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Motivation

Nowadays analog and mixed-signal integrated Circuits like low-dropout regulator (LDO regulator), analog-to-digital converters (ADC), DC to DC converters, digital-to-analog converters (DAC), and power management ICs; use voltage references extensively. The figure of merit of these circuits mainly depends on the accuracy and stability of the reference voltage. These circuits operate in different environmental conditions, where they are exposed to a wide temperature range. So, if the change in temperature or any other process variation changes the reference voltage then the performance of ADCs, DACs etc will deteriorate. One of the hottest applications of voltage reference is in modern communication system as shown in Fig:1. In these systems, the performance of data acquisition section strongly depends on stability of voltage reference.

Fig 1:  Modern communications System on Chip

A bandgap reference (BGR) is an electronic circuit which provides a stable voltage which is independent of temperature, supply voltage and process variation.

Problem statement

A first order conventional BGR is made by the summation of base to emitter (VBE) voltage of BJT transistor (which has indirect variance with absolute temperature, or “CTAT”) and of thermal voltage (VT) that has a direct relation with absolute temperature (“PTAT”).  But the output voltage of a first order BGR cannot achieve a very low Temperature coefficient (TC). Due to the non-linear behaviour of the base-emitter voltage (VBE) as shown in Fig:2, the output exhibits a curvature in reference voltage.

Fig 2: Curvature in the temperature dependence of a BGR voltage

The other important design parameters of BGR circuit are PSRR, line regulation, power consumption, start-up, area, and reference voltage value. So, in the proposed design we will mainly focus on curvature correction to reduce the TC, improvement in PSRR and will also include a start-up circuit to ensure that BGR work properly during supply voltage transient.

 

Proposed solution

In this proposed solution we will extract VGo as shown in equation (1) [1], from the base emitter voltage of a bipolar transistor. VGO is the silicon bandgap voltage at zero Kelvin, and it is a physical constant voltage independent of temperature.

                              

Where VGo is the bandgap energy of silicon independent of temperature, Zeta is a process dependent parameter, x refers to the temperature dependency order, Tr is reference temperature, K is boltzmann constant and q is charge of electron.

For VGo extraction, we must compensate the 2nd term, which is CTAT, and 3rd term which is non-linear in equation (1) to get constant VGo voltage which is independent of temperature. CTAT term will be compensated by adding PTAT term. To compensate the non-linear term, which causes curvature in reference generator output, we will use the concept of three diode connected model as shown in Fig:3 and self-bootstrapping concept.

Fig: 3 Illustration of using three BJT connected as diodes to extract the VGO.

In this model the difference of V1 and V2 generates A PTAT term and the difference of V2 and V3 generates a non-linear term which have opposite behaviour to the non-linear term in equation (1). Then we will add VBE, PTAT and non-linear term generated from the difference of V2 and V3 to extract the VGo from equation (1), which is temperature independent.

Fig :3  Weighted summation to extract the VGO

Block Diagram

In this proposed block diagram, we have applied the three-diode connected model and self-bootstrapping to the first order conventional BGR to cancel the non-linear behavior of VBE which causes curvature in the output voltage of reference generator and extract VGo.

Fig: 4 VGO extractor based on conventional BGR circuit

 

Table 1 Target Specifications

Specification

Target

Temperature coefficient (TC)

 <5 ppm/oC

 PSRR

 > -50 dB

Supply Voltage Range

2 to 4 V

Line regulation

< 2 mV

Temperature range

-40 to 125 oC

Team

Engr.Uzair Ahmad (i201317@nu.edu.pk)

Prof.Dr.Rashid  Ramzan (rashad.ramzan@nu.edu.pk)

Dr. Hassan Saif (hassan.saif@nu.edu.pk)

National University of Computer and Emerging Sciences (FAST-NUCES) introduced the first specialized MS Integrated Circuit (IC) design program in Pakistan in Spring 2020. We need your assistance to promote the IC design in Pakistan.  
 
The applied project is the MS thesis and we would like to tapeout the proposed circuit and then measure and publish and share the design in open source.  Your assistance in tapeout will be of real help. 
To know more about this nascent MS IC Design program please have a look at the link below. 
 

Deliverables

Schematic, Layout , GDS

References

[1]      Z. Liu and D. Chen, “A voltage reference generator targeted at extracting the silicon bandgap Vgo from Vbe,” Proc. - IEEE Int. Symp. Circuits Syst., Sep. 2017, doi: 10.1109/ISCAS.2017.8050750.

[2]      N. Liu, R. Geiger, and D. Chen, “Bandgap voltage VGO extraction with two-temperature trimming for designing sub-ppm/°C voltage references,” Proc. - IEEE Int. Symp. Circuits Syst., vol. 2019-May, 2019, doi: 10.1109/ISCAS.2019.8702697.

Owner

Uzair Ahmad

Organization URL

http://isb.nu.edu.pk/rfcs2/

Summary

A novel bandgap voltage reference with accurate curvature correction technique is proposed that target sub-ppm temperature coefficient for the wide range of temperature. In addition, the proposed design provides good line regulation and high-power supply rejection ratio. The design is an improvement of state-of-the-art bandgap voltage reference design published in ISCAS

Category

bg

Process

sky130A

Tags

BGR

Curvature Correction

Line Regulation

Opamp

PSRR