Design of a High PSRR Low-pressure Bandgap Reference Source with Curvature Compensation

Based on the NUVOTON 0.35 μm BCD process, a low voltage bandgap reference with an input voltage of 5 V and output voltage of 0.6 V is designed in this paper. The traditional low-voltage bandgap reference output voltage has a large temperature coefficient and poor ability to suppress power supply fluctuations. In this paper, a circuit with high order compensation is proposed to reduce the temperature coefficient of the circuit, and a pre-regulated circuit is used to increase the power supply rejection ratio. The simulation results are at -30 °C∼130 °C. The output voltage of reference voltage is 947.5 μV, the temperature coefficient is 9.58 ppm/°C, and the power suppression ratio is -104.62 dB, which meets the design requirements of the power management chip.


Introduction
In today's world, semiconductor chips and integrated circuits play an important role in modern electronics.Since the appearance of integrated circuits, people's life has been changed constantly.For the early radio, television, computers, mobile communication, mobile intelligent terminal, etc., the integrated circuit has promoted the development of application fields to some extent.With the emergence of the emerging consumer electronics market and the rapid development of the Internet of Things, the demand for chips has soared.
For some low-cost and low-power analog integrated circuits in the consumer electronics market, the automotive chip market and industrial chip market occupy a great proportion, such as medical equipment, intelligent wearable devices, and industrial equipment [1] .The bandgap reference source is an important part of the analog circuit.Some circuit modules, such as a low-voltage differential linear regulator, digital-to-analog converter, and transformer, will directly affect the performance of the overall circuit.Common reference source voltage varies with temperature obviously, and it is easy to be interfered with by power supply noise, thus affecting the stability of the whole circuit [2] .
With the development of integrated circuit technology, the application and performance requirements of bandgap reference circuits are constantly improved.It is of great practical value and significance to research bandgap reference sources in the aspects of low-temperature drift, high precision, low voltage, and high power suppression ratio., so the total current is:

Low-pressure bandgap reference source
It is factored out, then: ( This current is replicated through the current mirror to P3, then the voltage drop of R4 is: The resistance ratio can be adjusted so that the Vout has a zero temperature coefficient and the voltage can be reduced to less than 1.25 V [3] [4] .

Starting circuit
The starting circuit is essential to the bandgap reference circuit.There are one or more degeneracy points in the circuit.The circuit is in a zero current state at the degeneracy point, so it is necessary to jump out of the degeneracy point to start the circuit.When the circuit is just powered on, the capacitor voltage cannot change, so the P13 conduction will pull the amplifier output to the ground and start working.When the capacitor is fully charged, the grid of P13 will be pulled up to turn off the starting circuit.We can adjust the capacitance value of the capacitor to determine the power-on time.

Temperature compensation
The analysis of common low-voltage bandgap reference is based on the first-order temperature characteristic curve of Vbe.The Vbe temperature curve of the triode is not a sloping line with a constant slope, but a curve of higher-order terms [5] .As a result, the low-voltage band-gap reference source made according to the first-order temperature curve is not ideal.Considering the nonlinear change of voltage with temperature, it is necessary to compensate for the temperature curve [6] .The common temperature compensation methods include exponential curvature compensation, piecewise curvature compensation technology, BJT non-uniform current density compensation technology, and wi MOS compensation technology [7] .Of course, before compensation, it is very important to derive the Vbe characteristic curve of the transistor.The collector current and emitter current of the transistor is: and C and D are constants independent of temperature, which are substituted into Equation (4) to obtain: where E represents the constant independent of temperature.From the above equation, we can get: The collector current at the reference temperature Tr is: Dividing Equation ( 8) by Equation ( 6), and we can get: ( ) where Į is a constant dependent on temperature and can be obtained from Equation ( 9) Where VG0 represents the silicon bandgap energy of 1.12 eV at 0 K, T represents the temperature variable, Tr represents a specific temperature, Ș represents the constant related to carrier mobility in semiconductor physics, which is determined by process parameters, and Į represents the temperaturedependent order of the collector current of the triode [8] [9] .When PTAT current is input into the collector, Į=1; When the collector input current is zero temperature coefficient current, Į=0 [10] .
As can be seen from the above equation, the second term on the right of Equation ( 10) has a negative linear term, the third term has a high-order nonlinear term, and the second-order temperature coefficient is negative, so only the band-gap reference voltage curve with first-order compensation opens downward.The method used in this paper makes the design of a current or voltage structure that is linearly related to  The power suppression ratio is also an important index of bandgap reference, which represents the anti-interference ability of the circuit when the power supply voltage changes and whether the reference voltage can remain stable with the voltage fluctuation.To achieve a high power rejection ratio, the power supply voltage is usually processed by using an internal regulator, as shown in Figure 3.However, this structure increases chip area and power consumption and the resistance network used can vary depending on the process.Therefore, in this paper, a negative feedback circuit is used to regulate the voltage of the power supply.The resistance network is not required, but only the transistor is required to supply the processed power supply voltage to the band gap, thus improving the power suppression ratio of the whole circuit and greatly saving the area and power consumption, as shown in Figure 4.When the power supply voltage changes, if it rises, the drain voltage of P1 will also rise, and the total current will increase, while the drain voltage of P3 will increase to improve the grid voltage of N1, while the current of N1 will increase to reduce the current of P1 to keep the internal power supply voltage vddl stable.The advantages of this structure are that no resistance network is needed and the area is small.The overall circuit is shown in Figure 2. The output voltage of the band-gap reference designed in this paper is 600 mV (figures 5) and the variation range is 947.5 ȝV at the tt process angle (figures 6).The reference temperature drift coefficient is 19.3353 ppm/ႏ before compensation, 9.58 ppm/ႏ after compensation, and the power suppression ratio is -72 dB before adding the voltage regulator circuit (figures 7).After adding the voltage regulator circuit, it is -104.62 dB (figures 8).

Conclusion
In this paper, a band gap reference cell with a high power rejection ratio, high stability, and lowtemperature drift coefficient is designed based on the traditional low-voltage bandgap reference.Within -30 Υ-130 Υ, the temperature drift coefficient is 9.58 ppm/Υ, and the power rejection ratio is -104.62 dB at low frequency, which meets the needs of a low-temperature drift coefficient and high power rejection ratio.The reference source designed in this paper has better temperature stability and power noise suppression ability than ordinary low-voltage reference sources.The reference source is used in high-voltage ldo or power management chips.

,
which is composed of P7, R5, R6, and Q3.R5=R6 and the current flowing through R5 and R6 is We select appropriate resistance values of R5 and R6 to eliminate the high-order terms of Vbe.

Figure 3 .
Figure 3. Bandgap reference source with voltage regulator

Figure 4 .
Figure 4. Negative feedback voltage regulator circuit

Figure 7 .
Figure 5. Transient simulation Figure 6.Simulation of the temperature curve