RC oscillation circuit with wide voltage range and low temperature drift

Abstract

The application belongs to the technical field of RC oscillation circuits, and specifically provides a wide-voltage-range low-temperature-drift RC oscillation circuit, which comprises a vref standard circuit, an RC charging and discharging circuit and a comparator circuit; the vref standard circuit comprises an operational amplifier and a vref generating circuit, the vref generating circuit is connected in series by resistors between a power supply vdda and the ground; the positive input end of the operational amplifier is connected with vref, the negative input end of the operational amplifier is electrically connected with the RC charging and discharging circuit through a switch; the output end vopa of the operational amplifier is electrically connected with the positive input end of the comparator circuit, and the negative input end of the comparator circuit is electrically connected with the RC charging and discharging circuit through a switch. The calibration accuracy of the scheme is within 0.5% at room temperature, the absolute accuracy is within ±1% at a wide voltage (1.8-5.5V) and a full temperature (-45-125°), and the scheme has a low temperature drift characteristic in the oscillation frequency range.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of RC oscillation circuit, more particularly, to a RC oscillation circuit with wide voltage range and low temperature drift. BACKGROUND

[0002] SOC or MCU chip needs an output oscillation frequency independent of temperature and power supply voltage, existing chip companies can achieve normal temperature ± 1%, but under the condition of full voltage and full temperature, it can only reach ± 3% or so, and a few companies can approach ± 2%, which is mainly because the internal clock accuracy is more susceptible to voltage, especially temperature, in the application occasions where the clock accuracy is required, usually need to configure an on-board crystal, and use the crystal clock to ensure the clock accuracy of the chip, the crystal oscillator is affected by the power supply voltage and temperature fluctuation, but the crystal oscillator is large in size and expensive in price, which affects the use range of the crystal oscillator. SUMMARY

[0003] The present application aims at the technical problem of the existing technology that the crystal oscillator is large in size and expensive in price, which affects the use range of the crystal oscillator.

[0004] The present application provides a RC oscillation circuit with wide voltage range and low temperature drift, comprising:

[0005] vref standard circuit, RC charge-discharge circuit and comparator circuit;

[0006] The vref standard circuit comprises an operational amplifier and a vref generating circuit, and the vref generating circuit is connected in series between the power supply vdda and the ground;

[0007] The positive input end of the operational amplifier is connected with vref, and the negative input end of the operational amplifier is connected with the RC charge-discharge circuit through a switch; the output end vopa of the operational amplifier is connected with the positive input end of the comparator circuit, and the negative input end of the comparator circuit is connected with the RC charge-discharge circuit through a switch.

[0008] Preferably, the RC charge-discharge circuit and the comparator circuit each comprise two, and are symmetrically distributed relative to the negative input end of the operational amplifier.

[0009] Preferably, the RC oscillation circuit further comprises an RC active filter circuit, the RC active filter circuit comprises a resistor R2 and a capacitor C0, and the negative end of the operational amplifier is connected with the output end vopa of the operational amplifier through the capacitor C0 on one hand, and is connected with the RC charge-discharge circuit through the resistor R2 on the other hand.

[0010] Preferably, the RC charging and discharging circuit comprises a resistor R and a capacitor C, one end of the resistor R is connected to the power supply vdda through a switch, and the other end is connected to the ground through the capacitor C.

[0011] Preferably, the power supply vdda is a low dropout linear regulator (LDO).

[0012] Preferably, the resistor R is a 5-bit adjustable resistor with 32-step tuning steps.

[0013] Preferably, the RC oscillation circuit further comprises a frequency adjustment circuit, the frequency adjustment circuit comprises a resistor R0 and a resistor R1 connected in series, one end of the R0 is connected to the power supply vdda, one end of the R1 is connected to the ground, and the other end of the R0 and the other end of the R1 are both electrically connected to the positive input terminal of the operational amplifier.

[0014] Beneficial effects: the RC oscillation circuit with wide voltage range and low temperature drift provided by the application comprises a vref standard circuit, a RC charging and discharging circuit, and a comparator circuit; the vref standard circuit comprises an operational amplifier and a vref generating circuit, the vref generating circuit is connected in series between the power supply vdda and the ground; the positive input terminal of the operational amplifier is connected to the vref, the negative input terminal of the operational amplifier is electrically connected to the RC charging and discharging circuit through a switch; the output terminal vopa of the operational amplifier is electrically connected to the positive input terminal of the comparator circuit, and the negative input terminal of the comparator circuit is electrically connected to the RC charging and discharging circuit through a switch. The calibration accuracy of the scheme at room temperature is within 0.5%, the absolute accuracy is within ±1% at wide voltage (1.8-5.5V) and full temperature (-45-125°), and the oscillation frequency range has low temperature drift characteristics. BRIEF DESCRIPTION OF DRAWINGS

[0015] Figure 1 It is a traditional RC oscillation circuit diagram;

[0016] Figure 2 It is a RC oscillation circuit diagram with wide voltage range and low temperature drift provided by the application;

[0017] Figure 3 It is an oscillation waveform diagram of the RC oscillation circuit with wide voltage range and low temperature drift provided by the application;

[0018] Figure 4 It is a diagram of the oscillation frequency of the RC oscillation circuit with wide voltage range and low temperature drift provided by the application changing with k;

[0019] Figure 5 It is a schematic diagram of the operational amplifier and the comparator circuit of the RC oscillation circuit with wide voltage range and low temperature drift provided by the application. DETAILED DESCRIPTION

[0020] The specific embodiments of the present application are described in further detail below in conjunction with the accompanying drawings and examples. The following examples are used to illustrate the present application but are not intended to limit the scope of the present application.

[0021] Figure 1 The RC oscillation circuit with wide voltage range and low temperature drift provided by the present application comprises a vref standard circuit, an RC charge-discharge circuit and a comparator circuit.

[0022] The vref standard circuit comprises an operational amplifier and a vref generating circuit, and the vref generating circuit is connected in series between the power supply vdda and the ground.

[0023] The positive input end of the operational amplifier is connected to vref, and the negative input end of the operational amplifier is electrically connected to the RC charge-discharge circuit through a switch. The output end vopa of the operational amplifier is electrically connected to the positive input end of the comparator circuit, and the negative input end of the comparator circuit is electrically connected to the RC charge-discharge circuit through a switch.

[0024] The calibration accuracy of this scheme at room temperature is within 0.5%, the absolute accuracy is within ±1% at wide voltage (1.8-5.5V) and full temperature (-45-125°), and it has low temperature drift characteristics in the oscillation frequency range.

[0025] The power supply vdda is a low dropout linear regulator (LDO). The RC oscillation circuit further comprises an RC active filter circuit, which comprises a resistor R2 and a capacitor C0. The negative end of the operational amplifier is electrically connected to the output end vopa of the operational amplifier through the capacitor C0, and is also electrically connected to the RC charge-discharge circuit through the resistor R2.

[0026] The traditional RC oscillation circuit is shown in the attached Figure 1 The voltage on the capacitor is fixed between approximately VREFL and VREFH, and the current charging and discharging formula for the capacitor is When the voltage VC on the capacitor C is lower than VREFL, s=0, the PMOS switch is turned on, and the Ibias current source charges the capacitor. When the voltage on the capacitor is higher than VREFH, s=1, the NMOS switch is turned on, and the Inbias current source discharges the capacitor, so that the circuit outputs a periodic clock (only need to add some numbers buffer after S in the figure to generate a clock). However, this circuit has the following problems in actual implementation: 1. The different comparator delay times td at different operating voltages and temperatures will cause changes in the oscillator frequency. 2. The current sources Ibias and Inbias will change when the VC voltage changes, causing changes in the oscillation frequency.

[0027] The traditional RC oscillation circuit is shown in the attached Figure 2The RC oscillation circuit provided by the embodiment of the present application, the oscillation waveform is as shown in the accompanying Figure 3 , Figure 2 The comparator part and the RC charging and discharging circuit are completely symmetrical. Therefore, the working principle of the oscillation circuit is described by taking the left circuit as an example. The RC charging and discharging circuit includes a resistor R and a capacitor C, where the resistor R is a short form of Rtemp trim, and the capacitor C is a short form of Ctrim.

[0028] The RC charging and discharging circuit includes a resistor R and a capacitor C, one end of the resistor R is connected to the power supply vdda through a switch, and the other end is connected to the ground through the capacitor C.

[0029] The resistor R2 and the capacitor C0 form an active filter circuit. When the resistor R2 >> Rtemp trim, it can be seen that the voltage of Vosca during capacitor charging is:

[0030]

[0031] At the same time, the voltage across the resistor R2 at low frequency is equal. For simplicity, it is assumed that the operational amplifier is ideal, that is, the voltage at the positive and negative terminals of the operational amplifier is equal (the virtual short feature of the operational amplifier), and it can be obtained that

[0032]

[0033] That is, the integral area of Vosca with respect to time in half an oscillation period should be equal to the integral area of vref with respect to time.

[0034] The RC oscillation circuit further includes a frequency adjustment circuit, the frequency adjustment circuit includes a resistor R0 and a resistor R1 connected in series, one end of the R0 is connected to the power supply vdda, one end of the R1 is connected to the ground, and the other end of the R0 and the other end of the R1 are both electrically connected to the positive input terminal of the operational amplifier.

[0035] The RC oscillation circuit further includes a frequency adjustment circuit, the frequency adjustment circuit includes a resistor R0 and a resistor R1 connected in series, one end of the R0 is connected to the power supply vdda, one end of the R1 is connected to the ground, and the other end of the R0 and the other end of the R1 are both electrically connected to the positive input terminal of the operational amplifier. Let k = vref / vdda.

[0036] By combining equations (1) and (2), we have:

[0037]

[0038] The known frequency f rcosc = 1 / T, so from equation (3), it can be seen that f rcoscOnly related to k, Rtemptrim, Ctrim and chip operating voltage and the delay of the comparator td. In the actual implementation, considering that the op-amp is not ideal, the gain of the op-amp is weakly related to the voltage, so the RC oscillation circuit is placed under the low dropout linear regulator LDO, which is better. Only consider the temperature characteristics of the three parameters k, Rtemptrim, Ctrim. Since k=vref / vdda, the same type of resistance string voltage division can be used to ensure that k has zero temperature coefficient, and the temperature coefficient of C in CMOS process can be basically ignored (about 10 ppm), so only Rtemptrim needs to be designed to be close to zero temperature coefficient (in actual implementation, the temperature coefficient of Rtemptrim can be used to offset the slight temperature coefficient of Ctrim). In CMOS process, poly resistors have negative temperature coefficient characteristics and diffusion resistors have positive temperature coefficient characteristics, so a resistance close to zero temperature coefficient can be designed using these two types of resistors. Considering the actual process affected by the process corner, Rtemptrim is made into a 5-bit 32-step TRIM trimmer (i.e. a 5-bit adjustable resistance with 32-step trimming steps), with a temperature change of 0.5% per step, the entire temperature characteristic can cover ±8%, which is enough to cope with mainstream CMOS process (if the process is poor, the bit number of Rtemptrim can be appropriately increased). In this way, a wide voltage range low temperature drift RC oscillation circuit is designed.

[0039] After the value of Rtemptrim is fixed, the frequency is related to k and Ctrim. First, fix the value of Ctrim, and see the relationship between the oscillation frequency f rcosc and the coefficient k. Take the design of a 24MHz frequency oscillation circuit as an example, in the actual circuit, k is fixed between 0.25 and 0.75, and 9 bits are used for trimming, ensuring that the change of k is within 0.1% per step. As shown in the attached Figure 4As shown, when k = 0.25, the frequency is 63.13 MHz, when k = 0.5, the frequency is 24 MHz, and when k = 0.75, the frequency is 9.755 MHz. The range of the oscillation frequency is 9.755 M - 63.13 MHz (if a wider frequency range is needed, the range of k can be further expanded), and at any frequency point of the oscillation, the Rtemptrim is the same and is not affected by the comparator delay td, so the local oscillation circuit has low temperature drift at any frequency within the oscillation range. When k changes by 0.1%, the frequency changes by 0.3% - 0.5% per step, which presents a characteristic of small change in the middle and large change at both ends, so in the actual circuit, the Ctrim can be first coarsely adjusted, and 4 bits are used to coarsely adjust the oscillation frequency by about 10% per step (at the same time, the oscillation frequency range is further widened), so that k is near 0.5 to ensure that the designed circuit can oscillate to the desired target frequency at different process corners and has better fine adjustment step frequency accuracy.

[0040] Finally, the main circuit of the operational amplifier and the comparator is shown in Figure 4. Figure 5 The comparator circuit is similar to the operational amplifier circuit, except that the MOS capacitor Ml for phase compensation is removed and several current control gears are added to ensure that the comparator can work normally. The value of k is between 0.25 and 0.75, so the above operational amplifier needs a rail to rail input operational amplifier. The negative terminal of the comparator is charged to the power supply voltage vdda, and the voltage change will also change from 0 to a very high value, as shown in Figure 5. The voltage at the positive terminal of the comparator is the output of the operational amplifier OPA, which will also change greatly (depending on vref and the delay td of the comparator), so the comparator also needs a rail to rail input. Thus, a wide voltage range low temperature drift RC oscillation circuit is designed. Figure 3

[0041] Although the preferred embodiments of the present application have been described, those skilled in the art can make further changes and modifications to the embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including all the preferred embodiments and all changes and modifications falling within the scope of the present application.

[0042] Obviously, those skilled in the art can make various modifications and variations to the present application without departing from the spirit and scope of the present application. Thus, if these modifications and variations of the present application fall within the scope of the claims of the present application and their equivalents, the present application is also intended to include these modifications and variations.​

Claims

1. A wide voltage range, low temperature drift RC oscillation circuit, characterized in that, include: vref standard circuit, RC charging and discharging circuit and comparator circuit; The Vref standard circuit includes an operational amplifier and a Vref generation circuit, wherein the Vref generation circuit is generated by a resistor connected in series between the power supply Vdda and ground. The positive input terminal of the operational amplifier is connected to vref, and the negative input terminal of the operational amplifier is electrically connected to the RC charging and discharging circuit through a switch; the output terminal vopa of the operational amplifier is electrically connected to the positive input terminal of the comparator circuit, and the negative input terminal of the comparator circuit is directly connected to the RC charging and discharging circuit. The RC charging and discharging circuit includes a resistor R and a capacitor C. One end of the resistor R is connected to the power supply Vdda through a switch, and the other end is grounded through the capacitor C. The resistor R is an adjustable resistor with 5 bits and 32 steps for adjustment.

2. The RC oscillation circuit with wide voltage range and low temperature drift according to claim 1, characterized in that, The RC charging / discharging circuit and the comparator circuit each consist of two units, and are symmetrically distributed relative to the negative input terminal of the operational amplifier.

3. The RC oscillation circuit with wide voltage range and low temperature drift according to claim 1, characterized in that, The RC oscillation circuit also includes an RC active filter circuit, which includes a resistor R2 and a capacitor C0. The negative terminal of the operational amplifier is electrically connected to the output terminal vopa of the operational amplifier through the capacitor C0, and the negative terminal of the operational amplifier is also electrically connected to the RC charging and discharging circuit through the resistor R2.

4. The RC oscillation circuit with wide voltage range and low temperature drift according to claim 1, characterized in that, The power supply vdda is a low dropout linear regulator (LDO).

5. The RC oscillation circuit with wide voltage range and low temperature drift according to claim 1, characterized in that, The RC oscillation circuit also includes a frequency adjustment circuit, which includes a series resistor R0 and a resistor R1. One end of R0 is connected to the power supply Vdda, one end of R1 is grounded, and the other ends of R0 and R1 are both electrically connected to the positive input terminal of the operational amplifier.

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