Read circuit which uses a coarse-to-fine search when reading the threshold voltage of a memory cell
First Claim
1. A read circuit comprising:
- a driver which is operable in a first mode to ramp up an output voltage which is applied to a gate in a selected memory cell during a read process and operable in a second mode to ramp down the output voltage which is applied to the gate in the selected memory cell during the read process;
a sense circuit which generates a first signal indicating whether the selected memory cell conducts a first current; and
a control circuit coupled to the sense circuit and the driver, wherein during the read process, the control circuit switches the driver between the first mode and second mode in response to changes in the first signal from the sense circuit.
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Accused Products
Abstract
A read circuit includes a driver which changes a gate voltage of a memory cell and a sense circuit which identifies when the memory cell trips. The driver searches for the threshold voltage of the memory cell using stages which ramp up gate voltage and stages which ramp down the gate voltage. Each stage ends when the sense circuit senses that the memory cell trips, i.e. begins or stops conducting. Initial stages of the search have high ramp rates so that the gate voltage reaches the threshold voltage. These initial stages can give inaccurate threshold voltage readings because high ramp rates change the gate voltage during the period between the transistor tripping and sensing the trip. Later stages ramp the gate voltage slowly to provide an accurate threshold voltage reading. The low ramp rate of the last stage provides accuracy, and the high ramp rate of the initial stages reduces read time. To further reduce read time, the search process can begin at a median voltage for possible threshold voltages.
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Citations
25 Claims
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1. A read circuit comprising:
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a driver which is operable in a first mode to ramp up an output voltage which is applied to a gate in a selected memory cell during a read process and operable in a second mode to ramp down the output voltage which is applied to the gate in the selected memory cell during the read process; a sense circuit which generates a first signal indicating whether the selected memory cell conducts a first current; and a control circuit coupled to the sense circuit and the driver, wherein during the read process, the control circuit switches the driver between the first mode and second mode in response to changes in the first signal from the sense circuit. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. A memory comprising:
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an array of memory cells which includes word lines and bit lines, each word line being coupled to control gates of a row of memory cells in the array, each bit line being coupled to a column of memory cells in the array; a driver which is operable in a first mode to ramp up a voltage on a selected word line during a read process and operable in a second mode to ramp down the voltage on the selected word line during the read process; a sense circuit which generates a sense signal indicating whether a selected bit line conducts a first current; and a control circuit coupled to the sense circuit and the driver, wherein during the read process, the control circuit switches the driver between the first mode and second mode in response to changes in the sense signal. - View Dependent Claims (12, 13, 14, 15, 16, 17, 18, 19)
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20. A method for reading a memory cell, the method comprising:
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executing a series of stages including a first stage and a last stage, wherein each stage comprises; changing a gate voltage applied to the memory cell; and ending the stage after changing of the gate voltage causes the memory cell to trip, wherein for each stage except the first stage, a direction selected for changing the gate voltage during the stage is opposite to a direction selected for changing the gate voltage during a preceding stage; sampling the gate voltage after the last stage; and determining from a sampled gate voltage a value read from the memory cell. - View Dependent Claims (21, 22, 23, 24, 25)
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Specification