Start-up circuit for a current generator

1. A circuit comprising:

a current generator having;

an input node;

a start-up circuit coupled to provide a start-up current to the input node of the current generator during a start-up phase of the current generator;

a feedback node coupled to provide a feedback signal as a function of an output current of the current generator; and

a cut-off circuit coupled to both the current generator and the start-up circuit to provide a control signal that reduces the start-up current when the output current from the current generator exceed a threshold value, the cut-off circuit including a current mirror that outputs the control signal to the start-up circuit, coupled to the feedback node.

2. The circuit of claim 1 wherein the current generator comprises:

a first transistor coupled to the input node to control the output current.

3. The circuit of claim 1 wherein the current mirror controls the control signal as a function of the output current.

4. The circuit of claim 2 wherein the cut-off circuit comprises:

an input node coupled to receive the feedback signal from the current generator;

a first cut-off transistor coupled to the input node of the cut-off circuit to control a first cut-off circuit current; and

a control node providing the control signal as a function of the first cut-off circuit current.

5. The circuit of claim 4 wherein the cut-off circuit further comprises:

a second cut-off transistor coupled to the input node of the cut-off circuit, to control a second cut-off circuit current;

the current mirror coupled to the first and second cut-off transistors and to the control node.

6. The circuit of claim 5 wherein the current mirror controls the control signal as a function of the first cut-off circuit current.

7. The circuit of claim 4 wherein the start-up circuit comprises:

an input node receiving the control signal from the cut-off circuit; and

a first start-up transistor coupled to the input node of the start-up circuit to control the start-up current.

8. The circuit of claim 7 wherein the control signal causes the first transistor to reduce the start-up current to approximately zero.

9. The circuit of claim 1 wherein the current mittor comprises first and second cut-off circuit providing the signal as a function of the first cut-off circuit current.

10. A circuit comprising:

current generating means for generating an output current;

start-up means for providing a start-up current to the current generating means during a start-up phase; and

a cut-off circuit configured to reduce the staff-up current when the output current exceeds a threshold value, including:

a feedback node coupled to receive a feedback signal from the current generating means as a function of the output current,

first and second transistors coupled to the feedback node,

a cut-off current mirror configured to control current flowing in the first arid second transistors and outputting a control signal at a control node to reduce the current output by the start-up means.

11. The circuit of claim 10 wherein the cut-off circuit provides the control signal to the start-up means as a function of the feedback signal.

12. The circuit of claim 11 wherein the start-up means reduces the staff-up current as a function of the control signal.

13. The circuit of claim 12 wherein the start-up means reduces the staff-up current to approximately zero.

14. A method of starting a current generator, comprising:

outputting start-up current to the current generator during a start-up phase of the current generator;

receiving a feedback signal from the current generator as a function of an output current of the current generator;

producing a cut-off current and a corresponding mirror current as a function of the feedback signal:

producing a control signal as a function of the mirror current; and

reducing the staff-up current in response to the control signal.

15. The method of claim 14 wherein the feedback signal indicates the output current has exceeded a threshold value.

16. The method of claim 14 wherein reducing the start-up current comprises:

reducing the start-up current to approximately zero.

17. A start-up circuit for a current generator, comprising:

first and second power supply nodes for connection to an electrical power supply;

a feedback node for receiving a feedback signal from the current generator;

an output node for applying a start-up current to the current generator;

a first transistor connected to the feedback node for drawing a first current;

a second transistor connected to the first transistor for drawing a second current;

a current mirror connected to the first and second transistors for regulating the first and second currents and providing a control signal; and

a third transistor connected to the current mirror and the output node for drawing the start-up current in response to the control signal.

18. The start-up circuit of claim 17 wherein the current mirror comprises:

a fourth transistor connected to the first transistor; and

a fifth transistor connected to the second transistor and the fourth transistor.

19. The start-up circuit of claim 18 wherein the first, second and third transistors are each p-channel MOSFETs having a source, a drain and a gate; the first transistor having its source connected to the first power supply node, and its gate connected to the feedback node and the gate of the second transistor; the second transistor having its source connected to the first power supply node; and the third transistor having its source connected to the first power supply node, and its drain connected to the output node.

20. The start-up circuit of claim 19 wherein the fourth and fifth transistors are each npn bipolar junction transistors having a collector, an emitter and a base; the fourth transistor having its collector connected to its base and the drain of the first transistor, and its base connected to the base of the fifth transistor; and the fifth transistor having its collector connected to the drain of the second transistor and to the gate of the third transistor.

21. The staff-up circuit of claim 20 , further comprising:

a first resistor connected between the emitter of the fourth transistor and the second power supply node; and

a second resistor connected between the emitter of the fifth transistor and the second power supply node.

22. The start-up circuit of claim 18 wherein the first, second and third transistors are each n-channel MOSFETs having a drain, a source and a gate; the first transistor having its source connected to the second power supply node, and its gate connected to the feedback node and the gate of the second transistor; the second transistor having its source connected to the second power supply node; and the third transistor having its source connected to the second power supply node, and its drain connected to the output node.

23. The start-up circuit of claim 22 wherein the fourth and fifth transistors are each pnp bipolar junction transistors having an emitter, a collector and a base; the fourth transistor having its collector connected to its base and the drain of the first transistor, and its base connected to the base of the fifth transistor; and the fifth transistor having its collector connected to the drain of the second transistor and to the gate of the third transistor.

24. The staff-up circuit of claim 23 , farther comprising:

a diode connected between the gate of the third transistor and the second power supply node;

a first resistor connected between the first power supply node and the emitter of the fourth transistor; and

a second resistor connected between the first power supply node and the emitter of the fifth transistor.

25. The start-up circuit of claim 18 wherein the fourth and fifth transistors have a size ratio of 2:1.