15 April 2008

Short Circuit Power

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When dynamic power is analyzed the switching component of power consumption, an instantaneous rise time was assumed, which insures that only one of the transistors is ON. In practice, finite rise and fall times results in a direct current path between the supply and ground, GND, this exists for a short period of time during switching.



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Short circuit power [3]

2Consider an example of inverter. During switching both NMOS and PMOS transistors in the circuit conduct simultaneously for a short amount of time. Specifically, when the condition, VTn (lesser than) Vin (lesser than) Vdd - |VTp| holds for the input voltage, where VTn and VTp are NMOS and PMOS thresholds, there will be a conductive path open between Vdd and GND because both the NMOS and PMOS devices will be simultaneously on. This forms direct current path between the power supply and the ground. This current has no contribution towards charging of the output capacitance of the logic gate.

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When the input rising voltage exceeds the threshold voltage of NMOS transistor, it starts conducting. Similarly until input voltage reaches Vdd-|Vt,p| PMOS transistor remains ON. Thus for some time both transistors are ON. Similar event causes short circuit current to flow when signal is falling. Short circuit current terminates when transition is completed.


Assuming symmetric inverter with Kn=Kp=K and Vt,n=|Vt,p|=Vt and very small capacitive load and both rise and fall times are same we can write,

Pavg(short circuit) = 1/12.k.τ.Fclk.(Vdd-2Vt)3 [1]


Thus short circuit power is directly proportional to rise time, fall time and k. Therefore reducing the input transition times will decrease the short circuit current component. But propagation delay requirements have to be considered while doing so.


Short circuit currents are significant when the rise/fall time at the input of a gate is much larger than the output rise/ fall time. This is because the short-circuit path will be active for a longer period of time. To minimize the total average short-circuits current, it is desirable to have equal input and output edge times [2]. In this case, the power consumed by the short-circuit currents is typically less than 10% of the total dynamic power. An important point to note is that if the supply is lowered to be below 2the sum of the thresholds of the transistors, Vdd (lesser than) VTn + |VTp|, the short-circuit currents can be eliminated because both devices will not be on at the same time for any value of input voltage.


References

[1] Sung Mo Kang and Yusuf Leblebici, “CMOS Digital Integrated Circuits-Analysis and Design”, Tata McGraw Hill, Third Edition, New Delhi, 2003

[2] Anantha P. Chandrakasan, Samuel Sheng and Robert W.Broadersen, “Low Power CMOS Digital Design”, IEEE Journal of Solid State Circuits, vol. 27, no. 4, pp. 472-484, April 1992

[3] Michael Keating, David Flynn, Robert Aitken, Alan Gibsons and Kaijian Shi, “Low Power Methodology Manual for System on Chip Design”, Springer Publications, NewYork, 2007, www.lpmm-book.org, 4/9/2007

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