The present invention relates generally to flash memory, and more particularly to limiting bit line current of flash memory during programming.
Flash memory differs from other types of electrically erasable memories in that one cannot arbitrarily erase individual bits or words in flash memory in an efficient manner. Instead, memory cells in flash memories are organized into sectors such that all data within a sector must be erased (or “flashed”)—one cannot select individual bits or bytes within a flash sector for erasure. Although flash memory has a number of variations such as whether it is NAND-based or NOR-based, a number of features are common to the erasure process for flash memories.
For example, because flash memories erase all the memory cells in a sector during each erase cycle, there is a possibility that some of the memory cells may have been “over-erased” such that an over-erased cell has an undesirable threshold voltage (Vt). Thus, as part of each erasure cycle, a programming step (which may be denoted as a self-convergent programming step) occurs that corrects for any over-erased cells. For example,
There is a limit to the amount of current that can be drawn through node D from charge pump 120 before the voltage at node D drops to undesirable levels during programming events. However, during programming steps such as self-convergent programming, a transistor 130 coupled to source node S is turned on such that current may flow through memory cells 105 that have been over-erased. Transistor 130 thus acts as a switch. In turn, the current flow from activation of transistor 130 may pull the drain voltage at node D too low because charge pump 120 cannot meet the current demand while still maintaining the desired voltage at node D. Thus, it is conventional to couple node S to transistor 130 through a resistor R to limit the current flow from charge pump 120 in cyclical events such as the self-convergent programming (SCP) step.
The inclusion of resistor R, however, creates a number of problems. For example, because the resistance of resistor R is fixed, this resistance must be properly chosen. If the resistance is too low, the current is not properly limited such that the voltage provided by charge pump 120 may collapse. If the resistance is too high, however, the current is limited too much such that the effectiveness of the SCP step is reduced, thereby leaving over-erased memory cells uncorrected. In addition, an overly-high resistance increases the required time for SCP processing to undesirable levels. But selection of the proper resistance to avoid these issues is problematic because of inevitable semiconductor process variations, temperate effects, and other variables.
Accordingly, there is a need in the art for improved memory flash architectures that appropriately limit current during programming of the memory cells.
In accordance with an embodiment of the invention, a flash memory is provided that includes: a plurality of flash memory cells sharing a common drain node and a common source node; and a current source that controls the current into the common source node.
In accordance with another embodiment of the invention, a method of controlling current during flash memory programming events is provided in a flash memory having a plurality of flash memory cells sharing a common drain node and a common source node, the common source node being coupled to a transistor adapted to control a current through the common source node, the method comprising: generating a reference voltage using a reference circuit; and biasing the transistor with the reference voltage to control the current through the common source node.
In accordance with another embodiment of the invention, a flash memory within an integrated circuit is provided that includes: a plurality of flash memory cells sharing a common drain node and a common source node; a charge pump coupled to the common drain node and adapted to charge the common drain node to a desired voltage; and a current source adapted to provide a current into the common source node and through the common drain node to the charge pump, the current source being adapted to be substantially insensitive to semiconductor process variations and temperature variations.
Use of the same reference symbols in different figures indicates similar or identical items.
Reference will now be made in detail to one or more embodiments of the invention. While the invention will be described with respect to these embodiments, it should be understood that the invention is not limited to any particular embodiment. On the contrary, the invention includes alternatives, modifications, and equivalents as may come within the spirit and scope of the appended claims. Furthermore, in the following description, numerous specific details are set forth to provide a thorough understanding of the invention. The invention may be practiced without some or all of these specific details. In other instances, well-known structures and principles of operation have not been described in detail to avoid obscuring the invention.
An improved flash memory architecture is disclosed that avoids the problems of limiting current draw from a flash memory charge pump using a fixed resistance. An exemplary embodiment for such a flash memory 200 is illustrated in
The advantages of a current-source-based approach for limiting current during SCP and other programming events vs. a fixed-resistance-based approach of the prior art may be better understood with reference to
Further details for an exemplary implementation of current source 210 are illustrated in
The above-described embodiments of the present invention are merely meant to be illustrative and not limiting. For example, embodiments of the disclosed flash memory may be integrated with programmable logic devices to, for example, store corresponding configuration signals. It will thus be obvious to those skilled in the art that various changes and modifications may be made without departing from this invention in its broader aspects. Accordingly, the appended claims encompass all such changes and modifications as fall within the true spirit and scope of this invention.