High-precision Low-power Low-temperature Drift Dropout Voltage Cancellation Oscillator

As electronic products continue to develop, the requirements for the accuracy of integrated circuits continue to increase. Oscillators play a more important role in integrated circuits, and higher requirements are placed on the power consumption as well as the accuracy of oscillators. In this paper, a high-precision and low-power consumption RC oscillator is designed. The oscillator in this paper consists of a bias module and a capacitor charging and discharging module and provides bias current and voltage with temperature coefficient through bandgap to reduce the change of oscillator operating frequency caused by the increase of ambient temperature in traditional oscillators, which improves the oscillator operating state under different temperatures and the accuracy of the oscillator. The single comparator oscillator structure is also designed to effectively eliminate the effect of the dropout voltage on the oscillation period. The oscillator is simulated by using the MGN 0.18 μm process.


Introduction
Currently, there are four main types of oscillator structures: ring oscillators, RC oscillators, LC oscillators, and crystal oscillators.Of these, LC oscillators and crystal oscillators are not easily integrated inside the chip in CMOS processes.Ring oscillators have the advantage of simple structure and small area cost, but ring oscillators are more susceptible to temperature and supply voltage than RC oscillators, so RC oscillators have a greater advantage [1] .In 2021, Ma et al. proposed a fivecomparator RC oscillator structure that does not require a reference voltage but has a significant increase in design power consumption and area, and limited improvement in temperature stability of the output frequency.In response to the above problems in this paper, a single comparator RC oscillator with eliminated dropout voltage and a direct 50% duty cycle output is designed by using the MGN 0.18 ੇ process.The temperature shift rate between -40 ႏ and 125 ႏ is 0.46%, which is significantly improved compared to 1.09% for traditional RC oscillators.

Basic RC oscillator
Figure 1 shows the basic structure of the single comparator RC oscillator of this design, which consists of a comparator, two-branch capacitor charging and discharging circuits, and multiple switching controls [2] .Vref is the reference voltage, V1 is the output oscillation waveform, I1 and I3 are the charging current sources, I2 and I4 are the capacitor discharge current sources, and I1-I4 current source current size is the same so that the two capacitors charge and discharge the same time to achieve the effect of 50% duty cycle [3] .
Figure 1.The basic structure of a single comparator RC oscillator

Principle analysis
As shown in Figure 2, VB1, VB2, and Vref are the bandgap output reference voltages and the reference current source bias, so that the magnitude of the charge and discharge currents of the two branches are determined and the charge and discharge currents of both branches are the same.1) When the circuit is started, the overall circuit has not yet begun to operate, the comparator output voltage V2 is low and the inverter reverses to a high-level V1; 2) V2 low causes the PM6 tube to open and current source PM5 to charge capacitor C1.At the same time, high V1makes the NM1 tube on and the comparator positive input feeds the capacitor C1 voltage; 3) The V2 low-voltage PM2 tube opens, the reference voltage Vref is input to the negative input of the comparator, and the comparator compares the two voltages; 4) The voltage of capacitor C1 rises gradually.When it exceeds the reference voltage Vref at the negative input, the state is reversed, at which point V2 goes high and V1 goes low;  Conventional RC oscillators use a dual comparator and dual capacitor structure, while a single comparator and single capacitor circuit structure can be used to save area [4] .A typical single comparator structure voltage mode RC oscillator is shown in Figure 3 the output state of the comparator is reversed and a fast on-reset of the switch is prompted, bringing the capacitor voltage to zero and completing an oscillation cycle.Capacitor C continues to start charging and a new cycle begins [5] [6] .
The difference between this structure and a typical RC oscillator with a dual comparator structure is that instead of two capacitors charged and discharged in turn, only one capacitor is charged and discharged to form the oscillation period, but the output waveform is a narrow pulse with a small duty cycle.At the same time, the structure requires a sufficiently powerful driver buffer at the comparator output to enable the switch to turn on and the capacitor potential to be quickly reset [7] .
The single comparator structure of the voltage-mode RC oscillator differs from the typical single comparator structure in that two charging capacitors are used, and the output oscillation waveform is controlled by over-switching and by the amount of charging current to achieve a 50% duty cycle.In this circuit structure, whether the comparator output is 1 or 0, the process of charging the capacitors is compared with the reference voltage of Vref [8] .The switching control is used to alternately vary the reference voltage and the capacitor voltage input to the positive and negative inputs of the comparator so that the comparator can output different results under the same comparison conditions.Under the condition that the two branches are charged with the same current, if the two capacitors have the same capacitance C, the oscillation period of the oscillator is: OSC T = 2RC .However, when the oscillator is operated in the actual circuit, there are non-ideal factors and the oscillator is subject to deviations caused by non-ideal factors.In the circuit shown in Figure 2, capacitor C1 and capacitor C2 are not well matched to each other and the two charging current sources PM5 and PM3 are not well matched to each other, resulting in different charging times.
These objectively irrational factors may introduce errors in the output frequency.The small errors introduced by the assumed charging currents I1 and I2 and capacitors C1 and C2 are analyzed through calculations [9] .Capacitor C1 is charged when V2 is low: When V1 is low, the capacitor C2 is charged: Based on the two equations above, the period of oscillation of the oscillator can be obtained as: VOS is the comparator input detuning voltage and td is the comparator transmission delay.It has been assumed earlier that there are objective non-ideal factors for both capacitors as well as the two branch current mirrors, so let

I = I -I +
then the period of oscillation under objective conditions is obtained as: Under this equation, the oscillation period of the oscillator is affected by the detuned voltage.In the actual circuit, the voltage V1 across capacitor C1 is one VOS higher in magnitude than the reference voltage, and the voltage V2 across capacitor C2 is one VOS lower than the reference voltage [10] .So, by the analysis, it is concluded that the effect of the detuning voltage on the oscillation period does not occur and the effect of VOS is canceled out.Therefore, the oscillation period of the actual oscillation period is: (6)

Test results
The oscillator in this paper uses the MGN 0.18 ੇ process and is simulated by using cadence with a supply voltage of VDD = 5 V.The sweep was performed at a temperature of 27 ႏ and the cycle time of the output oscillation waveform was 421 ns as shown in Figure 4, with a frequency of 2.37 MHz.Simulations were also carried out at -40 ႏ and 125 ႏ for comparison, with a frequency of

Conclusions
This paper designs a single comparator RC oscillator based on the MGN 0.18 ੇ process, proposing a simple circuit structure to offset the effect of the detuning voltage on the oscillation period, effectively improving the accuracy of the oscillator.While the output frequency duty cycle is allowed to be closer to 50%, it effectively reduces the power consumption and area of the oscillator, with an independent bias supply with a temperature offset of 0.46%.This is an effective improvement over the 1.09% temperature drift of conventional RC oscillators.Compared with a five-comparator RC oscillator structure proposed by Ma et al., the power consumption and area are significantly reduced.Compared with the conventional oscillator comparator and RS flip-flop, the area is significantly reduced.At the same time, the accuracy of the output oscillation waveform is greatly improved with the offset of the dropout voltage, and the stability of the circuit is improved.

5 )
With high V2 and low V1, the NM5 tube opens to stop charging capacitor C1 and PM4 opens the current source PM3 to charge capacitor C2.At the same time, PM1 and NM2 tubes open the comparator positive input voltage for the reference voltage Vref negative input C2 voltage; 6) When the C2 voltage exceeds the reference voltage Vref, the comparator outputs a low level; 7) We repeat steps 2)-7) for the oscillator to start oscillating.

Figure 2 .
Figure 2. The complete circuit structure of the oscillator

C.
. This structure uses an external current source to generate a reference voltage Vref with Iref flowing through resistor R and Vref as the threshold comparison voltage for the single comparator.The other input of the comparator is linked to a capacitor C. The current source generates a current of Iref to charge the capacitor C. The voltage across the capacitor C ramps up at a gradual rate ref I When the voltage on capacitor C exceeds Vref,

Figure 3 .
Figure 3.Typical single comparator structure voltage mode RC oscillator