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Active Noise Regulation
 

Introduction

The increasing importance of effective power and supply integrity management for Nanoprocessors and other ULSI designs has been well documented recently. Yield loss and timing problems undetected by traditional verification and validation methods can be traced to a significant decrease in supply noise margin in components using advanced fabrication processes at and below the 90nm node. A combination of increased current density at lower supply voltages and supply pathway impedance results in large, on and off chip, relative supply variations called voltage droops in the literature [1]. These fluctuations make it more difficult to reduce static and dynamic power and energy consumption by further reductions in supply voltage, despite the scaling direction for semiconductors. Simultaneously, finer dimension nanometer processes (90nm and below) exhibit very substantial device property variances in manufacturing, necessitating a greater allocation of design margin to such variations. Therefore, the traditional process-voltage-temperature (PVT) validation methodology that accorded as much as +/- 10% variance to supply voltage in the past is now moving rapidly toward more stringent supply voltage control and lesser voltage variance tolerance. This trend requires that the combination of DC (static) and AC (dynamic) noise on-chip be contained within a narrower supply variance band of +/- 5% or lower in chips in the Nanotechnology era (100nm to 1nm).

Traditional techniques to minimize supply noise such as voltage positioning and on-chip decoupling capacitor integration are seen to become increasingly inapplicable in addressing power integrity needs. A voltage regulator module (VRM) is far too distant, both physically and electrically, to match the power supply bandwidth requirement of gigahertz processors for which the voltage positioning technique is usually employed. Due to the exponential rise of gate leakage in sub 100nm processes, on-chip decoupling is seen to be an unacceptable choice for dynamic noise mitigation. And, in any case, energy stored in these integrated capacitors diminishes quadratically with supply voltage. It has also been shown in the art that the scaling of package filter component characteristics, such as the loop inductance of on-package capacitors, and the series resistance of the power path, will be impractically exponential [Ref. 1, Power Delivery section]. In this paper, we introduce techniques of active noise regulation (ANR) and active VLSI packaging (AVP) developed at ComLSI. These methods take advantage of proximity to the load component and quadratic increase in stored capacitive energy with voltage to place stable charge reservoirs where they are most needed, Very close to the high current density and high-speed, transient loads.

A key requirement in ensuring effectiveness of this technology is a rigorous understanding of dynamic noise behavior in a high-performance ULSI component power grid. Tools analyzing the full stack of multiple on-chip power grids, distributed loads, leakage, and decoupling capacitance, that also include the package grid, external connectivity and on-package components are critical for this understanding. Such tools give the designer the ability to visualize the spatial and temporal variation of noise throughout the system, providing a detailed view of the interaction of on-chip dynamic noise with critical path activity. In addition, these tools provide a dynamic view of the noise minimization impact of ANR and other on-package active/passive components. They provide the means to carefully design the placement and temporal activation of ANR's, passive decoupling caps and other components in single or multi-chip systems. True dynamic noise analysis requires the ability to model all key elements of a power grid including power loop inductance for all segments of the power grid, on and off chip standing wave resonances and resistive energy loss. The authors have made extensive use of such a tool, PI-FP, developed for this rigorous analysis of dynamic noise in high-performance systems.

ANR

Active Noise Regulation is a non-disruptive Sysytem-in-Package technology that addresses power integrity for high-performance ULSI systems and components such as microprocessors, SoC's, SiP's and multi-chip assemblies. High-performance, high-power components undergo very frequent transitions in operational state through a number of different 'power states' so as to minimize power consumption while maintaining required performance. These state transitions occur when applications are running on a processor, and may induce very sudden, and large swings in power supply current requirement, exciting the power delivery network into resonance while simultaneously depleting the local charge stored in the vicinity of the high-bandwidth load. ANR's address this problem through rapid, controlled and local supply of charge into the load component power grid. Figure 1 illustrates the implementation of an ANR component shown simply as a FET switch device. The ANR is associated with a dedicated 'Reservoir' capacitor that is either fed by a supply line connecting to an external, high-voltage power supply or is charge-pumped as determined by the system design. This provices the ANR with a resevoir of charge that is many times greater than charge stored at the load component's operating voltage.

Figure 1: Illustrative Schematic of an ANR Component Implementation (patent pending)

In this paper, we discuss and illustrate the impact an ANR has on a high-performance chip power grid. The ANR (or an array of ANR's) connects to the load component by very short lengths of interconnect as in figure 6. The ANR is therefore fully cognizant of the spatial and temporal variation of the load component power supply voltages. When the ANR detects (or is intimated of) a substantial change (say a voltage droop event) in the conditions at the load component power grid point or region to which it is connected, it initiates compensating flow of charge from the Reservoir Cap into the load power grid. After a brief duration of high current and charge flow, the ANR shuts the current flow in a controlled manner, allowing the Reservoir Cap to be replenished and prepared for another such transient event.

Figure 2: Simulation results using a distributed model on the use of an ANR in a high speed system. The plot shows the supply voltage change delta(Vdd-Vss) at all points across the chip surface.

Figure 2 displays simulation results on the use of ANR in a high speed system with load1 and load2 currents as shown in figure 1. The animation shows the response of the system's power delivery stack to a power state transition. This simulation is carried out in a distributed model that emulates the on-chip grid and all components of the power delivery system. Various downward excursions of the grid differential supply voltage are seen, and are referred to as 'droops'. These droops reduce the voltage available to the circuits within the chip regions they occur in and impede their ability to perform required functions at the chip's operational frequency.

The load on the right hand side of the plot operates without the benefit of an ANR device. The left hand side of the plot shows the grid response to an identical load current with ANR functionality included. For the duration that the ANR component is active, it may be seen that the voltage droop, or performance-degrading noise, is reduced substantially. Figure 3 shows the change in supply voltage at two locations on the on-chip grid close to the center of each load.

Figure 3: Load current induced power noise with and without ANR at single points on the chip surface. This ANR has a filter that tunes the device to the lower frequency system level transients that occur when blocks are switched on and off during power saving mode changes.

It is evident from these results that this ANR device is particularly effective at controlling low frequency system level transients. While droops have been specifically inspected, ANR's can just as effectively address overshoots. Low frequency droops and overshoots are related to supply path inductances and capacitors on-package and on system boards and is often the most important power noise component impacting performance in high speed systems. ANR devices can be employed to reduce noise amplitudes throughout the frequency range. In figre 4, trace (a) is connected to the package grid close to the ANR circuit and trace (b) is located near the load without active noise regulation.

 

(a)

 

(b)

Figure 4: Distributed Vdd and Vss supply variations along two transmission lines.

Leakage and voltage dependence of dynamic noise

Consideration of the spectral content of power noise is important in the trade-off between speed and power in the system. Figure 5 shows a simplified model of the system power grid.

Figure 5: Simple power grid model.

 

In this model the voltage drop at the load is given by:

where:

Substituting (2) into (1) gives:

In general, the load current I is a non-linear function of the power noise v and equation (3) has to be solved numerically. However, we can gain some insight into the performance of a typical power grid using quasi-static approximations for the load current. For example, today's advanced processes have considerably higher static leakage than previous process generations. This leakage falls off rapidly during dynamic voltage drops providing negative feedback that acts to reduce the overall noise level. Substituting the first order approximation to the system load current (linear increase in transistor leakage with supply voltage):

into equation (3), where Io and v are slowly varying functions of time, gives:

or:

Thus a large static leakage component can provide an overall reduction in system noise. However, relying on the damping effect of static leakage also means an acceptance that the system power consumption will be much higher than it needs to be. And, in any case, even with a large static leakage contribution to the overall power consumption, the instantaneous peak dynamic current density within any particular region of the IC is likely to be much larger than the static current per unit area at this location.

Note that this feedback will be negative whenever the leakage current increases monotonically with voltage (e.g. mosfets). This ensures that the last two terms in equation (3) remain greater than or equal to zero at all times.

At higher frequencies the remaining terms in equation (3) can no longer be ignored. Suppose we now switch off the load current. The power noise can then be written as:

Where vo is the noise level at switch off (t=0) and

In power networks with Q>0.5 the grid continues to oscillate for approximately Q cycles after the noise source has been switched off. The system Q depends on the ratio of the energy stored in L and C to the energy dissipated in R. In systems designed for efficient power delivery (high Q), a larger proportion of the noise energy generated in one cycle remains in the grid during subsequent cycles. This energy is available to power active loads in the IC. However, low dissipation systems also have much greater dynamic voltage drops, particularly at frequencies around the resonant frequency wo. Any efforts made by designers to reduce power loss such as the use of low leakage processes and circuit design techniques will inevitably lead to an increase in dynamic noise. ANR provides designers with the means to reduce this noise at higher system speeds without excessive heat generation. Using ANR the system Q is increased, not by reducing resistance, but by taking advantage of the quadratic increase in capacitive energy with voltage to place large amounts of energy at the load. The advantages of low loss power transport at high voltage, followed by voltage conversion at the load, have been well known since the early days of power distribution. This advantage can now be realised for high-speed systems using ANR.

AVP

A severe limitation in the capability of on-chip capacitors in storing charge follows from the relationship:

where charge and capacitance are per unit area and E is the electric field in the charge storage device. The maximum capacitance per unit area integrated within chips is typically in a MOS-capacitor. Most fabrication processes push the MOS capacitor dimension (gate oxide thickness) to being fine enough that the reliability limit for the gate oxide is approached. Therefore it is not feasible to store greater charge per unit-area (and energy) within an integrated MOS capacitor using higher voltage, since high-voltage tolerant devices within the fabrication process must necessarily have a thicker gate oxide, thereby reducing their capacitance per unit area approximately in proportion to the higher voltage desired.

The importance of package capacitors has been amply demonstrated in experiments conducted at a high-volume microprocessor manufacturer that showed that in the presence of even one land-side package capacitor, the quantum of on-die capacitance integrated did not seem to matter to component performance (maximum frequency). Land-side package capacitors are mounted on the opposite side of the processor package substrate and immediately below it such that the physical thickness of the package substrate separates the capacitor from the processor circuits. This is one of the closest practical assemblies of capacitors and large values of stored charge to a processor both from a physical and an electrical standpoint. In other words, integrated on-chip capacitance may be too far laterally and therefore, electrically, to be of as much value in mitigating noise as much as properly positioned package capacitors. Package capacitors are therefore seen to be more effective in maintaining processor power integrity.

In other experiments, it has been demonstrated that removing many of the package capacitors while retaining an appropriately placed few also seemed to have little impact on the performance of the processor. This result indicates the importance of understanding the exact spatial and temporal nature of dynamic noise in a chip grid; a package capacitor at a location of low dynamic noise, or a location where dynamic noise does not coincide with a critical circuit or circuit path on the die may not be of significant benefit in optimal power integrity management.

A key limitation of passive devices such as package capacitors is that they are 'reactive' devices. In other words, they react to changing electrical conditions around them. A capacitor therefore only provides charge flow when there is a substantial rate of change of voltage across its terminals. Therefore, while a capacitor serves as a reservoir of electric charge, it cannot pro-actively provide copious amounts of charge to suppress a sudden or transient voltage variation. It only supplies charge when it encounters a substantial voltage variation or droop.

Additionally, the effective series resistance (ESR) and inductance (ESL) of these capacitors is fixed in value, and while manufacturing and device design improvements reduce the value of these parasitic elements, that is not necessarily advantageous. While lower ESR values assist in minimizing the voltage and power dropped within the capacitor as it supplies charge, low ESR does not effectively damp supply grid oscillations excited by the switched nature of the load. Passive devices therefore do not provide mechanisms to assist in the suppression of resonant power supply voltage variations.

In Active VLSI Packaging, package capacitors are combined with land-side mounted ANR devices (figure 6). These structures locate the high voltage reservoir capacitor and control circuitry within a distance of a package substrate thickness of the processor or SoC die. The ANR device makes use of the large energy capacity of these reservoirs to pro-actively restore charge to the on-die power grid. This technique thus allows active control of dynamic power noise with minimal power consumption. Additionally, active noise regulators provide a means for introducing dynamic damping impedances into a chip power delivery system, pro-actively controlling supply resonances.

Figure 6: ANR and LVR devices can be package or PCB mounted. This ensures a low impedance, minimal delay path to the high performance IC (patent pending).

The natural evolution of ANR's is into local voltage regulators (LVR's) that provide extremely high-bandwidth, on-package, high-efficiency power conversion. LVR's use the parasitic elements associated with package capacitors and the supply pathways into the chip power grid to provide extremely high frequency switched power conversion. An array of LVR's augment the externally supplied low voltage power supply and dramatically increase the bandwidth of the full power delivery system. This allows a high-power SoC component to rapidly modulate the supply voltage provided to it's circuits in order to minimize average power supply consumption. The proximity of LVR's tothe load component enables fast communication between the SoC and the LVR array, thereby enabling the rapid transitions in supply voltage and the significant power consumption reduction that results from dynamic energy management.

Conclusion

System level simulation methods have shown that active noise regulation can be used to control noise in low loss power grids. These tools and design methodologies allow system designers increased flexibility when designing low noise, high speed systems with minimal power consumption. In addition, the authors believe a package for a chip can do a lot more than provide interconnect pathways for power and signal connections. It is increasingly clear in RF and high-speed design that package components provide high-performance passives that enhance the performance of integrated circuits. Packaging for RFID components captures energy to power the circuits. Given the proximity to the integrated circuits, package components will soon function actively and symbiotically with SoC chips, providing efficient, cost-effective systems solutions to power and signal integrity management. ANR and LVR devices and arrays are capable of non-disruptive change in the established integrated circuit and system packaging architectures addressing power integrity and management. Electronic circuit and systems packaging will play an 'active' role in system function and performance to push integration further in the Nanotechnology era.

 

[1] R. Mahajan, Raj Nair et al., "Emerging Directions for Packaging Technologies", ITJ '02.



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