Non-volatile memory (NVM) with dynamically adjusted reference current
View Patent ↗A sense amplifier is configured to sense a current from a selected bit cell of a non-volatile memory array and compare the sensed current to a reference current to determine a logic state stored in the bit cell. A controller is configured to perform a program/erase operation on at least a portion of the memory array to change a logic state of at least one bit cell of the portion of the memory array; determine a number of program/erase pulses applied to the at least one bit cell during the program/erase operation to achieve the change in logic state; and when the number of program/erase pulses exceeds a pulse count threshold, adjust the reference current of the sense amplifier for a subsequent program/erase operation.
1. A memory system, comprising:
a non-volatile memory array comprising a plurality of bit cells;
a sense amplifier coupled to the non-volatile memory array and configured to sense a current from a selected bit cell of the non-volatile memory array and compare the sensed current to a reference current to determine a logic state stored in the selected bit cell; and
a controller coupled to the non-volatile memory array and sense amplifier and configured to:
perform a program or erase operation on at least a portion of the memory array to change a logic state of at least one bit cell of the portion of the non-volatile memory array;
determine a number of program or erase pulses applied to the at least one bit cell during the program or erase operation to achieve the change in logic state; and
when the number of program or erase pulses exceeds a pulse count threshold, adjust the reference current of the sense amplifier for a subsequent program or erase operation,
wherein the pulse count threshold corresponds to a predetermined temperature range.
2. The memory system of claim 1 , further comprising storage circuitry configured to store a plurality pulse count thresholds corresponding to the at least a portion of the non-volatile memory array, wherein each of the plurality of pulse count threshold corresponds to a unique temperature range.
3. The memory system of claim 1 , wherein the at least a portion of the non-volatile memory array corresponds to a memory block of the memory array, wherein all memory cells of the memory block are erased during an erase operation.
4. The memory system of claim 1 , wherein the sense amplifier is configured to use a previously stored reference current as the reference current during a program or erase verify performed during the program or erase operation and the adjusted reference current as the reference current during a program or erase verify performed during the subsequent program or erase operation.
5. The memory system of claim 1 , wherein the sense amplifier is further configured to perform a program or erase verify operation after each program or erase pulse during the program or erase operation.
6. The memory system of claim 1 , wherein the program or erase operation is further characterized as an erase operation, the number of program or erase pulses is further characterized as a number of erase pulses, and wherein adjusting the reference current of the sense amplifier for the subsequent program or erase operation is further characterized as decreasing the reference current of the sense amplifier.
7. The memory system of claim 1 , wherein the program or erase operation is further characterized as a program operation, the number of program or erase pulses is further characterized as a number of program pulses, and wherein adjusting the reference current of the sense amplifier for the subsequent program or erase operation is further characterized as increasing the reference current.
8. The memory system of claim 1 , wherein the controller is further configured to adjust the pulse count threshold when the reference current of the sense amplifier is adjusted for the subsequent program or erase operation.
9. In a non-volatile memory system, a method comprising:
performing a program or erase operation by applying program or erase pulses to a memory cell until the memory cell achieves a target logic state, wherein after each program or erase pulse, a verify is performed to determine whether the target logic state has been achieved, wherein performing the verify includes providing a verify reference current to a sense amplifier for comparison with a sensed current from the memory cell at a verify gate voltage to determine whether the target logic state has been achieved;
comparing a number of program or erase pulses applied to the memory cell to achieve the target logic state to a threshold value;
when the number of program or erase pulses exceeds the threshold value, adjusting the verify reference current;
performing a subsequent program or erase operation to achieve the target logic state, wherein verification during the subsequent program or erase operation includes providing the adjusted verify reference current to the sense amplifier for comparison with the sensed current from the memory cell at the verify gate voltage;
sensing a temperature of the non-volatile memory system; and
obtaining the threshold value from storage circuitry based on the sensed temperature, wherein the threshold value corresponds to a predetermined temperature range which includes the sensed temperature.
10. The method of claim 9 , wherein adjusting the verify reference current comprises decreasing the verify reference current when the program or erase operation is an erase operation.
11. The method of claim 9 , wherein adjusting the verify reference current comprises increasing the verify reference current when the program or erase operation is a program operation.
12. The method of claim 9 , wherein the threshold value is one of a plurality of threshold values stored in the storage circuitry, in which each threshold value corresponds to a unique temperature range.
13. The method of claim 9 , wherein obtaining the threshold value from the storage circuitry is further based on an address range which includes the memory cell.
14. The method of claim 9 , further comprising:
when the verify reference current is adjusted, adjusting the threshold value.
15. A memory system, comprising:
a non-volatile memory array comprising a plurality of bit cells; and
a controller coupled to the non-volatile memory array and configured to:
determine whether at least one of programming performance or erasing performance of the non-volatile memory array has degraded beyond a corresponding degradation threshold; and
when the at least one of the programming performance or erasing performance of the non-volatile memory array has degraded beyond the corresponding degradation threshold, adjusting a verify reference current that is provided to a sense amplifier during a program verification or an erase verification, wherein the controller is configured to determine a number of erase pulses required to erase a first bit cell of the plurality of bit cells and determine a number of program pulses required to program a second bit cell of the plurality of bit cells, wherein the controller is configured to determine whether at least one of the programming or erasing performance has degraded beyond the corresponding degradation threshold based on at least one of the number of erase pulses or the number of program pulses.
16. The memory system of claim 15 , wherein the controller is configured to increase a verify reference current that is provided to a sense amplifier coupled to the second bit cell for program verification during a subsequent programming of the second bit cell when the programming performance has degraded beyond the corresponding degradation threshold.
17. The memory system of claim 15 , wherein the controller is configured to decrease a verify reference current that is provided to a sense amplifier coupled to the first bit cell for erase verification during a subsequent erasing of the first bit cell when the erasing performance has degraded beyond the corresponding degradation threshold.