CR2450 battery.Application Analysis of CMOS Integrated Circuit Resistors

　　Currently, there are three main types of resistors used in design: polysilicon, MOS tubes, and capacitor resistors. In the design, these three types of resistors should be used flexibly according to needs to optimize the chip design.

　　1Performance and characteristics of CMOS integrated circuits

　　1.1 Low power consumption CMOS integrated circuits use field effect transistors, and they are all complementary structures. During operation, two series-connected field effect transistors are always in a state where one tube is on and the other is off. The static power consumption of the circuit is theoretically zero. . In fact, CMOS circuits still consume a small amount of static power due to leakage current. The typical power consumption of a single gate circuit is only 20mW, and the dynamic power consumption (at an operating frequency of 1MHz) is only a few mW.

　　1.2 Wide operating voltage range CMOS integrated circuits provide simple power supply, small power supply, and basically do not require voltage stabilization. Domestic CC4000 series integrated circuits can operate normally at 3~18V voltage.

　　1.3 Large logic swing The logic high level 1 and logic low level 0 of the CMOS integrated circuit are close to the power supply high potential VDD and the film low potential VSS respectively. When VDD=15V and VSS=0V, the output logic swing is approximately 15V. Therefore, the voltage utilization coefficient of CMOS integrated circuits is relatively high among various types of integrated circuits.

　　1.4 Strong anti-interference ability The typical value of the voltage noise tolerance of CMOS integrated circuits is 45% of the power supply voltage, and the guaranteed value is 30% of the power supply voltage. As the supply voltage increases, the absolute value of the noise margin voltage will increase proportionally. For a supply voltage of VDD=15V (when VSS=0V), the circuit will have a noise margin of about 7V.

　　1.5 High input impedance The input terminal of a CMOS integrated circuit is generally a protection network composed of a protection diode and a series resistor, so the input resistance is slightly smaller than that of a general field effect transistor. However, within the normal operating voltage range, these protection diodes are in reverse. In the biased state, the DC input impedance depends on the leakage current of these diodes. Normally, the equivalent input impedance is as high as 103~1011Ω, so the CMOS integrated circuit consumes almost no power from the driving circuit.

　　1.6 Good temperature stability. Because CMOS integrated circuits have low power consumption and low internal heat generation, and the CMOS circuit circuit structure and electrical parameters are symmetrical, certain parameters can automatically compensate when the temperature environment changes. , so the temperature characteristics of CMOS integrated circuits are very good. Generally, the working temperature range of ceramic metal packaged circuits is -55~+125℃; the working temperature range of plastic packaged circuits is -45~+85℃.

　　1.7 Strong fan-out capability Fan-out capability is expressed by the number of input terminals that the output terminal of the circuit can drive. Since the input impedance of CMOS integrated circuits is extremely high, the output capability of the circuit is limited by the input capacitance. However, when a CMOS integrated circuit is used to drive the same type, it can generally drive more than 50 input terminals, regardless of speed.

　　2Applications of CMOS integrated circuit resistors

　　2.1 Polysilicon resistors The monolithic resistors in integrated circuits are far from the ideal resistance. In the standard MOS process, the most ideal passive resistor is a polysilicon strip.

　　In the formula: ρ is the resistivity; t is the thickness of the sheet; R□= (ρ/t) is the resistivity of the sheet, the unit is Ω/□; L/W is the aspect ratio. Since the commonly used sheet resistance is very small, usually the maximum resistivity of polysilicon is 100Ω/□, and the design rules determine the minimum value of the polysilicon strip width, so high-value resistors require a large size. Due to the limitation of chip area, In fact it is very difficult to achieve. Of course, diffusion strips can also be used to make thin-layer resistors, but due to the instability of the process, they are usually easily affected by temperature and voltage, and it is difficult to accurately control their absolute values. The parasitic effect is also very obvious. Regardless of polysilicon or diffusion layer, their resistance varies widely, which is related to the impurity concentration in the implanted material. It is not easy to calculate the exact value. For the reasons mentioned above, active resistors are often used in integrated circuits.

　　2MOS tube resistor

　　The MOS tube is a three-terminal device. When these three terminals are properly connected, the MOS tube becomes an active resistor at both ends. The main principle of this resistor is to use the equivalent resistance of the transistor under a certain bias. Can replace polysilicon or diffused resistors to provide DC voltage drop, or small signal AC resistance that is linear over a small range. In most cases, the area required to obtain small-signal resistance is much more important than linearity. A MOS device is an analog resistor that is much smaller than an equivalent polysilicon or transistor resistor. Simply connect the gate of an n-channel or p-channel enhancement MOS transistor to the drain to obtain an active resistance similar to a MOS transistor. For n-channel devices, the source should be connected to the most negative power supply voltage as much as possible to eliminate the influence of the substrate. Similarly, the source of p-channel devices should be connected to the most positive power supply voltage. At this time, VGS=VDS, as shown in Figure 1 (a), (b). The MOS transistor bias in Figure 1(a) operates in the linear region, and Figure 2 shows the large signal characteristics of the active resistance transconductance curve ID-VGS. This curve is applicable to both n-channel and p-channel enhancement-mode devices. It can be seen that the resistance is non-linear. But in practice, since the amplitude of the signal swing is small, this resistor can actually work very well. Among them: K′=μ0C0X. It can be seen that if VDS (VGS-VT), the relationship between ID and VDS is linear (assuming that VGS has nothing to do with VDS, resulting in an equivalent resistance R=KL/W, K=1/[μ0C0X (VGS -VT)], μ0 is the surface mobility of carriers, C0X is the gate-channel capacitance density; the K value is usually 1000~3000Ω/□. Experiments have proved that at VDS0.5 (VGS-VT), the approximate situation is very Good. Figure 1 (c), (d) Although the linearity of the resistivity can be improved, the area is sacrificed and the complexity is increased.

　　Sometimes AC resistance is used in design, and its DC current should be zero. The active resistor shown in Figure 1 cannot meet this condition because its resistance is required to be infinite. Obviously this is impossible. At this time, the switching characteristics of the MOS tube can be used to achieve this.

　　3 capacitor resistor

　　AC resistance can also be implemented using switches and capacitors. Experience has shown that if the clock frequency is high enough, the combination of switch and capacitor can be used as a resistor. Its resistance depends on the clock frequency and capacitor value.

　　Under certain conditions, according to sampling system theory, the variation within the period is negligible.

　　Among them, fc=1/T is the frequency of signals φ1 and φ2. This method can obtain a large resistance on a small silicon chip. For example, assuming that the capacitor is polysilicon type, the clock frequency is 100kHz, and a resistance of 1MΩ is required, find its area. According to equation (3), the capacitance is 10pF. Assuming that the capacitance per unit area is 0.2pF/mil2, the area is 50mil2. If using polysilicon, taking the maximum possible value of 100Ω and taking its minimum width, then 900mil2 is required. Of course, in the switched capacitor resistor, in addition to the capacitor area, two MOS tubes with extremely small areas are required for switching. It can be seen that the area of the capacitor resistor is much smaller than that of the polysilicon resistor. This is very important in integrated circuit design.

　　The effective RC time constant is proportional to the ratio of capacitors, so that capacitors and switched capacitor resistors can be used to accurately achieve the time constant required in the circuit; and the use of resistors from active devices can minimize the resistor size. Polysilicon resistors are the simplest. These three different methods should be used flexibly in design.

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