Power Gating is yet another effective implementation employed in Low Power Designs. Unlike Clock Gating, which saves the dynamic power, Power Gating saves the leakage power. As we move from micron (i.e. greater than 90nm) technology nodes to sub-micron (i.e. less than 90nm) technology nodes, leakage power dissipation dominates the dynamic power dissipation. It is therefore employed very frequently in modern SoCs. We shall talk about the structure of a power gate in this post.
Consider any CMOS digital logic circuit consisting of Pull-Up Network (made from PMOS transistors) and Pull-Down Network (made from NMOS transistors), as shown in the figure.
At any point of time, if a direct path would exist from the power supply (VDD) and the ground (GND), the circuit would continue to dissipate leakage power. What is the possible turnaround? One can gate the power and ground terminals from the circuit when it is not intended to be used. That's what is accomplished by a power gate! Let's take a look at the circuit.
- During normal operation, SLEEP = 0. Both the PMOS and NMOS Sleep Transistors (in blue and green respectively) are ON. And we have Virtual Power Rails and Virtual Ground which ensure normal circuit operation.
- However, during periods of low activity, SLEEP = 1. Hence the Sleep Transistors turn OFF. And a direct path from power rails to ground is broken and hence no leakage power is dissipated due to the Pull-up and Pull-down networks.
Note that, during normal operation, Sleep Transistors contribute to some extra leakage power because they are still ON. Though, the leakage power due to these two transistors would be extremely small compared to that of the Pull-up and Pull-down networks, nevertheless, these transistors are custom designed in a way such that they have high Vt (the threshold voltage) to reduce any excess leakage power during normal mode of operation.