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Power management for optimal design

Posted: 19 Mar 2009 ?? ?Print Version ?Bookmark and Share

Keywords:eSilicon? management power? design optimal?

From this, we see that the main parameters controlling the power are the threshold voltage Vth, oxide thickness tox, transistor length L and width W, the power supply voltage Vdd, and the back-gate bias Vbs. Since active power varies as the square of Vdd, reducing Vdd has the most impact on reducing active power. In fact, the reduction in power is twice the amount of reduction in voltage, i.e., a 20 percent reduction in Vddwill yield a 40 percent reduction in active power. The remaining parameters only affect active power linearly. Any significant change in L, W, or Vth has adverse effects on the performance on the transistor. As a result, these parameters can only be changed by small amounts and therefore have only a small role in reducing active power. However, they do have a significant impact in reducing leakage power, since they are exponentially related to it. From equation (5), we can see that


If Vgs = - nT, the equation becomes


This means that the subthreshold current is reduced by a factor e (2.71828), for every nT reduction in effective gate to source voltage. Since n is typically between 1 and 2.5 for a technology, and T is 26mV at room temperature, this means for every 50mV to 75mV change in gate to source voltage, we can see a reduction of 2.7 in subthreshold current. Increasing the threshold voltage Vth has the same effect. Thus for every 50mV to 75mV increase in threshold voltage, the leakage current is reduced by a fact of 2.7. A 100mV to 150mV increase in threshold voltage reduces leakage by a factor of 7.4.

We can get further reduction in leakage current by increasing the back-gate bias. The gains as not as significant due to the presence of the body bias coefficient . Reducing Vdd also helps to reduce leakage current. Increasing the channel length of the transistor helps to reduce leakage current in two ways. Not only does it directly impact leakage current as in equation (5), it also helps to increase the threshold voltage as seen in equation (2).

The subthreshold current has an exponential dependence on temperature. Since the term nT appears on the denominator of the negative exponent, as the temperature increases, the current increases significantly. This poses a major challenge, as leakage power becomes a significant component of total power at high temperatures. The total power at high temperature for fast process corner devices must be considered for worst case power analysis.

Now that we understand the parameters that affect active and leakage power, let's examine how we can control these using process technology and design methods.

The role of technology

Proper technology selection is one of the key aspects of power management. The goal of each technology advance is to improve performance, density and power consumption. The typical approach in developing a new generation of technology is based on constant electric field scaling. Both the applied voltage and the oxide thickness are scaled to maintain the same electric field. In this approach power is reduced by about 50 percent with every new technology node. However as the voltage gets smaller, the threshold voltage also needs to be scaled down to meet the performance targets of that technology. This unfortunately increases the subthreshold current and hence the leakage power. In order to overcome this, constant scaling is no longer applied for 65nm and beyond, instead a more generalized form of scaling is adopted.

Since it is not possible to optimize a technology for performance and leakage at the same time, each technology usually has two variants. One variant is targeted for high performance while the other is targeted for low leakage. The primary differences between the two are in the oxide thickness, supply voltage and threshold voltage. The technology variant with the thicker gate oxide is targeted for low leakage design and needs to supports a higher voltage in order to achieve a reasonable performance.

From equation 2, we see that there are two technology dependent parameters, and Sthat can be manipulated to control the threshold voltage. They are dependent on the doping concentration which can be adjusted using an additional implant mask. This allows one to create devices with multiple threshold voltages in a technology which can then be used by circuit designers to control leakage power in their designs.

When selecting a technology to optimize the power for a given design, therefore both aspects need to be taken into considerationthe need to use a smaller geometry to reduce active power and the need to use a low leakage variant to reduce leakage. There is a trade-off to be made with cost and risk.

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