Method and apparatus for sensing the state of floating gate memory cells by applying a variable gate voltage
First Claim
1. An apparatus for determining a state of a memory cell that is programmed to one of a plurality of states, comprising:
- a voltage circuit that applies a variable voltage to a gate of the memory cell during a data reading operation, wherein the memory cell generates a cell current that is proportional to the variable voltage;
a reference current circuit that provides a fixed reference current;
a current comparing circuit coupled to the memory cell and the reference current circuit, the current comparing circuit detecting when the cell current is substantially equal to the fixed reference current, wherein the cell current is substantially equal to the fixed reference current when the variable voltage assumes a first voltage value that reflects the state of the memory cell; and
a state sensing circuit coupled to the current comparing circuit, the state sensing circuit determining the state of the memory cell according to the first voltage value of the variable voltage at which the cell current is substantially equal to the fixed reference current.
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Accused Products
Abstract
A method and apparatus for sensing the state of floating gate memory cells in a memory array. Because of its stability and accuracy, the sensing apparatus may be used for sensing the state of multi-bit floating gate memory cells. The state of a memory cell is sensed by applying a variable gate voltage to the top gate of the floating gate memory cell and comparing the cell current to a fixed reference current. A circuit detects when the cell current is equal to the reference current. When the currents are equal, the value of the variable gate voltage indicates the state of the memory cell. For one embodiment, an analog-to-digital converter converts the variable gate voltage to a digital value that is latched when the currents are equal. The latched digital value indicates the state of the memory cell. For this embodiment, a ramp voltage or other suitable variable voltage may be used as the variable gate voltage. For another embodiment, a digital-to-analog converter is used to generate the variable gate voltage. A counter generates digital values to step the variable gate voltage. When the cell current equals the fixed reference current, the digital counter value is latched to indicate the state of the memory cell.
248 Citations
26 Claims
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1. An apparatus for determining a state of a memory cell that is programmed to one of a plurality of states, comprising:
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a voltage circuit that applies a variable voltage to a gate of the memory cell during a data reading operation, wherein the memory cell generates a cell current that is proportional to the variable voltage; a reference current circuit that provides a fixed reference current; a current comparing circuit coupled to the memory cell and the reference current circuit, the current comparing circuit detecting when the cell current is substantially equal to the fixed reference current, wherein the cell current is substantially equal to the fixed reference current when the variable voltage assumes a first voltage value that reflects the state of the memory cell; and a state sensing circuit coupled to the current comparing circuit, the state sensing circuit determining the state of the memory cell according to the first voltage value of the variable voltage at which the cell current is substantially equal to the fixed reference current. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13)
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14. An apparatus, comprising:
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a memory cell having a floating gate, wherein the memory cell is programmed to one of a plurality of states corresponding to an amount of charge stored on the floating gate, the memory cell having a top gate, a source, and a drain; a bias circuit coupled between the drain and source of the memory cell, for establishing a fixed voltage across the source and drain of the memory cell; a variable voltage circuit coupled to the top gate of the memory cell, the variable voltage circuit applies a variable voltage to the top gate of the memory cell during a data reading operation, wherein the memory cell generates a cell current proportional to the variable voltage; a reference current circuit that provides a substantially constant reference current; a sensing circuit coupled to the reference current circuit and the memory cell, the sensing circuit indicating when the cell current is substantially equal to the reference current, wherein the cell current is substantially equal to the reference current when the variable voltage assumes a voltage value that reflects the state of the memory cell; a first circuit coupled to the variable voltage circuit, the first circuit generating a digital value corresponding to the voltage value of the variable voltage; and a latch coupled to the first circuit and responsive to the sensing circuit, the latch latching the digital value when the cell current is substantially equal to the reference current, wherein the latched digital value represents the state of the memory cell. - View Dependent Claims (15, 16, 17, 18, 19)
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20. An apparatus for determining a state of a memory cell that is programmed to one of a plurality of states, comprising:
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means for applying a variable voltage to a gate of the memory cell during a data reading operation, wherein the memory cell generates a cell current that is proportional to the variable voltage; means for providing a fixed reference current; means for detecting when the cell current is substantially equal to the fixed reference current, wherein the cell current is substantially equal to the fixed reference current when the variable voltage assumes a first voltage value that reflects the state of the memory cell; and means for determining the state of the memory cell according to the first voltage value of the variable voltage at which the cell current is substantially equal to the fixed reference current. - View Dependent Claims (21, 22)
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23. A method for sensing a state of a memory cell that is programmed to one of a plurality of states, comprising the steps of:
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applying a variable voltage to a gate of the memory cell during a data reading operation, wherein the memory cell generates a cell current that is proportional to the variable voltage; providing a fixed reference current; detecting when the cell current is substantially equal to the fixed reference current, wherein the cell current is substantially equal to the fixed reference current when the variable voltage assumes a first voltage value that reflects the state of the memory cell; and determrining the state of the memory cell according to the first voltage value of the variable voltage at which the cell curent is substantially equal to the fixed reference current. determining the state of the memory cell according to a value of the variable gate voltage when the cell current is substantially equal to the fixed reference current. - View Dependent Claims (24, 25)
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26. A method for sensing the state of a memory cell, comprising the steps of:
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selecting a memory cell to be sensed; applying a variable gate voltage to a gate of the selected memory cell during a data reading operation, wherein the memory cell generates a cell current proportional to the variable gate voltage; providing a fixed reference current; determining whether the cell current is substantially equal to the fixed reference current, wherein the cell current is substantially equal to the fixed reference current when the variable gate voltage assumes a first voltage value that reflects the state of the memory cell; providing a digital value corresponding to the first voltage value of the variable gate voltage when the cell current is substantially equal to the fixed reference current; and latching the digital value when the cell current is substantially equal to the fixed reference current, wherein the latched digital value represents the state of the memory cell.
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Specification