POWER SUPPLY NOISE REDUCTION USING ACTIVE RESISTORS


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Introduction

Continual scaling in integrated circuit technology has made it possible to achieve higher transistor density
in a given die area. Even with the lowering of the power supply voltage for newer process nodes, the
maximum power consumption of high-end microprocessors has been projected to continue to increase for
the foreseeable


Resistive Damping

The oscillations in the power supply voltage across the chip are due to the underdamped nature of the power
grid distribution network. Underdamped systems inherently produce oscillations when excited by step
responses. Clock gating can be considered as a step disturbance on the power grid since it causes a sudden
change in the current demand of the chip. Additional damping may be provided by introducing dissipative
elements, such as resistors, into the system. As described by Kundert [5], resistors can be introduced in
various ways to the network.


Active Resistors for Noise Reduction
Concept

Havinga nalyzed the second order parallel RLCc ircuit, one can see that if minimumov ershoot is desired,
the damping ratio has to be as high as possible. This can be achieved by lowering R; however, since R is
between the Vda and Gnd rails, low R will cause excessive power dissipation. If the passive resistor is
replaced with an equivalent active resistor that will act as a dissipative device at the frequency of
oscillations, one can achieve significant damping without excessive power dissipation.