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Steve Flannagan's
IC Design Journal
Issue #2    January-February 2004

Published by Flannagan IC Technical Services & Consulting, Inc.
WWW.FLANIC.COM


Printable Version:  Issue2 PDF
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Dear Friends and Colleagues,

In the last issue, I emphasized that good opportunities can be found even in rough times, for those with insight. In this issue I have for you:
 

  1. How to Build a World-Class Business
  2. Direct your Career with World-Class Quality, and
  3. Today's Technical Feature: Understanding Temperature Independence.


The theme of the next few issues is "To be the Very Best." I will review the key requirements for building a world-class business, and show how the careers of individuals contribute to this enterprise.

For our Technical Topic, we return to the On-Chip regulator begun in the previous issue, and explore how temperature independence actually arises in a design.

Thank you all very much for your comments and feedback. Be assured that I read every letter and email individually, and will continue to be your partner for a bright future in Technology!


Sincerely,
Steve



Business Outlook:

To Be the Very Best

Have you ever asked the question, "What does it take to be the very best in the entire world?"

Generally, it's believed that taking an enterprise to the world leadership position requires about five years. As many times as I've observed this process, and had the privilege to participate in it, the path has been different each time. But certain essential components are always present.

Define responsibilities coherently

The most important advantage is a culture that contains a good spirit of co-operation, where the individual is totally dedicated to the success of everyone involved: their co-workers, superiors, subordinates and customers. This is best achieved when responsibilities are defined coherently. Job definitions should be flexible and coordinated with the mission. Especially, they must not be in conflict with one another.

Know where you are

Intensity of effort can never be the whole story. Success also depends on timing, positioning, and wise choice of the mission. Often a specific idea will be ripe at a given time, when the market needs it and the industry has not yet exploited the best architecture for the product. Your own organization may be in a better position to excel at one particular enterprise and not others. A careful choice for the first product is very important.

For similar reasons, study carefully the existing state-of-the-art. Especially look for key difficulties for which prior designs may not have the strongest solution. Very often, this is the best opportunity to gain an extremely significant advantage. Provide for frequent communication between your designers and product engineers. Testing and evaluation of products (both your own and those from other companies) is essential for making good design improvements and breakthroughs.

Allow time for exploration

Early in the project, the team needs the flexibility to study and understand design needs. This phase of design must be adequately staffed. Its goals need to be energetic, but adaptive at the same time. The managers should protect the technical contributors from organizational distractions during this period of preparation.

Make the first product a significant winner

Growing an enterprise to world-class should begin with a significant break-through application or design for high demand. Typically this will take about a year of very careful work and background preparation.

Make every component a quality design

High-quality technical knowledge is widely available. But it's surprising how seldom all the necessary technical attributes are assembled for a successful project. Make sure that every key aspect of the project is understood and designed correctly, with good immunity to variations in parameters, and tolerance for marginality in other components.

Rework and debug cost a lot more later than earlier. Invest the effort to utilize existing knowledge as early as possible in the project.

If your process includes an approach of quick implementation followed by later improvement, that method can also be managed to better provide the results you desire. Break the enterprise down into cleanly demarcated segments, and then thoroughly debug the parts individually before assembling them. The points of interaction must then be completely defined, understood and checked. The choice of how to partition your project thus becomes a key input into the quality.

Another reason that this compartmentalization is important, is that bugs have a way of interacting with each other. When there are several problems happening at once, it's sometimes difficult to separate out the specific cause and effect relationships.

Promote co-operation

There should be synergy and exchange of information among different groups, and a certain willingness to get the best benefit from available resources.

Your team includes individuals who are highly competitive by nature. Inspire them to transcend that inclination and to co-operate. An important natural unifying force is the perception of a common adversary. Make sure that the team recognizes the opponent as your company's competitors, and not some other group across the hall. Regularly compare and benchmark your products' performance with the rest of the industry. Inspire professionalism by example; speak well of your own equals and superiors within your organization. Be consistently supportive of all the contributors on the project.


Career View:

The Quality is the Key

To build a world-class business over a period of a few years, the quality must be present at the very beginning. This means that the managers must have available to them a key advantage or application that will carry them forward. One of the best things you can do for your career is to provide them with such a key advantage.

Always ask your superiors and co-workers about their significant needs. Find out what are the major problems facing their organization, and how you can assist them in achieving the world-class success that they desire.

Our seminar Future-Oriented Career Development explores key ideals for being the best possible in your technical career, and supporting your organization.

Do you know the answers to these key career questions?

      1. What is the most important function of my supervisor?

      2. How do I find a compass when there's no clear direction?

      3. How do I recognize if my manager is a strong leader?

      4. How do I know when I have enough data?

      5. What should I do, and avoid, when entering a new organization?

      6. What is the number one cause of delay in technical work?

      7. What are the major sources of conflict within an organization?

These are a few of the 15 key career issues covered in our seminar. Please see the Careers section of our web site for a more complete description.

The career seminar consists of a series of talks and discussion groups focused on methods to help professionals to maximize their potential by renewing their teamwork ideals. It begins with business goals, introduces technical priorities and management methods, and explores the dynamics of respect and integrity for individuals and groups. This seminar can be tailored to the specific needs of your department.


Today's Technical Feature:

Temperature Invariance for the On-Chip Regulator in CMOS

The first part of this analysis was given in our previous issue.

In that discussion, we identified the three major parts of an on-chip regulator; (1) the Bandgap Generator, (2) the Reference Generator, and (3) the Output Driver. We developed the methods to design the second and third parts with good stability and gain.

Today we complete the study by looking into the "Bandgap Generator," from which temperature independence originates.

A qualitative exploration of the temperature independence of Widlar-style circuits is available in most advanced bipolar circuit design textbooks. Here I will expand the equations more extensively, giving individual terms as a function of temperature, so that we can see how they vary algebraically.

In most circuit design activities, equations are empirical rather than analytical. For this range of operation, an empirical expression taking the form of the general diode equation is applicable:

Js is also dependent on temperature. Using the subscript "o" to denote the room-temperature value of any parameter, the saturation current density (Js) relative to its room temperature value (Jso) is

Neglecting the 'minus one' in eq. (1), and substituting eq. (2) into eq. (1), gives the forward diode current density as a function of temperature:

Rearranging gives an expression for the diode voltage as a function of temperature:

Equations (3) and (4) can be expressed in terms of the currents (I) or current densities (J).

Now we go to the familiar bandgap circuit (Fig. 1) and apply the nodal equations at V1.


Since at equilibrium V1 = V3, we write

Using eq. (4) to substitute expressions for V2 and V3, this becomes

Then since we're using equal sizes for Q1 and Q2, we allow Iso1=Iso2 to cancel in eq. (6). Then substitute I1/I2 = R2/R1, into (6) and the result is

From the schematic we get

then substituting V1 = V3 results in

Then use eq. (4) to get an expression for V3 in terms of I1. Substitute that expression for V3 into eq. (8b) and then substitute I1 = I2R2/R1. The resulting equation for VBG will be:

Substituting (7) into (9) gives

which provides an algebraic expression for VBG in terms of temperature. Some typical parameters values might be:

      kTo =      0.0257 V
       Eg =        1.12 V
        a =        5.52
       To =         298 K
     Iso1 =     8.5E-17 A
        k =  8.6242E-05 V/K
       R1 =      34,900 ohm
       R2 =      14,250 ohm
       R3 =       1,000 ohm


In this above example, the values of R1, R2 and R3 are selected to minimize the variation of VBG. Because of the ratios appearing in equation (10), for a given value of one of the resistors, the optimum point is reached with an optimum ratio of the other two. Choose larger resistance values to reduce the current consumption of the circuit.

The actual value of Is is dependent on many factors. There is some controversy about the best value of a, but small changes in the value used for Eg can be made to achieve an empirical fit. Early textbooks had used a = 1.5 and Eg = 1.2, but later empirical analyses have adjusted Eg to the value of 1.12 and compensated with larger values of a.

Equation (10) details the variation of the terms of VBG. Term 5 (Eg) variation is neglected in this range. The temperature independence is realized as terms 3 and 4 decrease while terms 1 and 2 increase (Fig. 3.)


Summary

Temperature independent circuits can be created using the classical double-diode technique. Empirical design techniques make it straight-forward to implement these methods in a modern design. Good bus routing techniques are appropriate to ensure a quiet supply over the entire chip.


Technical References
 

[1] R. J. Widlar, "New developments in IC voltage regulators," IEEE J. Solid-State Circuits, Vol. SC-6, pp. 2-7, February 1971
[2] Gray and Meyer, Analog Integrated Circuits, Second Edition, Wiley, 1984, ch. 4



This concludes the second edition of Steve Flannagan's IC Design Journal. We hope you have found this interesting and useful. Feel free to send me your comments, suggestions, or ideas for future issues. Please contact me for assistance in your technical business, electronic design, planning, training or team development.


Regards,
Steve


About the writer:

Steve Flannagan is a highly respected electronics designer and consultant, widely acclaimed for his leadership and professionalism. He is especially regarded for his teamwork and dedication to the success of individuals. Steve has designed some of the world’s leading SRAM and memory circuits, is widely published in the field and holds more than 40 United States Patents in the area of circuit design and evaluation. He has worked professionally as a designer at Intel, and also at Motorola where he held the title of Fellow.


Printable Version:  Issue2 PDF
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Flannagan IC Technical Services & Consulting, Inc.
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