avsdbgp_3v3_sky130_v1

Implementation of bandgap reference circuit on Skywater's open-source 130nm pdk.

Bandgap Reference Design

Table of Contents

1. Introduction
2. Requirements
3. Tools
   • XSCHEM
   • ngspice
   • Magic
4. Schematic
5. Pre-Layout Simulations
   • Voltage Variation Check
   • Temperature Variation Check
   • Voltage Coefficient Check
   • Temperature Coefficient Check
   • Start-up Circuit Check
   • Enable Logic Check
6. Layout
7. Post-Layout Simulations
   • Voltage Variation Check
   • Temperature Variation Check
   • Voltage Coefficient Check
   • Temperature Coefficient Check
   • Start-up Circuit Check
   • Enable Logic Check
8. Bandgap Performance Parameters [Post-Layout]
9. Future Work
10. References
11. Acknowledgement
12. Contact Information

Introduction

A bandgap reference circuit implementation on Skywater’s open-source 130nm process design kit (pdk) is proposed here. A simple bandgap topology is valuated with a set of simulations and then implemented in open-source EDA tools. In the design, a current mirror without operational amplifier is used to keep the topology simple. For more information, please check here.

Requirements

Design specifications and requirements are listed here.

Bandgap Performance Parameters [Post-Layout]

Parameter	Description	Min	Type	Max	Unit	Condition
Technology	130 nm CMOS Process
RL	Load resistance at Vbgp terminal	100			Mohm	VDD=3.3V, T=27C
Vbgp	Output Reference voltage	1.197642	1.201057	1.212476	V	T=-40 to 140C, VDD=3.3V
Vbgp	Output Reference voltage	1.178285	1.201057	1.211677	V	VDD=2.7V to VDD=3.6V, T=27C
TC_vbgp	Temperature Coefficient of Vbgp		6.8717		ppm/C	T=-40 to 140C, VDD=3.3V
VC_vbgp	Voltage Coefficient of Vbgp		2.7802		%/V	VDD=2.7V to 3.6, T=27C
VDD	Supply Voltage	3.2	3.3	3.6	V	T=-40C to 140C
IDD	Supply Current		3.954		uA	EN=1
IDD	Supply Current		607.836		nA	EN=0

Tools

Open-source tools are used to design and simulate bandgap reference circuit.

XSCHEM

XSCHEM is a schematic capture and netlisting EDA tool. In addition to Xschem, a version named Xschem_sky130 which is optimised to work with Skywater's 130nm pdk is used. For more information, please check this link.

Although XSCHEM is not required to run simulation files in this repository, it can be installed with built-in ngspice tool and support for sky130 pdk. Please follow set up steps mentioned in this video.

ngspice

ngspice offers a spice simulator to model active, passive, analog and digital elements of a device according to parameters provided by semiconductor manufacturer. For more information, please check this link.

Installation on Ubuntu

sudo apt-get install -y ngspice

Magic

Magic is a opensource VLSI layout tool. Please check link.

Installation on Ubuntu

sudo wget "http://opencircuitdesign.com/magic/archive/magic-8.3.122.tgz"
tar -xvzf magic-8.3.122.tgz
cd magic-8.3.122
sudo ./configure
sudo make
sudo make install

Schematic

A simple current mirror topology is implemented to achieve stable reference voltage output.

Pre-Layout Simulations

Here are the steps to run simulation files,

git clone https://github.com/swarup-p/bandgap-ref-on-sky130.git
cd avsdbgp_3v3_sky130_v1/pre-layout/spice-deck/
ngspice

Run below commands in ngspice terminal to get the simulation results.

Voltage Variation Check

Reference voltage (vbgp) vs variations in supply voltage Vdd (2V to 4V)

source pre_vv_check.spice

Temperature Variation Check

Reference voltage (vbgp) vs variations in temperature (-40 to 140) degree

source pre_tv_check.spice

Voltage Coefficient Check

source pre_vc_check.spice

Temperature Coefficient Check

source pre_tc_check.spice

Start-up Circuit Check

source pre_startup_check.spice

Enable Logic Check

Check the current values when enable terminal is at logic 1 and when enable terminal is at logic 0.

source pre_enable_check.spice

Layout

Check the layout file in magic,

cd avsdbgp_3v3_sky130_v1/layout/

magic -T ../lib/sky130A.tech avsdbgp_3V3.mag &

Post-Layout Simulations

Here are the steps to run simulation files,

git clone https://github.com/swarup-p/bandgap-ref-on-sky130.git
cd avsdbgp_3v3_sky130_v1/post-layout/spice-deck/
ngspice

Run below commands in ngspice terminal to get the simulation results.

Voltage Variation Check

Reference voltage (vbgp) vs variations in supply voltage Vdd (2V to 4V)

source post_vv_check.spice

Temperature Variation Check

Reference voltage (vbgp) vs variations in temperature (-40 to 140) degree

source post_tv_check.spice

Voltage Coefficient Check

source post_vc_check.spice

Temperature Coefficient Check

source post_tc_check.spice

Start-up Circuit Check

source post_startup_check.PNG

Enable Logic Check

Check the current values when enable terminal is at logic 1 and when enable terminal is at logic 0.

source post_enable_check.spice

Future Work

1. Add cascode transistors in the design to improve stability of output reference voltage.

2. Add a resistor at the gate terminal of enable transistor.

3. Improve enable logic to consume low current when enable logic is 0.

4. Perform PNR of the design using openLANE flow.

References

Here are some reference links used to learn to design in open source EDA tools and about bandgap reference circuit design.

1. https://github.com/nickson-jose/vsdstdcelldesign

2. https://github.com/praharshapm/vsdmixedsignalflow

3. https://github.com/sherylcorina/avsdbgp_3v3

4. https://github.com/an3ol/General-purpose-Bandgap-Reference-avsdbgp_3v3

5. https://www.udemy.com/course/vlsi-academy-custom-layout/

Acknowledgement

• Kunal Ghosh - Co-founder (VSD Corp. Pvt. Ltd)
• Tim Edwards - Senior Vice President of Analog and Design at efabless corporation
• Anmol Purty - VSD Intern
• Sheryl Serrao - VSD Intern

Contact Information

• Swarup Pulujkar - VSD Intern