This handbook is intended to help and assist analog IC designers in their day to day design work. This handbook describes the general TOP-DOWN design methodology and some related techniques that can be used during most design phases. It looks at the design activity as a hole and tries to figure out some general principles and to give an overview on the design process.
The document is structured logically and normally defines words and concepts before using them. However, basic knowledge of electronics is required for using this handbook efficiently in day to day design. The general method developed in this handbook is not intended to change the way you work, it is intended to help you and to speed up your experience.
Let’s start with a very simple example from day to day life: Building a new house. What can be the process to perform this task?
Basically there are two opposite methods:
What the TOP-DOWN method suggests is that the second method is a better approach. Of course, in real life, one often has to mix these two extreme methods. However, this mix always benefits from putting emphasis on defining before doing.
Common arguments against the TOP-DOWN method are that it is less “creative” or that it takes time to define what has to be done or that people know what they have to do and don't need to write specifications before designing. In this book we will show that:
We can state that most experienced people practicing BOTTOM-UP design in fact use an implicit TOP-DOWN approach. Finally, another common situation is when people do not know what they want. The BOTTOM-UP method allows to start and see what gets out. But is that the best path to a successful product?
The confusion is that most of the time, the designer knows more or less what he has to do. The specification is often implicit and inaccurate. What the TOP-DOWN method suggests is that clarifying the specification is important and that expressing implied things is important. This is of particular importance in the common situation where several designers have to work together on the same projects. Another very common situation is that it is difficult to specify everything from the beginning. The TOP-DOWN method does not require that every detail is defined before starting, it recommends that things should be defined before being done. So the specification can be louse at the beginning and can be refined along the design process. There is a big margin between defining nothing and defining everything. This margin is a space for step by step implementing the TOP-DOWN design method. And anyway, it does not much take time to define what you know!
For instance, what about the location of the different rooms in the house? What about the wallpaper colour?
Obviously the sequential aspect is important. Some questions have to be addressed first, other questions depend on the answers to the first ones. Some topics are important, others are details.
This very simple example is aimed at showing two things that are very common in projects:
The TOP-DOWN design methodology consists globally in:
Of course, TOP-DOWN design is possible only for designers who know about the design techniques, the laws of their art, the behavior and properties of basic building blocks and some of the classic solutions in their field. This is basically why dedicated sections in this book deals with components, building blocks and basic electronics laws.
Before considering the details of the TOP-DOWN method, a good start point is looking at the overall product life, taking a big picture. After all, the best approach for introducing the TOP-DOWN method is the TOP-DOWN method itself!
To some extend, what is shown here for integrated circuits can apply to many other products. Looking at the entire product life gives a good overview and shows what happens before and after the design phase this book focuses on. And knowing the context, the origin and the goal really helps the designer in his day to day work as it gives some of the always missing data that are required to make the best choice between the design options.
The product life can be figured as linear over time and it divides in four major phases:
The other good reasons for speeding up Development are competition and limited product life time on the market. The shorter the definition and development phases, the sooner the product gets on the market and the more likely it can be ahead of competitors, but also the longer the product can stay on the market before it is outdated or replaced by another product. This can make the difference between a successful project and a disaster.
As briefly said in the introduction, it is always a good practice to define the goal before doing anything. For most products, definition consists mainly in defining two things:
A product definition is usually a list of functions it must achieve together with numbers specifying all the required performances. It is often a good practice to define also the undesired behaviors and their characteristics so as this gives a better picture of the product. The definition phase goal is translating a product idea into a formal product description. The definition phase ends when the product requirement specification is available. A common situation is that not all the functions and performance can be specified at the very beginning of a project but a good practice is to open questions even though answers can be “TBD” (To Be Defined) for a while. TOP-DOWN consists in specifying before doing, but it does not say that everything must be defined during the very first minute...
Development will not be detailed here. In the next chapter, we will zoom on the development phase that includes design, our primary focus with this book. The development phase goal is translating a product description formalized in the requirement specification into an actual working product. The Development really starts when the product requirement specification is available and ends when the product is ready for volume production.
Production is usually manufacturing and distributing the product in volume. The three main constraints during the production phase are:
One element of the IC market to consider is the price pressure that leads to reduce the sales price along time. Maintaining revenue in a context of price reduction is a hard challenge that must be addressed every day. Initial “Return On Investment” calculations must take into account the price pressure that can be as high as 5 to 15 % price reduction per year. Designing a good product that can be manufacture in volume with good yield is important in such a context. The good news is that it is one of the TOP-DOWN method goals. The production phase goal is making money selling the product. The production phase starts after a pre-production phase has demonstrated the product manufacturability in good conditions and stops when the product is no more economically interesting, either because the market prices have dropped or because volumes have dropped.
Most products are kept in production as long as possible since it is a good way to pay for the initial development costs. This is possible as long as the sales price is higher than the production cost to ensure a reasonable margin. This implies in many cases that the production volumes are large enough. For various reasons, volumes may drop after some time:
In this case, the product is often no longer interesting and production has to stop. Depending on the market or the application, some products cannot be stopped without notice:
For these reasons, production stop has to be planned, customers have to be noticed to define their needs. This requires some delay that are often defined in the contracts. Of course, this completely depends on the addressed market. Military products have very long life cycles. Customer products are often stopped without notice and without spare parts for maintenance.
Another aspect is that the product life time may differ from its manufacturing technology life time. In such a situation that can occur for several reasons, the product has to be redesigned. The reasons for such a situation can be:
Whatever the reason, in situations where a product has to be redesigned, the TOP-DOWN method is really helpful or, more precisely, can be helpful if it has been used for the initial design...
This introduction aims at defining the context around the design activity that will be detailed all along the book. It is important to understand the reasons for developing a method or a technique in order to use them properly and benefit from them. In this introduction, we have seen that there are many reasons for the TOP-DOWN design method to be used, for speeding up the development process and for creating better products. Before going further, it might be good to list the reasons in order to make sure they have been understood and will be remembered.