STABLE MEMORY SOURCE BIAS OVER TEMPERATURE AND METHOD

• US 20130170287A1
• Filed: 10/30/2012
• Published: 07/04/2013
• Est. Priority Date: 01/03/2012
• Status: Active Grant

• Current Assignee: Medtronic Incorporated (Medtronic Plc)
• Original Assignee: Medtronic Incorporated (Medtronic Plc)
• Inventors: Walsh, Kevin K., Tyler, Larry E., Scott, Brandon P.
• Granted Patent: US 9,378,805 B2
• Time in Patent Office: Days
• US Class Current: 365/154
• CPC Class Codes: G11C 11/417 for memory cells of the field-effect type

Abstract

Random access memory having a plurality of memory cells, each of the plurality of memory cells having a memory element and a first electrical characteristic being variable based, at least in part, on temperature and a bias circuit operatively coupled to at least one of the plurality of memory cells, the bias circuit being configured to generate a bias voltage for the at least one of the plurality of memory cells. The bias circuit has a second electrical characteristic being variable based, at least in part, on temperature. The first electrical characteristic is approximately proportional to the second electrical characteristic over a predetermined range of temperatures, the predetermined range of temperatures being greater than zero. The bias voltage on each of the plurality of memory cells is approximately proportional with variations in the first electrical characteristic over the predetermined range of temperatures.

Claims

1. A random access memory, comprising:

a plurality of memory cells, each of said plurality of memory cells having a memory element and a first electrical characteristic being variable based, at least in part, on temperature; and

a bias circuit operatively coupled to at least one of said plurality of memory cells, said bias circuit being configured to generate a bias voltage for said at least one of said plurality of memory cells;

said bias circuit having a second electrical characteristic being variable based, at least in part, on temperature;

said first electrical characteristic being approximately proportional to said second electrical characteristic over a predetermined range of temperatures, said predetermined range of temperatures being greater than zero;

whereby said bias voltage on each of said plurality of memory cells is approximately proportional with variations in said first electrical characteristic over said predetermined range of temperatures.

2. The random access memory of claim 1 wherein said bias circuit is a source bias circuit.

3. The random access memory of claim 2 further comprising a supply bias circuit operatively coupled to said at least one of said plurality of memory cells, said supply bias circuit being configured to generate a bias voltage for at least one of said plurality of memory cells, said supply bias circuit having said second electrical characteristic being variable based, at least in part, on temperature.

4. The random access memory of claim 1 wherein said bias circuit is a supply bias circuit.

5. The random access memory of claim 4 further comprising a voltage buffer operatively coupled between said supply bias circuit and said at least one of said plurality of memory cells.

6. The random access memory of claim 5:

wherein said voltage buffer comprises an input and an output configured to supply said bias voltage, said input being coupled to said supply bias circuit and said output being coupled to said at least one of said plurality of memory cells; and

wherein another one of said plurality of memory cells is coupled to said input of said voltage buffer and configured to supply, in combination with said supply bias circuit, an input voltage related to said bias voltage to said input to said voltage buffer.

7. The random access memory of claim 1, further comprising a plurality of said bias circuits.

8. The random access memory of claim 7 wherein each of said plurality of bias circuits comprise a bias element with said bias element having said second electrical characteristic being variable based, at least in part, on temperature.

9. The random access memory of claim 7 wherein each one of said plurality of bias circuits corresponds to exactly one of said plurality of memory cells.

10. The random access memory of claim 7 wherein said bias element of each of said bias circuits comprise metal oxide semiconductor transistors.

11. The random access memory of claim 10 wherein said bias element of each of said plurality of bias circuits comprise P-channel metal oxide semiconductor transistors.

12. The random access memory of claim 11 wherein said random access memory is operatively coupled to a power source, wherein each memory element of said plurality of memory cells comprises a P-channel metal oxide semiconductor transistor having a body and wherein said body said P-channel metal oxide semiconductor transistor of at least one of said plurality of memory cells is operatively coupled to said power source.

13. The random access memory of claim 12 wherein said random access memory is operatively coupled to a reference ground, wherein each memory element of said plurality of memory cells comprises an N-channel metal oxide semiconductor transistor having a body and wherein said body of said N-channel metal oxide semiconductor transistor of said at least one of said plurality of memory cells is operatively coupled to said reference ground.

14. The random access memory of claim 11 wherein each memory element of said plurality of memory cells comprises a P-channel metal oxide semiconductor transistor having a body and wherein said body of said P-channel metal oxide semiconductor transistor of at least one of said plurality of memory cells is biased by said bias voltage of said bias element.

15. The random access memory of claim 14 wherein said random access memory is operatively coupled to a reference ground, wherein each memory element of said plurality of memory cells comprises an N-channel metal oxide semiconductor transistor having a body and wherein said body of said N-channel metal oxide semiconductor transistor of said at least one of said plurality of memory cells is operatively coupled to said reference ground.

16. The random access memory of claim 10 wherein said bias element of each of said plurality of bias circuits comprise N-channel metal oxide semiconductor transistors.

17. The random access memory of claim 16 wherein said random access memory is operatively coupled to a reference ground, wherein each memory element of said plurality of memory cells comprises an N-channel metal oxide semiconductor transistor having a body and wherein said body of said N-channel metal oxide semiconductor transistor of at least one of said plurality of memory cells is operatively coupled to said reference ground.

18. The random access memory of claim 16 wherein each memory element of said plurality of memory cells comprises an N-channel metal oxide semiconductor transistor having a body and wherein said body of said N-channel metal oxide semiconductor transistors of at least one of said plurality of memory cells is biased by said bias voltage of said bias element.

19. The random access memory of claim 7 wherein each of said plurality of bias circuits comprise a current mirror, said bias element being a component of said current mirror.

20. The random access memory of claim 19 wherein said memory element of each of said plurality of memory cells has a source node and a power node, said bias circuit being operatively coupled between said power node and a power source.

21. The random access memory of claim 20 wherein said bias circuit is a first bias circuit and wherein a second bias circuit is operatively coupled between said source node and a reference node.

22. The random access memory of claim 21 further comprising:

a supply bias voltage buffer operatively coupled between said first bias circuit and said supply node; and

a source bias voltage buffer operatively coupled between said second bias circuit and said source node.

23. The random access memory of claim 19 wherein said memory element of each of said plurality of memory cells has a source node and a power node, said power node being coupled to said power source and said bias circuit being operatively coupled between said source node and said reference node.

24. The random access memory of claim 23:

wherein said current mirror comprises said bias element and a current element operatively coupled to said bias element;

wherein a current source induces a current through said current element; and

whereby a bias current is induced through said bias element and said memory element is approximately equivalent to said current and whereby said bias voltage on each source node of said plurality of memory cells is properly maintained.

25. The random access memory of claim 24 wherein each of said plurality of memory cells has a read mode and a storage mode, wherein said bias current is induced through said bias element when said memory cell is in said storage mode and wherein said bias current is not induced through said bias element when said memory cell is in said read mode.

26. The random access memory of claim 25 wherein each of said plurality of memory cells also has a write mode, wherein said bias current is not induced through said bias element when said memory cell is in said write mode.

27. The random access memory of claim 1 wherein said first electrical characteristic is a threshold voltage and wherein a current generated by said bias circuit induces said bias voltage based, at least in part, on said supply voltage and said threshold voltage.

28. The random access memory of claim 27 wherein said threshold voltage is inversely variable to variation in said temperature.

29. The random access memory of claim 27 wherein said bias voltage is based, at least in part, on said supply voltage less said threshold voltage less a voltage margin.

30. The random access memory of claim 27 wherein said bias voltage maintains said digital bit in at least one of said plurality of memory cells.

31. The random access memory of claim 1 wherein said bias circuit generates said bias voltage for more than one of said plurality of memory cells.

32. The random access memory of claim 31 wherein said bias circuit generates said bias voltage for all of said plurality of memory cells.

33. The random access memory of claim 1 wherein said bias circuit comprises at least one of said plurality of memory cells.

34. A method of maintaining a relatively stable bias voltage of a random access memory over a range of temperatures, comprising the steps of:

providing a plurality of memory cells each having a memory element, each of said plurality of memory cells being configured to store a digital bit and having a first electrical characteristic being variable based, at least in part, on temperature; and

providing a bias circuit operatively coupled to at least one of said plurality of memory cells and being configured to bias said at least one of said plurality of memory cells;

said bias circuit having a second electrical characteristic being variable based, at least in part, on temperature;

said first electrical characteristic being approximately proportional to said second electrical characteristic over a predetermined range of temperatures;

whereby a bias voltage of at least one of said plurality of memory cells is approximately proportional with variations over said range of temperatures.

35. The method of claim 34 wherein said providing said bias circuit step comprises providing a source bias circuit.

36. The method of claim 35, further comprising the step of providing a supply bias circuit operatively coupled to said at least one of said plurality of memory cells, said supply bias circuit being configured to generate a bias voltage for at least one of said plurality of memory cells, said supply bias circuit having said second electrical characteristic being variable based, at least in part, on temperature.

37. The method of claim 34 wherein said bias circuit is a supply bias circuit.

38. The random access memory of claim 37 further comprising a voltage buffer operatively coupled between said supply bias circuit and said at least one of said plurality of memory cells.

39. The random access memory of claim 38:

wherein said voltage buffer comprises an input and an output configured to supply said bias voltage, said input being coupled to said supply bias circuit and said output being coupled to said at least one of said plurality of memory cells; and

wherein another one of said plurality of memory cells is coupled to said input of said voltage buffer and configured to supply, in combination with said supply bias circuit, an input voltage related to said bias voltage to said input to said voltage buffer.

40. The method of claim 34 wherein said providing said bias circuit step provides said bias circuit operatively coupled to each of said plurality of memory cells.

41. The method of claim 34 wherein said bias voltage of each of said memory cells is approximately proportional with variations over said range of temperatures.

42. The method of claim 34 wherein said plurality of bias circuits generates said bias voltage of at least one of said plurality of memory cells.

43. The method of claim 42 wherein said plurality of bias circuits generates said bias voltage of each of said plurality of memory cells.

44. The method of claim 34 wherein said providing said bias circuit provides a plurality of bias circuits operatively coupled to at least one of said plurality of memory cells.

45. The method of claim 44 wherein at least one of said plurality of bias circuits being configured to bias at least said at least one of said plurality of memory cells, each of said plurality of bias circuits having a second electrical characteristic being variable based, at least in part, on temperature.

46. The random access memory of claim 45 wherein said random access memory is operatively coupled to a power source, wherein each memory element of said plurality of memory cells comprises a P-channel metal oxide semiconductor transistor having a body and wherein said body said P-channel metal oxide semiconductor transistor of at least one of said plurality of memory cells is operatively coupled to said power source.

47. The random access memory of claim 46 wherein said random access memory is operatively coupled to a reference ground, wherein each memory element of said plurality of memory cells comprises an N-channel metal oxide semiconductor transistor having a body and wherein said body of said N-channel metal oxide semiconductor transistor of said at least one of said plurality of memory cells is operatively coupled to said reference ground.

48. The random access memory of claim 45 wherein each memory element of said plurality of memory cells comprises a P-channel metal oxide semiconductor transistor having a body and wherein said body of said P-channel metal oxide semiconductor transistor of at least one of said plurality of memory cells is biased by said bias voltage of said bias element.

49. The random access memory of claim 48 wherein said random access memory is operatively coupled to a reference ground, wherein each memory element of said plurality of memory cells comprises an N-channel metal oxide semiconductor transistor having a body and wherein said body of said N-channel metal oxide semiconductor transistor of said at least one of said plurality of memory cells is operatively coupled to said reference ground.

50. The method of claim 44 wherein said providing said plurality of bias circuits step provides said plurality of bias circuits operatively coupled to each of said plurality of memory cells, at least one of said plurality of bias circuits being configured to bias of each respective one of said plurality of memory cells.

51. The random access memory of claim 50 wherein said random access memory is operatively coupled to a reference ground, wherein each memory element of said plurality of memory cells comprises an N-channel metal oxide semiconductor transistor having a body and wherein said body of said N-channel metal oxide semiconductor transistor of at least one of said plurality of memory cells is operatively coupled to said reference ground.

52. The random access memory of claim 50 wherein each memory element of said plurality of memory cells comprises an N-channel metal oxide semiconductor transistor having a body and wherein said body of said N-channel metal oxide semiconductor transistors of at least one of said plurality of memory cells is biased by said bias voltage of said bias element.

53. The method of claim 44 wherein said plurality of bias circuits generates said bias voltage of at least one of said plurality of memory cells.

54. The method of claim 53 wherein said plurality of bias circuits generates said bias voltage of each of said plurality of memory cells.

55. The method of claim 44:

wherein said providing said plurality of memory cells provides each of said plurality of memory cells having said memory element with a source node and a power node, said power node being coupled to a power source configured to generate a supply voltage; and

wherein said providing said bias circuit provides said bias circuit operatively coupled between said power node of each of said plurality of memory cells and said power source, each of said bias circuits being configured to generate a bias current through said power node of each respective one of said plurality of memory cells;

whereby said bias voltage on each power node of said plurality of memory cells is relatively stable over variations in temperature.

56. The method of claim 55 wherein said providing said bias circuit provides said bias circuit operatively coupled between said source node and said reference node, whereby said bias voltage on each power node of said plurality of memory cells is relatively stable over variations in temperature.

57. The random access memory of claim 56, further comprising:

a supply bias voltage buffer operatively coupled between said first bias circuit and said supply node; and

a source bias voltage buffer operatively coupled between said second bias circuit and said source node.

58. The method of claim 44:

wherein said providing said plurality of memory cells provides each of said plurality of memory cells having said memory element with a source node and a power node, said power node being coupled to a power source configured to generate a supply voltage; and

wherein said providing said bias circuit provides said bias circuit operatively coupled between said source node of each of said plurality of memory cells and a reference node, each of said bias circuits being configured to generate a bias current through said source node of each respective one of said plurality of memory cells;

whereby said bias voltage on each source node of said plurality of memory cells is relatively stable over variations in temperature.

59. The method of claim 58 wherein each of said plurality of bias circuits comprise a bias element with said bias element having said second electrical characteristic being variable based, at least in part, on temperature.

60. The method of claim 59 wherein said bias element of each of said bias circuits comprise metal oxide semiconductor transistors.

61. The method of claim 60 wherein said bias element of each of said plurality of bias circuits comprise P-channel metal oxide semiconductor transistors.

62. The method of claim 60 wherein said bias element of each of said plurality of bias circuits comprise N-channel metal oxide semiconductor transistors.

63. The method of claim 59 wherein each of said plurality bias circuits comprise a current mirror, said bias element being a component of said current mirror.

64. The method of claim 63:

wherein said current mirror comprises said bias element and a current element operatively coupled to said bias element;

wherein a current source induces a current through said current element; and

whereby a bias current is induced through said bias element approximately equivalent to said current and said bias voltage on each source node of said plurality of memory cells is properly maintained.

65. The method of claim 64 wherein each of said plurality of memory cells has a read mode and a storage mode, wherein said bias current is induced through said bias element when said memory cell is in said storage mode and wherein said bias current is not induced through said bias element when said memory cell is in said read mode.

66. The method of claim 65 wherein each of said plurality of memory cells also has a write mode and wherein said bias current is not induced through said bias element when said memory cell is in said write mode.

67. The method of claim 58 wherein said first electrical characteristic is a threshold voltage and wherein bias current induces said bias voltage based, at least in part, on said supply voltage and said threshold voltage.

68. The method of claim 67 wherein said threshold voltage is inversely variable to variation in said temperature.

69. The method of claim 67 wherein said bias voltage is based, at least in part, on said supply voltage less said threshold voltage less a voltage margin.

70. The method of claim 67 wherein said bias voltage maintains said digital bit in at least one of said plurality of memory cells.