Copyright 2004, 2005 Hans Camenzind. February 2005 This book is can be downloaded without fee from www.designinganalogchips.com Re-publishing of any part or the whole is prohibited. The author is indebted to the following for comments, suggestions and corrections: Bob Pease, Jim Feit, Ted Bee, Jon Fischer, Tim Camenzind, Jules Jelinek, Brian Attwood, Ray Futrell, Beat Seeholzer, David Skurnik, Barry Schwartz, Dale Rebgetz, Tim Herklots, Jerry Gray, Paul Chic, Mark Leonard, Yut Chow, Gregory Weselak, Lars Jespersen and Wolfgang Horn. Camenzind: Designing Analog Chips Table of Contents Table of Contents Analog World 1 Devices 1-1 Semiconductors 1-1 The Diode 1-5 The Bipolar Transistor 1-6 The Integrated Circuit 1-13 Integrated NPN Transistors 1-14 The Case of the Lateral PNP Transistor 1-22 CMOS Transistors 1-23 The Substrate PNP Transistor 1-27 Diodes 1-27 Zener Diodes 1-28 Resistors 1-29 Capacitors 1-32 Other Processes 1-33 CMOS vs. Bipolar 1-34 2 Simulation 2-1 What You Can Simulate 2-2 DC Analysis 2-2 AC Analysis 2-3 Transient Analysis 2-4 The Big Question of Variations 2-6 Models 2-8 The Diode Model 2-8 The Bipolar Transistor Model 2-10 The Model for the Lateral PNP Transistor 2-13 MOS Transistor Models 2-14 Resistor Models 2-16 Models for Capacitors 2-17 Pads and Pins 2-17 Just How Accurate is a Model? 2-18 3 Current Mirrors 3-1 4 The Royal Differential Pair 4-1 5 Current Sources 5-1 Bipolar 5-1 CMOS 5-7 The Ideal Current Source 5-7 6 Time Out: Analog Measures 6-1 dB 6-1 RMS 6-2 Noise 6-4 Fourier Analysis, Distortion 6-6 Frequency Compensation 6-9 7 Bandgap References 7-1 Low-Voltage Bandgap References 7-11 Edition February 2005 All rights reserved Camenzind: Designing Analog Chips Table of Contents CMOS Bandgap References 7-13 8 Op Amps 8-1 Bipolar Op-Amps 8-1 CMOS Op-Amps 8-9 Auto-Zero Op-Amps 8-15 9 Comparators 9-1 10 Transconductance Amplifiers 10-1 11 Timers and Oscillators 11-1 Simulation of Oscillators 11-14 LC Oscillators 11-15 Crystal Oscillators 11-16 12 Phase-Locked Loops 12-1 13 Filters 13-1 Active Filters, Low-Pass 13-1 High-Pass Filters 13-6 Band-Pass Filters 13-6 Switched-Capacitor Filters 13-8 14 Power 14-1 Linear Regulators 14-1 Low Drop-Out Regulators 14-4 Switching Regulators 14-8 Linear Power Amplifiers 14-12 Switching Power Amplifiers 14-15 15 A to D and D to A 15-1 Digital to Analog Converters 15-1 Analog to Digital Converters 15-7 The Delta-Sigma Converter 15-8 16 Odds and Ends 16-1 Gilbert Cell 16-1 Multipliers 16-3 Peak Detectors 16-5 Rectifiers and Averaging Circuits 16-7 Thermometers 16-10 Zero-Crossing Detectors 16-12 17 Layout 17-1 Bipolar Transistors 17-1 Lateral PNP Transistors 17-5 Resistors 17-6 CMOS Transistors 17-7 Matching 17-9 Cross-Unders 17-10 Kelvin Connections 17-11 Metal Runs and Ground Connections 17-11 Back-Lapping and Gold-Plating 17-12 DRC and LVS 17-12 References Index Edition February 2005 All rights reserved Camenzind: Designing Analog Chips Analog World Analog World "Everything is going digital". Cell phones, television, video disks, hearing aids, motor controls, audio amplifiers, toys, printers, what have you. Analog design is obsolete, or will be shortly. Or so most people think. Imminent death has been predicted for analog since the advent of the PC. But it is still here; in fact, analog ICs have been growing at almost exactly the same rate as digital ones. A digital video disk player has more analog content than the (analog) VCR ever did. The explanation is rather simple: the world is fundamentally analog. Hearing is analog. Vision, taste, touch, smell, analog all. So is lifting and walking. Generators, motors, loud-speakers, microphones, solenoids, batteries, antennas, lamps, LEDs, laser diodes, sensors are fundamentally analog components. The digital revolution is constructed on top of an analog reality. This fact simply won't go away. Somewhere, somehow you have to get into and out of the digital system and connect to the real world. Unfortunately, the predominance and glamour of digital has done harm to analog. Too few analog designers are being educated, creating a void. This leaves decisions affecting analog performance to engineers with a primarily digital background. In integrated circuits, the relentless pressure toward faster digital speed has resulted in ever-decreasing supply voltages, which are anathema to high-performance analog design. At 350nm (3.3V) there is still enough headroom for a high-performance analog design, though 5 Volts would be better. At 180nm (1.8V) the job becomes elaborate and time-consuming and performance starts to suffer. At 120nm (1.2V) analog design becomes very difficult even with reduced performance. At 90nm, analog design is all but impossible. There are "mixed signal" processes which purportedly allow digital and analog circuitry on the same chip. A 180nm process, for example, will have some devices which can work with a higher supply voltage (e.g. 3 Volts). While such an addition is welcome (if marginal), the design data (i.e. models) are often inadequate and oriented toward digital design. Edition February 2005 All rights reserved Camenzind: Designing Analog Chips Analog World Hence this book. It should give you an overview of the world of analog IC design, so that you can decide what kind of analog function can and cannot, should and should not be integrated. What should be on the same chip with digital and what should be separate. And, equally important, this book should enable you to ask the right questions of the foundry, so that your design works. The first time. * * * You will find that almost all analog ICs contain a number of recognizable circuit elements, functional blocks with just a few transistors. These elements have proven useful and thus re-appear in design after design. Thus it makes sense to first look at such things as current mirrors, compound transistors, differential stages, cascodes, active loads, Darlington connections or current sources in some detail and then examine how they are best put together to form whole functions. * * * Academic text books on IC design are often filled with mathematics. It is important to understand the fundamentals, but it is a waste of time to calculate every detail of a design. Let the simulator do this chore, it can do it better and faster than any human being. An analysis will tell you within seconds if you are on the right track and how well your circuit performs. Assuming that you have competent models and a capable simulator, an analysis can teach you more about devices and circuits than words and diagrams on a page. Edition February 2005 All rights reserved Camenzind: Designing Analog Chips Chapter 1: Devices 1 Devices Let's assume your IC design needs an operational amplifier. Which one? If you check the data-books of linear IC suppliers, you'll find hundreds of them. Some have low current consumption, but are slow. Others are quite complex, but feature rail-to-rail inputs and outputs. There are inputs which are factory-trimmed for low offset voltages, outputs for high currents, designs for a single supply voltage, very fast devices, etc. Here is the inherent problem with analog building blocks: there are no ideal designs, just configurations which can be optimized for a particular application. If you envisioned a library from which you can pull various analog building blocks and insert them into your design, you are about to experience a rude shock: this library would have to be very large, containing just about every operational amplifier (and all other linear functions) listed in the various data-books. If it doesn't, your IC design is bound to be inferior to one done with individual ICs. In short: There are no standard analog cells. If your application is the least bit demanding, you find yourself either modifying previously used blocks or designing new ones. In either case you need to work on the device level, connecting together transistors, resistors and rather small capacitors. To do this you need to know what devices are available and what their limitations are. But above all you need to understand devices in some detail. The easiest way to learn about complex technical things is to follow their discovery, to have the knowledge gained by the earlier men and women (who pioneered the field) unfold in the same way they brought it to light. Edition February 2005 1-1 All rights reserved Camenzind: Designing Analog Chips Chapter 1: Devices Semiconductors In 1874 Ferdinand Braun was a 24-year old teacher in Leipzig, Germany. He published a paper which was nothing short of revolutionary: he had found that some materials violated Ohm's law. Using naturally formed crystals of Galena (lead sulfite, the chief ore mineral of lead) and other sulfites, he pressed a spring-loaded metal tip against their surfaces and observed that the current through this arrangement was dependent on the polarity of the applied voltage. Even more puzzling was the fact that, in the direction which had better conduction, the resistance decreased as the current was increased. What Braun (who later would give us the CRT) had discovered, we now know as the diode, or rectifier. It was not a very good one, there was only a 30% difference between forward and reverse current. And there were no practical applications.