Fast regulator architecture having transistor helper

1. A voltage regulator comprising:

an error amplifier configured to compare a first reference voltage with a representation of an output voltage of the voltage regulator;

an output transistor coupled to a supply voltage and configured to receive an output of the error amplifier and to provide the output voltage;

an auxiliary-current circuit including a helper transistor having a terminal coupled to the output voltage, the helper transistor having a control node responsive to a second reference voltage, the helper transistor configured to turn on when the output voltage drops due to current demand from the load and to provide auxiliary current to the load in addition to current provided by the output transistor; and

wherein the auxiliary-current circuit includes a feedback amplifier having an output coupled to the control node of the helper transistor.

2. The voltage regulator of claim 1 , wherein the feedback amplifier is configured to hold the control node of the helper transistor at a constant voltage level using the second reference voltage.

3. The voltage regulator of claim 2 , wherein the auxiliary-current circuit includes a current mirror configured provide a representation of the current provided by the helper transistor to a first input of the feedback amplifier.

4. The voltage regulator of claim 3 , wherein the auxiliary-current circuit includes a reference generator configured to provide the second reference voltage to a second input of the feedback amplifier.

5. The voltage regulator of claim 4 , wherein the reference generator includes a current source and a resistor configured to provide the second reference voltage using current supplied by the current source.

6. The voltage regulator of claim 1 , wherein an integrated circuit includes the error amplifier, the output transistor, an output capacitor and the auxiliary-current circuit.

7. A method comprising:

comparing a first reference voltage with a representation of an output voltage of a voltage regulator using an error amplifier;

receiving an output signal of the error amplifier directly by an output transistor coupled to a supply voltage;

providing the output voltage using the output transistor;

turning on a helper transistor of an auxiliary-current circuit coupled to the output voltage when the output voltage drops due to current demand from the load and to provide auxiliary current to the load in addition to current provided by the output transistor;

limiting current provided by the helper transistor when the output voltage is within a threshold of a desired output voltage using a second reference voltage; and

holding a control node of the helper transistor at a constant level using a feedback amplifier of the auxiliary-current circuit.

8. The method of claim 7 , including sensing the auxiliary current provided by the helper transistor using a sense transistor of a current mirror of the auxiliary-current circuit.

9. The method of claim 8 , including providing a mirrored current using a second transistor of the current mirror.

10. The method of claim 9 ; including generating a feedback voltage using the mirrored current.

11. The method of claim 10 , including comparing the feedback voltage to the second reference voltage using a feedback amplifier;

providing a helper control signal at an output of the feedback amplifier, the helper control signal based on the comparison of the feedback voltage and the second reference voltage; and

receiving the helper control signal at a control node of the helper transistor.

12. The method of claim 11 ; including generating the second reference voltage using a reference generator of the auxiliary-current circuit.

13. The method of claim 11 , wherein the generating the second reference voltage includes generating a reference current using a current source of the auxiliary-current circuit, and passing the reference current through a reference resistor; and

receiving the second reference voltage generated across the reference resistor at the feedback amplifier.

14. The method of claim 7 , including providing a small current from the auxiliary-current circuit correlated with the current source when the output voltage does not substantially include high frequency transients.

15. The method of claim 7 , wherein the helper transistor includes a bipolar transistor.

16. The method of claim 7 , wherein the helper transistor includes a metal-oxide semiconductor (MOS) transistor.

17. A low-dropout regulator configured to couple to a load, the low-drop-out regulator comprising:

an error amplifier configured to provide an output voltage error signal;

an output transistor coupled between a supply node and the load, the output transistor configured to receive the output voltage error signal directly from the error amplifier, and to provide first output current at an output voltage based on the output voltage error signal; and

auxiliary current circuit for providing auxiliary output current in addition to the output current, the auxiliary current circuit responsive to a drop in the output voltage due to current demand from the load,

wherein the auxiliary current circuit includes a helper transistor having a terminal coupled directly to the output voltage, the helper transistor having a control node responsive to a second reference voltage, the helper transistor configured to turn on when the output voltage drops due to current demand from the load and to provide auxiliary current to the load, via the terminal, in addition to the first output current provided by the output transistor,

wherein the output of the error amplifier is directly coupled to a control node of the output transistor.