Electronic digital logic circuitry – Interface – Current driving
Reexamination Certificate
2001-12-19
2004-08-24
Le, Don (Department: 2819)
Electronic digital logic circuitry
Interface
Current driving
C326S027000
Reexamination Certificate
active
06781416
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to drive circuits (or drivers). More particularly, the present invention defines an improved push-pull driver having edge conditioning and non-overlap control. The present invention further describes circuits and techniques for actively tuning the output of a push-pull driver.
2. Description of the Related Art
Push-pull circuits are well known and have been adapted to digital and analog applications as varied as stepping motor control, audio loudspeakers, and memory systems. In the present context, push-pull circuits have been used in bus systems including one or more devices that output data onto a common bus. As used throughout, the term “bus” refers to one or more conductive paths communicating electrical signals between two points.
Push-pull circuits have excellent drive characteristics. That is, push-pull circuits routinely provide clean rising and falling edges for high speed data signals being driven onto a bus. This capability is realized by effective control of two stages generically illustrated in FIG.
1
.
In
FIG. 1
, a push-pull drive circuit is shown as implemented in CMOS and comprises a PMOS-transistor first stage
1
and NMOS-transistor second stage. In theory, the effective switching of the first and second stages controls a current path between a voltage source (V
SS
) and ground. Ideal switching by the input signal
3
of ideal first and second stages (i.e., perfectly sized and implemented CMOS devices) produces an ideal output signal
4
, shown as curve “A” in the graph of FIG.
2
. The production of this ideal output signal requires an exact actuation timing relationship between the first and second stages of the push-pull driver. This relationship requires that the switching input signals turn OFF one stage of the push-pull driver while simultaneously turning ON the other stage.
However, as one would expect, process variations in the fabrication of the first and second stage CMOS devices, as well as variations in device performance due to operating voltage and temperature variations, (collectively and generically referred to hereafter as “PVT” for process, voltage and temperature), result in very different output curves. For example, curve “B” shown in
FIG. 2
illustrates an occurrence in which both stages of the push-pull driver are simultaneously OFF and a voltage knee momentarily forms in the output signal before one of the stages turns ON. Curve “C” in
FIG. 2
illustrates an occurrence in which both stages of the push-pull driver are simultaneously ON and current momentarily “shoots-through” the channel between V
SS
and ground.
In a digital system, this shoot-through phenomenon is well understood and results in considerable noise being transmitted onto the bus, absent some design remedy. Historically the remedy has come in the form of a large by-pass capacitor shunting the shoot-through current to a ground plane in the CMOS substrate. Unfortunately, as bus systems are required to run at ever increasing data rates this brute force method of dealing with shoot-through becomes less and less acceptable. This is particularly true where bus widths are wide and where data signals are driven onto the bus using multiple clocks and/or multiple clock edges.
Many conventional double-data-rate (“DDR”) memory systems use push-pull drivers to communicate data between bus system devices and the bus. This approach differs from other bus systems having integrated circuit using simpler, open-drain output drivers. As DDR memory systems and similar data communication systems push the envelop for high-speed data transfer, push-pull shoot-through noise and the corresponding charge dump via by-pass capacitors becomes increasing unacceptable.
It is further understood that by placing a “pre-driver circuit” in front of a push-pull driver performance of the push-pull driver may be enhanced. Looking at the simplified circuit shown in
FIG. 3
as an example, an adjustable pre-driver
20
precedes the push-pull driver
21
. This combination is shown in greater detail in
FIG. 4
, wherein the push-pull driver is formed by the combination of P
0
and N
0
connected between a voltage source and ground.
Conventionally, selected control signals sampled from the pre-driver circuit are used to monitor (or sense) the integrity of the switching signal(s) applied to the push-pull driver. For example, by comparing the timing of a voltage waveform taken at point—A—in the PMOS driver
22
of
FIG. 4
with the timing of a voltage waveform taken at point—A′—in the NMOS driver
23
of
FIG. 4
, one may roughly understand the quality of the switching signals. However, such pre-driver sensing techniques do not account for PVT affects at the PMOS and NMOS output transistors. Nor does pre-driver sensing detect or address the problem of shoot-through.
BRIEF SUMMARY OF THE INVENTION
At a minimum, performance of the conventional push-pull driver would be greatly benefitted from edge conditioning and/or improved non-overlap protection. Performance of the conventional push-pull driver would also be enhanced by providing slew rate control.
Edge conditioning prevents undershoot and overshoot at the terminal stages of the output waveform. The term “overlap” refers to the condition where both stages of the push-pull driver are ON (or conductive) and shoot-through occurs. Thus, non-overlap is a desired performance characteristic since shoot-through results in increased substrate (or backplane) noise and increased supply noise. Furthermore, shoot-through creates a requirement for larger by-pass capacitors. Increased by-pass capacitor size may result in a larger overall die size. Additionally, shoot-through results in increased power (and heat) dissipation within the semiconductor device.
The present invention provides greater non-overlap control, thus eliminating shoot-through. Power is conserved, as power previously lost to shoot-through is now applied to driving the output load. The number and/or size of by-pass capacitors may be reduced and die size saved, accordingly. Power (P=I*Vds) is further conserved because the present invention provides faster output transitions by applying a boot-strap circuit utilizing positive feedback.
In another aspect, the present invention provides an actively tuned, CMOS, push-pull driver. Conventional push-pull drivers are generally open loop systems. That is, they sense and set, or periodically adjust, rather than actively monitor and control. The conventional approaches to shoot-through control or skew rate adjustment, which tend to be complicated yet imprecise, are also not scalable with frequency.
In one aspect, the present invention uses a process detector to form a control loop by which shoot-through is prevented and skew rate is controlled. The process detector may take many forms, but as presently preferred a Delay Lock Loop (DLL) is used. Many high speed bus systems already incorporate DLLs or PLLs to adjust clock signals in relation to a fixed frequency reference. By advantageously using an existing set of DLL reference signals, a control loop may be implemented which tracks and adjusts slew rate on a clock cycle by clock cycle basis.
Thus, a closed loop, shoot-through control, feedback loop may be implemented which actively tunes the switching signals in a push-pull driver. The closed loop may be implemented with a filter or delay constant capable of being digitally adjusted. The closed loop feedback sensing points may be implemented with adjustable gain.
The approach taken by the present invention to shoot-through control and slew rate tracking is scalable with frequency. Where a DLL is used as a process detector, timing skews may be controlled by digitally adding or subtracting value(s) from a digital code derived from the DLL reference signals.
By the means set forth above, and as further explained in the brief description of the presently preferred embodiments which follows, the present invention provides slew rate control and shoot-through protection, along wit
Lau Benedict
Nguyen Huy M.
Vu Roxanne
Le Don
Morgan & Lewis & Bockius, LLP
Rambus Inc.
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