Low noise sense amplifier array and method for nonvolatile memory
View Patent ↗In sensing a page of nonvolatile memory cells with a corresponding group of sense modules in parallel, as each high current cell is identified, it is locked out from further sensing while others in the page continued to be sensed. The sense module involved in the locked out is then in a lockout mode and becomes inactive. A noise source from the sense module becomes significant when in the lockout mode. The noise is liable to interfere with the sensing of neighboring cells by coupling through its bit line to neighboring ones. The noise can also couple through the common source line of the page to affect the accuracy of ongoing sensing of the cells in the page. Improved sense modules and method isolate the noise from the lockout sense module from affecting the other sense modules still active in sensing memory cell in the page.
1. A method of sensing a group of non-volatile memory cells, comprising:
providing a corresponding group of sensing circuits for sensing the group of non-volatile memory cells, with each sensing circuit having a node and an intermediate circuit through which the node is coupled to an associated memory cell;
coupling a voltage boosting circuit to the node of each sensing circuit to boost the node from an initial voltage by a predetermined amount;
sensing a conduction current by a rate of voltage discharge at the node; and
whenever the conduction current of a memory cell of the group is sensed to be higher than a predetermined value, isolating the voltage boosting circuit from the intermediate circuit of the memory cell until at least sensing for the group of memory cells is completed.
2. A method as in claim 1 , wherein:
each memory cell of the group is accessible by an associated bit line; and
the intermediate circuit is coupled to the associated bit line.
3. A method as in claim 1 , wherein:
each memory cell of the group includes a source coupled to a common source line for the group; and
the intermediated circuit is coupled to the common source line.
4. A method as in claim 1 , wherein:
said isolating the voltage boosting circuit includes disabling a transfer gate circuit disposed between the node and the intermediate circuit.
5. A method as in claim 1 , wherein:
said isolating the voltage boosting circuit includes disabling a transfer gate circuit disposed between the voltage boosting circuit and the node.
6. The method as in claim 1 , wherein said sensing is a part of a program operation to verify if any of the memory cells of the group has been programmed to a target state.
7. The method as in claim 1 , wherein said sensing is a part of a read operation on the group of non-volatile memory cells.
8. A sensing circuit for sensing a conduction current of a memory cell among a group of nonvolatile memory cells being sensed in parallel, comprising:
a node;
an intermediate circuit coupled between the memory cell and said node;
a voltage boosting circuit coupled to the node to boost an initial voltage on the node by a predetermined amount;
a transfer gate disposed between the voltage boosting circuit and said intermediate circuit;
a comparator coupled to said node to determine the conduction current by a rate of voltage discharge at the node; and
said transfer gate turning off in response to the conduction current being determined to be higher than a predetermined value, thereby isolating the voltage boosting circuit from the intermediate circuit of the memory cell and wherein said transfer gate remains off until at least sensing for the group of memory cells is completed.
9. A sensing circuit as in claim 8 , wherein:
each memory cell of the group is accessible by an associated bit line; and
said intermediate circuit is coupled to the associated bit line.
10. A sensing circuit as in claim 8 , wherein:
each memory cell of the group includes a source coupled to a common source line for the group; and
said intermediated circuit is coupled to the common source line.
11. A sensing circuit as in claim 8 , wherein:
said transfer gate is disposed between the voltage boosting circuit and said node.
12. A sensing circuit as in claim 8 , wherein said transfer gate further comprises:
a pair of p-transistor and n-transistor each having a source, a drain and a gate;
a first node formed by coupling the source of the p-transistor to the drain of the n-transistor;
a second node formed by coupling the drain of the p-transistor to the source of the n-transistor;
whereby a pair of complementary signals on the gates of the pair of p-transistor and n-transistor controls passage between the first and second nodes.
13. A sensing circuit as in claim 12 , further comprising:
a supply voltage source for precharge operation;
wherein the transfer gate is also coupled to said supply voltage source, said p-transistor when enabled by the pair of complementary signals serves to pull up the first and second nodes to the supply voltage source during the precharge operation.
14. A sensing circuit as in claim 8 , wherein:
said transfer gate is disposed between the node and the intermediate circuit.
15. A sensing circuit as in claim 8 , wherein the group of nonvolatile memory cells are a portion of a flash EEPROM.
16. A sensing circuit as in claim 15 , wherein the flash EEPROM is of NAND type.
17. A sensing circuit as in claim 8 , wherein the nonvolatile memory cells are embodied in a memory card.