Power gating is one of the most effective ways of reducing leakage current in the circuit and sleep transistors. Nowadays sleep transistors are widely used as components in this power-gating scheme. Traditionally, sleep transistors are sized as a trade-off between reduction of leakage power and performance degradations.
Beside those factors, integrity of power supply should be considered critical factor when determining the size of sleep transistors.Due to current increased density, power supply noise has become a major problem in integrated circuits with high performance capacity. Power supply noise may lead to a series of logic errors and thus, may negatively affect circuit performance. Actually, power supply noises are of two types: IR-drop noise and inductive noise.In this paper, we study the damping effect caused by the current flowing through the sleep transistors and suggest proper circuit techniques of utilizing this effect by controlling dynamically the Sizing of Sleep Transistor When Considering Damping EffectIn this section we discuss the damping effect of sleep transistors. In active mode the sleep transistor can be modeled as an equivalent channel resistance Ractive.
Ractive is claimed to be inversely proportional to the size of sleep transistors. The increase of sleep transistor size results in increase of the IR-drop. IR drop is also proportional to the amount of current flowing through sleep transistor. Thus, the maximum instantaneous current (MIC) is investigated in accordance with the size of sleep transistors in terms of IR drop.Besides IR drops on sleep transistors, inductive noise should also be taken into account in sleep transistor sizing. One of the possible ways to reduce inductive noise is to add decoupling capacitance, which maintains low impedance in power supply. However, the power mesh has certain high impedance peaks and the current variation at this resonant frequency can cause large noise on power supply.
Fig.2 shows the simplified model of a power supply mesh. Rcircuit results from the continuous switching of current and leakage current of the circuit. At the resonance frequency, the impedance becomes Rwire//Rcircuit.
However, sometimes additional resistance has to be added on the power supply network, since damping effect from the Rcircuit is not large enough to be able to suppress the peak at the resonance frequency. Sleep transistors could be used to provide more damping as shown in Fig.3.The magnitude of voltage drop is illustrated in Fig.4. It is show in dependence from the sleep transistor size for different parallel resistor values.
If the size of sleep transistor reduces, the supply voltage drop reduces as well due to increasing damping being caused by sleep transistors. However, the voltage drop of d) increased, simply because the damping effect from the Rcircuit is large enough.Adaptive Controlling of Sleep TransistorsIn this section, we propose new circuit schemes designed to control the damping ratio. Two efficient methods of controlling are available. The first one is to turn on and off the sleep transistors dynamically as shown in the Fig.
5. Initially all the transistors are turned on when the circuit is in its active mode. Then, if resonant noise is detected on the virtual Vdd, some of the sleep transistors are turned off depending on the level of noise.The second one is to control the Vgs of sleep transistors as shown in the Fig.6. During active mode, sleep transistors are fully turned on with Vgs=Vdd.
Only when resonant noise is detected, the control circuit of sleep transistor increases the Vgs until the damping is enough to be able reduce the resonant noise.