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Last updated on 2024年6月29日

How can you optimize CMOS circuit design for power consumption and area?

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CMOS circuits are widely used in electronic engineering for their low power consumption and high integration density. However, designing CMOS circuits that are optimized for both power and area can be challenging, as they often involve trade-offs and constraints. In this article, you will learn some basic principles and techniques that can help you improve your CMOS circuit design for power consumption and area.

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1 Power Consumption Sources

The first step to optimize your CMOS circuit design is to understand the sources of power consumption. There are three main components of power dissipation in CMOS circuits: static power, dynamic power, and short-circuit power. Static power is the power consumed by leakage currents when the circuit is idle. Dynamic power is the power consumed by switching activities when the circuit is active. Short-circuit power is the power consumed by the direct path between the supply and the ground when both the pull-up and the pull-down networks are on during the transition. You can reduce the static power by using low-leakage transistors, such as high-threshold voltage (HVT) or subthreshold devices. You can reduce the dynamic power by minimizing the switching frequency, the capacitance, and the supply voltage. You can reduce the short-circuit power by using tapered sizing, minimizing the input rise and fall times, and avoiding glitches.

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Trevor Newlin

Senior Principal Engineer at NXP Semiconductors - retired
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Where we have multiple oxide transistors within a design, it is important that we move the level shifts to the last stages of the circuit. This reduces the number of high voltage devices switching reducing the power loss. In power switchers the level shift prop delay is the most critical component, and often innovative designs are required to achieve the pico-second prop delays. These delay move directly to the non-overlap delays that add power loss. Similarly in the power stages, care should be taken to optimize the stage delay. If we have to burn die area, this is often the biggest bang for the buck!

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2 Area Optimization Methods

The second step to optimize your CMOS circuit design is to apply some methods that can reduce the area of your circuit. There are two main approaches to achieve area optimization: logic synthesis and layout design. Logic synthesis is the process of transforming a high-level description of your circuit into a netlist of gates and wires. You can use techniques such as logic minimization, technology mapping, and gate sharing to reduce the number of gates and wires in your netlist. Layout design is the process of arranging the gates and wires on a silicon substrate. You can use techniques such as placement, routing, compaction, and folding to reduce the area of your layout.

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Trevor Newlin

Senior Principal Engineer at NXP Semiconductors - retired
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Be sure to use enough layers of metal, sometimes the overall area reduction pays for the cost of the extra metals, this is most critical when we have very high digital content mixed signal die.

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3 Power-Area Trade-offs

The third step to optimize your CMOS circuit design is to consider the trade-offs between power and area. Sometimes, reducing the power consumption may increase the area, or vice versa. For example, using HVT transistors can reduce the static power, but also increase the area and the delay. Using smaller transistors can reduce the area, but also increase the leakage and the dynamic power. Therefore, you need to balance the power and area objectives according to your design specifications and constraints. You can use tools such as power estimation, power analysis, and power optimization to evaluate and improve your power-area trade-offs.

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Trevor Newlin

Senior Principal Engineer at NXP Semiconductors - retired
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The mantra here is size matters, size = capacitance = leakage. The lowest voltage transistors give the lowest overall power requirement. (That’s what the digital guys have been ding for years) The demand for precision is often at odds with speed due to the large area of the devices required. Lower gate voltage devices tend to match better reducing the area and the current requirements. Adding Auto zero to analog circuits can dramatically reduce the size of the devices, reducing capacitance and current required, with optimal design, the die area for these circuits can be similar or smaller than the non Auto zero cells. The lower the offset required the grater the area and current saving.

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4 Design Hierarchy

The fourth step to optimize your CMOS circuit design is to use a hierarchical approach. Design hierarchy is the organization of your circuit into different levels of abstraction, such as system, module, block, and cell. By using design hierarchy, you can simplify your design process, reuse existing components, and modularize your design. You can also apply different optimization techniques at different levels of hierarchy, depending on the scope and the granularity of your design. For example, you can use system-level techniques such as clock gating, power gating, and voltage scaling to reduce the power consumption of your entire circuit. You can use module-level techniques such as pipelining, parallelism, and resource sharing to reduce the power consumption and the area of your functional units. You can use block-level techniques such as logic restructuring, gate reordering, and transistor sizing to reduce the power consumption and the area of your combinational and sequential logic. You can use cell-level techniques such as layout optimization, transistor folding, and device stacking to reduce the power consumption and the area of your standard cells.

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Trevor Newlin

Senior Principal Engineer at NXP Semiconductors - retired
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Clock gating is an effective way of controlling power loss in digital at the larger geometry nodes, however as the nodes gets smaller, consider power gating instead, since leakage can dominate clocking loss.

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Swarupa Kolli
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To optimize CMOS circuit design for both area and power, consider the following approaches: 1. Use larger transistors for critical paths and smaller transistors for non-critical paths. 2. Implement voltage scaling techniques to reduce power consumption. 3. Deactivate unused circuit blocks to reduce leakage power. 4. Use multiple threshold voltage transistors for a balance between performance, power, and area. 5. Disable clock signals to unused circuitry to save power. 6. Minimize overall logic complexity to reduce area and power consumption.

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5 Design Verification

The fifth step to optimize your CMOS circuit design is to verify your design. Design verification is the process of checking whether your design meets your specifications and constraints. You can use techniques such as simulation, testing, and formal methods to verify your design. Simulation is the technique of using software tools to emulate the behavior of your circuit under different inputs and conditions. Testing is the technique of using hardware tools to measure the performance of your circuit on a physical device or a prototype. Formal methods are the techniques of using mathematical logic and algorithms to prove the correctness of your circuit. You can use design verification to ensure that your optimization techniques do not compromise the functionality, reliability, and robustness of your circuit.

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Trevor Newlin

Senior Principal Engineer at NXP Semiconductors - retired
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Don’t kid yourself that you can convert all the blocks to algorithms to speed things up, the chances of missing a critical delay are too high. While these techniques are good starting points, there is nothing like extracted silicon simulation. With high digital content this is not so practical and there has to be some compromise. What we would really like to do is Monte Carlo our top simulation to ensure full chip spec compliance, however due to trimming requirements in analog design this falls down very fast. Using Auto-zero and self calibration can actually result in faster high level simulations (at the expense of more cores).

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6 Design Examples

The sixth step to optimize your CMOS circuit design is to learn from some design examples. Design examples are the practical applications of your optimization techniques to real-world problems. You can use design examples to gain insights, inspiration, and feedback for your own design. You can find design examples in various sources, such as textbooks, journals, conferences, websites, and blogs. Some examples of CMOS circuit design that are optimized for power consumption and area are low-power microprocessors, memory circuits, analog-to-digital converters, and radio-frequency circuits.

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7 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?

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How can you optimize CMOS circuit design for power consumption and area?

1

2

3

4

5

6

7

1 Power Consumption Sources

2 Area Optimization Methods

3 Power-Area Trade-offs

4 Design Hierarchy

5 Design Verification

6 Design Examples

7 Here’s what else to consider

Electronic Engineering

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How can you optimize CMOS circuit design for power consumption and area?

1

2

3

4

5

6

7

1 Power Consumption Sources

2 Area Optimization Methods

3 Power-Area Trade-offs

4 Design Hierarchy

5 Design Verification

6 Design Examples

7 Here’s what else to consider

Electronic Engineering

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