Electronic digital logic circuitry – Interface – Current driving
Reexamination Certificate
1999-02-25
2001-05-22
Tokar, Michael (Department: 2819)
Electronic digital logic circuitry
Interface
Current driving
C326S083000, C326S017000, C326S087000, C326S020000
Reexamination Certificate
active
06236237
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to semiconductor devices, and more particularly to output buffers for semiconductor devices.
An important aspect of semiconductor devices is the switching speed of the output drivers. The switching speed of the output drivers are a major factor in the rate at which data can be transferred out of the device, and thus its operating frequency. Programmable logic devices are one type of semiconductor device that use output buffers, although most—if not all—semiconductor devices use output buffers, also.
An output predriver may be designed to adjust the pull-up and pull-down speed of the output buffer thereby altering the switching speed of the output driver.
FIG. 1
illustrates a typical output buffer
100
with an output predriver such as may be included in currently available semiconductor devices. Output buffer
100
has a pull-up predriver
110
, a pull-down predriver
120
and an output driver
130
. Output driver
130
is further comprised of a pull-up transistor
134
and a pull-down transistor
138
.
Both output predrivers
110
and
120
comprise a series of inverting buffers
142
-
146
. The inverting buffers of pull-up predriver
110
may be appropriately sized to favorably adjust the switching speed for the low-to-high transition of pull-up driver
134
. Similarly, the inverting buffers of pull-down predriver
120
may be sized to favorably adjust the switching speed for the high-to-low transition of pull-down transistor
138
.
A disadvantage associated with output buffer
100
is that when the inverting buffers are sized to favor both switching from high-to-low and from low-to high in terms of speed, the design may cause a “crowbar current.” A crowbar current is a short circuit current caused by both the pull-up driver and the pull-down drivers being on concurrently. This occurs because when the predrivers are advantageously sized to turn on the pull-up or pull-down drivers as fast as possible, the drivers are more difficult to turn off quickly. Thus, when either driver is turning on, there is a period of time when the opposing driver is still on due to predriver's inability to switch the opposing driver into the off state. This causes a crowbar current. The crowbar current is effectively analogous to a short circuit from power to ground through the concurrently “on” output transistors. Some detrimental effects of such a crowbar current include increased power bus noise, increased power consumption for active devices and, in some cases, non-optimal output switching speed.
SUMMARY OF THE INVENTION
The present invention greatly reduces the crowbar current of output buffers. It provides an output buffer with delayed feedback to a predriver circuit such that the effective size of the predriver buffers are momentarily adjusted to favor a particular transition (i.e., low-to-high or high-to-low). The delayed output is used to selectively alter the input threshold characteristic of the predriver circuit to favor the appropriate transition. Thus, the time during which the output drivers are subject to a crowbar current is greatly reduced over previously available devices.
According to one embodiment of the present invention, the output buffer comprises a pull-up and pull-down driver, the output node of the output buffer being the common node between the two drivers. A first predriver circuit is coupled to the pull-up driver and a second predriver circuit is coupled to the pull-down driver. Each predriver circuit includes an inverting buffer with a pull-up and pull-down transistor coupled to its output. A delay circuit provides a delayed output signal from the output node of the output buffer. The delayed output signal controls a pass transistor or other switching device to selectively couple a signal from the input of the output buffer to the pull-up or pull-down transistors. Effectively, this causes the input threshold characteristics of the predriver circuit to vary during the delay period, causing the output drivers to switch more quickly. In another embodiment of the present invention, the predriver circuit is replicated for multiple stages of buffers to increase the switching speed even more.
A further understanding of the nature and advantages of the inventions presented herein may be realized by reference to the remaining portions of the specification and the attached claims.
REFERENCES:
patent: 5239211 (1993-08-01), Jinbo
patent: 5414375 (1995-05-01), Tsuboi
patent: 5583460 (1996-12-01), Dohi et al.
patent: 5633600 (1997-05-01), Ohnishi
patent: 5654652 (1997-08-01), Raza et al.
patent: 5717342 (1998-02-01), Lotfi et al.
patent: 5831447 (1998-11-01), Chaw
Chan Mark
Wong Myron W.
Altera Corporation
Tam Vibol
Tokar Michael
Townsend and Townsend / and Crew LLP
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