Voltage reference circuit

1. A reference current generator comprising:

a current generator comprising a plurality of p-n junction elements for providing said reference current;

a current injector arranged to provide a control current to a first node of said current generator for increasing the magnitude of said reference current; and

a comparator arranged to provide a control signal based upon comparing the difference between a first voltage derived from the voltage across one of the p-n junction elements and a second voltage proportional to the reference current, said difference being indicative of the current in said one of the p-n junctions, wherein

said current injector is controlled by said control signal to provide current to said first node while the current in said one of the p-n junctions is below a predetermined level.

2. A reference current generator according to claim 1 further comprising a resistance element in series with one of said p-n junction elements.

3. A reference current generator comprising:

a current generator comprising a plurality of p-n junction elements for providing said reference current;

a current injector arranged to provide a control current to a first node of said current generator for increasing the magnitude of said reference current;

a resistance element in series with one of said p-n junction elements; and

a comparator arranged to provide a control signal based upon comparing the voltage across said resistance element to a predetermined level, said voltage being indicative of the current in said one of the p-n junction elements, wherein

said current injector is controlled by said control signal to provide current to said first node while the voltage across said resistance element is below said predetermined level.

4. A reference current generator according to claim 2 wherein said p-n junction elements are arranged as:

a first element comprising one or more p-n junctions arranged in parallel and

a second element comprising one or more p-n junctions arranged in parallel and including said p-n junction element in series with said resistive element, wherein

the total emitter area of the one or more p-n junctions in said first element is less than the total emitter area of the one or more p-n junctions in said second element.

5. A reference current generator according to claim 4 further comprising a current mirror arranged to provide substantially identical currents: to said first element; to said second element; and as said reference current output.

6. A reference current generator according to claim 3 wherein said p-n junction elements are arranged as:

a first element comprising one or more p-n junctions arranged in parallel and

a second element comprising one or more p-n junctions arranged in parallel and including said p-n junction element in series with said resistive element, wherein

the total emitter area of the one or more p-n junctions in said first element is less than the total emitter area of the one or more p-n junctions in said second element.

7. A reference current generator according to claim 6 further comprising a current mirror arranged to provide substantially identical currents: to said first element; to said second element; and as said reference current output.

8. A reference current generator according to claim 1 wherein said first node is provided on one of said p-n junction elements to provide current to the p-n junction element.

9. A reference current generator according to claim 3 wherein said first node is provided on one of said p-n junction elements to provide current to the p-n junction element.

10. A reference current generator according to claim 1 wherein said current generator is a bandgap voltage reference circuit.

11. A reference current generator according to claim 2 wherein said current generator is a bandgap voltage reference circuit.

12. A reference current generator according to claim 3 wherein said current generator is a bandgap voltage reference circuit.

13. A start-up controller for a bandgap reference cell having a first voltage reference element comprising a first voltage reference device and a second voltage reference element comprising a second voltage reference device arranged in series with a resistance element, wherein said start-up controller comprises:

a comparator arranged to provide a control signal by comparing the voltage on a node of said bandgap reference cell with a voltage proportional to an output reference current generated by said bandgap reference cell, said measured voltage difference corresponding to the current through one of said first or second voltage reference elements; and

a current injector arranged to provide current to one of said first and second voltage reference elements while said measured voltage difference is below a predetermined level.

14. A start-up controller according to claim 13 wherein said voltage reference devices are p-n junction devices.

15. A start-up controller according to claim 13 , wherein said first and second voltage reference devices are bandgap voltage reference devices.

16. A start-up controller according to claim 13 , wherein said voltage reference circuit further comprises:

a resistance element in parallel with each of said voltage reference elements;

a current source for producing mirrored currents to said first voltage reference device, said second voltage reference device and a reference current output; and

an amplifier arranged to control said current source to maintain the voltage across the first and second voltage reference devices at the same level.

17. A start-up controller for a bandgap reference cell having a first voltage reference element comprising a first voltage reference device and a second voltage reference element comprising a second voltage reference device arranged in series with a resistance element, wherein said start-up controller comprises:

a comparator arranged to provide a control signal by comparing the voltage across said resistance element to determine if it exceeds a predetermined value, said measured voltage difference corresponding to the current through the second voltage reference element; and

a current injector arranged to provide current to one of said first and second voltage reference elements while said measured voltage is below said predetermined level.

18. A start-up controller according to claim 17 wherein said voltage reference devices are p-n junction devices.

19. A start-up controller according to claim 17 , wherein said first and second voltage reference devices are bandgap voltage reference devices.

20. A start-up controller according to claim 17 , wherein said voltage reference circuit further comprises:

a resistance element in parallel with each of said voltage reference elements;

a current source for producing mirrored currents to said first voltage reference device, said second voltage reference device and a reference current output; and

an amplifier arranged to control said current source to maintain the voltage across the first and second voltage reference devices at the same level.

21. A start-up controller according to claim 13 wherein the comparator includes a predetermined offset.

22. A start-up controller according to claim 17 wherein the comparator includes a predetermined offset.

23. A start-up controller for a voltage reference circuit having first and second voltage reference devices, the circuit having a first stable operating state and one or more other stable operating states in which the current flowing through the first and second voltage reference devices is below a predetermined level, the start-up controller comprising:

a current monitor arranged to determine if a device current in one of said voltage reference devices is below a predetermined threshold comprising comparing a voltage proportional to the voltage across one of the first and second voltage reference devices with a voltage proportional to the output voltage of said voltage reference circuit;

a current injector arranged to inject current to an injection node of said voltage reference circuit to cause the current in said voltage reference devices to increase, wherein

said current injector injects current while said device current is determined to be below said predetermined threshold.

24. A start-up controller according to claim 23 wherein said voltage proportional to the output voltage of said voltage reference circuit is provided by a voltage divider arranged to provide a fixed proportion of the output voltage.

25. A start-up controller according to claim 23 wherein said voltage proportional to the voltage across said one of the first and second voltage reference devices circuit is provided by a voltage divider arranged to provide a fixed proportion of the said voltage across said one of the first and second voltage reference devices.

26. A start-up controller according to claim 23 , wherein one of said first and second voltage reference devices also includes a resistance element in series with it.

27. A start-up controller according to claim 24 , wherein one of said first and second voltage reference devices also includes a resistance element in series with it.

28. A start-up controller according to claim 25 , wherein one of said first and second voltage reference devices also includes a resistance element in series with it.

29. A start-up controller according to claim 23 wherein said voltage reference devices are p-n junction devices.

30. A start-up controller according to claim 23 , wherein said first and second voltage reference devices are bandgap voltage reference devices.

31. A start-up controller for a voltage reference circuit having first and second voltage reference devices, the circuit having a first stable operating state and one or more other stable operating states in which the current flowing through the first and second voltage reference devices is below a predetermined level, the start-up controller comprising:

a current monitor arranged to determine if a device current in one of said voltage reference devices is below a predetermined threshold comprising determining said device current by reference to a voltage across a resistive element in series with said one of said first and second voltage reference devices;

a current injector arranged to inject current to an injection node of said voltage reference circuit to cause the current in said voltage reference device to increase, wherein

said current injector injects current while said device current is below said predetermined threshold.

32. A start-up controller according to claim 31 wherein said voltage reference devices are p-n junction devices.

33. A start-up controller according to claim 31 , wherein said first and second voltage reference devices are bandgap voltage reference devices.

34. A start-up controller according to claim 23 wherein the current monitor comprises a comparator which includes a predetermined offset.

35. A start-up controller according to claim 31 wherein the current monitor comprises a comparator which includes a predetermined offset.

36. An integrated circuit including a start-up controller according to claim 1 .