Logical signal voltage converter

1. A circuit for converting a lower voltage logical signal to a higher voltage logical signal, the circuit comprising:

two current mirror structures each comprising a first branch and a second branch, each branch comprising at least a first transistor of a first kind, an input transistor of a second kind, and a second transistor of the first kind coupled between the respective first transistor and input transistor;

wherein in each of the current mirror structures:

the first transistor of the first branch and the first transistor of the second branch are arranged as a current mirror so as to mirror the current in the first branch in the second branch;

the input transistor of each branch is arranged to be driven using a logical signal at the lower voltage, so as to control the current mirror structure to output a corresponding logical signal at the higher voltage;

the second transistor of each branch is arranged to be driven by a fixed intermediate reference voltage so as to reduce the operating voltage of the input transistor; and

the first kind is tolerant of a higher operating voltage than the second kind; and

wherein the current mirror structures are coupled together by a pair of cross coupled transistors connected in parallel with the current mirrors.

2. The circuit of claim 1 , wherein the first kind has a gate oxide layer substantially thicker than the second kind.

3. The circuit of claim 1 , wherein the input transistor of one of the first and second branches is connected to be driven by the lower voltage logical signal, and the other input transistor is connected to be driven by an inverse of the lower voltage logical signal.

4. The circuit of claim 1 , comprising latching circuitry operable to force the current mirror structures to output a specified value irrespective of the lower voltage logical signal.

5. The circuit of claim 1 , comprising latching circuitry operable to freeze the output of the current mirror structures.

6. A circuit for converting a lower voltage logical signal to a higher voltage logical signal, the circuit comprising:

at least one current mirror structure comprising a first branch and a second branch, each branch comprising at least a first transistor of a first kind, an input transistor of a second kind, and a second transistor of the first kind coupled between the respective first transistor and input transistor;

wherein the first transistor of the first branch and the first transistor of the second branch are arranged as a current mirror so as to mirror the current in the first branch in the second branch;

the input transistor of each branch is arranged to be driven using a logical signal at the lower voltage, so as to control the current mirror structure to output a corresponding logical signal at the higher voltage;

the second transistor of each branch is arranged to be driven by an intermnediate reference voltage so as to reduce the operating voltage of the input transistor;

the first kind is tolerant of a higher operating voltage than the second kind; and

each branch comprises a third transistor of the first kind coupled between the respective first and second transistors.

7. The circuit of claim 6 , wherein one of the third transistors is arranged to be driven in dependence on a feedback signal from said output of the current mirror structure.

8. The circuit of claim 6 , wherein one of the third transistors is arranged to be driven in dependence on an enable signal for enabling and disabling the respective branch.

9. A method of converting a lower voltage logical signal to a higher voltage logical signal in a circuit having two current mirror structures each comprising a first branch and a second branch, each branch comprising at least a first transistor of a first kind, an input transistor of a second kind and a second transistor of the first kind coupled between the respective first transistor and input transistor, the first kind being tolerant of a higher operating voltage than the second kind, the method comprising in each of the current mirror structures:

using the first transistor of the first branch and the first transistor of the second branch as a current mirror so as to mirror the current in the first branch in the second branch;

driving the input transistor of each branch using a logical signal at the lower voltage, so as to control the current mirror structure to output a corresponding logical signal at the higher voltage; and

driving the second transistor of each branch with a fixed intermediate reference voltage so as to reduce the operating voltage of the input transistor;

wherein the two current mirror structures are coupled together by a pair of cross coupled transistors connected in parallel with the current mirrors.

10. The method of claim 9 , wherein the first kind has a gate oxide layer substantially thicker than the second kind.

11. The method of claim 9 , comprising driving the input transistor of one of the first and second branches with the lower voltage logical signal, and driving the other input transistor with an inverse of the lower voltage logical signal.

12. The method of claim 9 , comprising using latching circuitry to force the current mirror structures to output a specified value irrespective of the lower voltage logical signal.

13. The method of claim 9 , comprising using latching circuitry to freeze the output of the current mirror structures.

14. A method of converting a lower voltage logical signal to a higher voltage logical signal in a circuit having at least one current mirror structure comprising a first branch and a second branch, each branch comprising at least a first transistor of a first kind, an input transistor of a second kind and a second transistor of the first kind coupled between the respective first transistor and input transistor, the first kind being tolerant of a higher operating voltage than the second kind, the method comprising:

using the first transistor of the first branch and the first transistor of the second branch as a current mirror so as to mirror the current in the first branch in the second branch;

driving the input transistor of each branch using a logical signal at the lower voltage, so as to control the current mirror structure to output a corresponding logical signal at the higher voltage; and

driving the second transistor of each branch with an intermediate reference voltage so as to reduce the operating voltage of the input transistor;

wherein each branch comprises a third transistor of the first kind coupled between the respective first and second transistors.

15. The method of claim 14 , comprising driving one of the third transistors in dependence on a feedback signal from said output of the current mirror structure.

16. The method of claim 14 , comprising driving one of the third transistors in dependence on an enable signal for enabling and disabling the respective branch.

17. A circuit for converting a lower voltage logical signal to a higher voltage logical signal, the circuit comprising:

at least one current mirror structure comprising a first branch and a second branch, each branch comprising at least a first transistor of a first kind, an input transistor of a second kind, and a second transistor of the first kind coupled between the respective first transistor and input transistor;

wherein the first transistor of the first branch and the first transistor of the second branch are arranged as a current mirror so as to mirror the current in the first branch in the second branch;

the input transistor of each branch is arranged to be driven using a logical signal at the lower voltage, so as to control the current mirror structure to output a corresponding logical signal at the higher voltage;

the second transistor of each branch is arranged to be driven by a fixed intermediate reference voltage so as to reduce the operating voltage of the input transistor;

the first kind is tolerant of a higher operating voltage than the second kind; and

wherein the circuit comprises latching circuitry operable to force the current mirror structure to output a specified value irrespective of the lower voltage logical signal.

18. A circuit for converting a lower voltage logical signal to a higher voltage logical signal, the circuit comprising:

at least one current mirror structure comprising a first branch and a second branch, each branch comprising at least a first transistor of a first kind, an input transistor of a second kind, and a second transistor of the first kind coupled between the respective first transistor and input transistor;

wherein the first transistor of the first branch and the first transistor of the second branch are arranged as a current mirror so as to mirror the current in the first branch in the second branch;

the input transistor of each branch is arranged to be driven using a logical signal at the lower voltage, so as to control the current mirror structure to output a corresponding logical signal at the higher voltage;

the second transistor of each branch is arranged to be driven by a fixed intermediate reference voltage so as to reduce the operating voltage of the input transistor;

the first kind is tolerant of a higher operating voltage than the second kind; and

wherein the circuit comprises latching circuitry operable to freeze the output of the current mirror structure.

19. A method of converting a lower voltage logical signal to a higher voltage logical signal in a circuit having at least one current mirror structure comprising a first branch and a second branch, each branch comprising at least a first transistor of a first kind, an input transistor of a second kind and a second transistor of the first kind coupled between the respective first transistor and input transistor, the first kind being tolerant of a higher operating voltage than the second kind, the method comprising:

using the first transistor of the first branch and the first transistor of the second branch as a current mirror so as to mirror the current in the first branch in the second branch;

driving the input transistor of each branch using a logical signal at the lower voltage, so as to control the current mirror structure to output a corresponding logical signal at the higher voltage;

using latching circuitry to force the current mirror structure to output a specified value irrespective of the lower voltage logical signal; and

driving the second transistor of each branch with a fixed intermediate reference voltage so as to reduce the operating voltage of the input transistor.

20. A method of converting a lower voltage logical signal to a higher voltage logical signal in a circuit having at least one current mirror structure comprising a first branch and a second branch, each branch comprising at least a first transistor of a first kind, an input transistor of a second kind and a second transistor of the first kind coupled between the respective first transistor and input transistor, the first kind being tolerant of a higher operating voltage than the second kind, the method comprising:

using the first transistor of the first branch and the first transistor of the second branch as a current mirror so as to mirror the current in the first branch in the second branch;

driving the input transistor of each branch using a logical signal at the lower voltage, so as to control the current mirror structure to output a corresponding logical signal at the higher voltage;

using latching circuitry to freeze the output of the current mirror structure; and

driving the second transistor of each branch with a fixed intermediate reference voltage so as to reduce the operating voltage of the input transistor.