length of the corresponding tube in the op amp.
The op amp is one of the key components in the bandgap voltage reference circuit. The loop gain and circuit offset determine the accuracy and stability of the reference source output. In order to increase the stability of the circuit and reduce the complexity of the circuit, a single-stage operational amplifier with high gain is used here instead of the secondary compensation operational amplifier. High-gain single-stage op amps include both telescopic and folding op amps. Because the op amp is connected to the feedback loop, the telescopic op amp is not used because the output swing is too small, and a folding op amp is used here.
2 Simulation results analysis
The circuit shown in Figure 3 was simulated in a 0.35 μm BSIM 3v3 CMOS process using Cadence Spectre software to obtain the following simulation results.
2.1 Reference output and power supply voltage relationship
Figure 4 is a plot of the reference output versus supply voltage (0 to 3.3 V). The simulation results show that the minimum supply voltage of the bandgap reference structure under normal operating conditions can reach 1.6 V, and the output reference voltage V ref = (1.174 43 ± 0.000 43 V), in the range of -40 to +100 °C. The temperature coefficient of the output voltage of the bandgap reference is r TC =2.077 ppm/°C. At 25 ° C, 3.3 V, the power consumption is less than 110 μW (the total power consumption of the circuit is 109.89 μW). At 25 ° C, 1.6 V, the power consumption is less than 9 μW (the total power consumption of the circuit is 8.453 μW).
Simulating the supply voltage rejection ratio (PSRR) for this bandgap voltage reference source yields -65 dB at 100 Hz at room temperature and without filter capacitors at 3.3 V supply voltage. To get a better PSRR, you can increase the PSRR by adding a filter capacitor to the output of the reference. Vref is passed through an RC low-pass filter circuit output, which improves the power supply rejection of the output reference voltage, reduces noise interference, and reduces the reference voltage transient overshoot when the circuit is powered up.
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