Reference voltage circuit

1. A method, comprising:

changing an output impedance value coupled to an output of a reference voltage circuit; and

detecting when an output of the reference voltage circuit reaches a predetermined voltage level, wherein detecting when the output of the reference voltage circuit reaches the predetermined voltage level comprises comparing an external voltage to a voltage from the reference voltage circuit, wherein the comparison of the two voltages is used to set the output impedance value, and wherein changing the output impedance value comprises tuning the output of the variable resistance circuit using a delay circuit.

2. The method of claim 1 , wherein changing the output impedance value comprises switching an output impedance value of the reference circuit to an alternate output impedance value.

3. An apparatus, comprising:

a reference voltage circuit having a first output; and

a variable resistance circuit coupled to the first output of the reference voltage circuit, the variable resistance circuit to adjust an output load of the reference voltage circuit, wherein the variable resistance circuit comprises:

a voltage detection circuit to compare a voltage from the reference voltage circuit to an external reference voltage;

a delay circuit coupled to an output of the voltage detection circuit, the delay circuit to tune an output of the variable resistance circuit; and

a variable resistor circuit comprising a plurality of resistance values coupled to an output of the delay circuit.

4. The apparatus of claim 3 , wherein an output of the voltage detection circuit is coupled to an input of the delay circuit, the voltage detection circuit to activate the delay circuit.

5. The apparatus of claim 3 , wherein an output of the delay circuit is coupled to an input of the circuit comprising a plurality of resistance values, the delay circuit comprising one or more delay elements, and the delay circuit to tune an output of the variable resistor circuit.

6. The apparatus of claim 3 , the voltage detection circuit comprises:

an electronic comparator circuit having a first and second input;

the first input of the electronic comparator circuit coupled to a second output of the reference circuit; and

the second input of the electronic comparator circuit coupled to the external reference voltage.

7. The apparatus of claim 6 , wherein voltage levels of the first and second inputs of the electronic comparator circuit are used to determine an impedance value of the output load of the reference voltage circuit.

8. An apparatus, comprising:

a reference voltage circuit having a first output; and

a variable resistance circuit coupled to the first output of the reference voltage circuit, the variable resistance circuit to adjust an output load of the reference voltage circuit, wherein the variable resistance circuit comprises:

a voltage detection circuit to compare a voltage from the reference voltage circuit to an external reference voltage;

a delay circuit coupled to an output of the voltage detection circuit, the delay circuit to tune an output of the variable resistance circuit; and

a digital-control switch coupled to an output of the delay circuit.

9. The apparatus of claim 8 , wherein the digital-control switch comprises:

a plurality of resistive elements coupled to a plurality of digitally-controlled switches; and

a control-logic circuit to control the activation of the digitally-controlled switches.

10. The apparatus of claim 9 , wherein the control-logic circuit couples the plurality of resistive elements to an output of the reference voltage circuit via the digitally-controlled switches, and wherein a resistive element is coupled to the output of the reference voltage circuit via the digitally-controlled switch in response to the output of the voltage detection circuit.

11. An apparatus, comprising:

means for adjusting an output load of a reference voltage circuit to achieve a predetermined level of power supply rejection for the reference voltage circuit; and

means for detecting when an output of the reference voltage circuit reaches a predetermined voltage level, wherein the means for adjusting comprises:

means for comparing a voltage from the reference voltage circuit to an external reference voltage; and

means for tuning an output of the means for adjusting the output load of the reference voltage circuit.

12. The apparatus of claim 11 , wherein the means for adjusting the output load further comprises a means for optimizing for a transient response of the reference voltage reference circuit.

13. The apparatus of claim 11 , wherein the wherein the means for adjusting the output load decreases start-up time of the reference voltage circuit.