Low power line driver and method thereof

1. A line driver for a communications system requiring multiple power sources for different modes of operation, comprising:

a current source having a first terminal and a second terminal;

a first source resistor coupled to the first terminal of the current source;

a second source resistor coupled to the second terminal of the current source; and

a voltage source that, in series combination with the first and second source resistors, is coupled in parallel with the current source,

wherein the current source provides a driving current and the voltage source provides a driving voltage at the same time during operations of the communications system.

2. The line driver as claimed in claim 1 , wherein the communications system is an Ethernet system.

3. The line driver as claimed in claim 2 , wherein the driving current and the driving voltage are both provided to the communications system in each of a 10 Base-T, 100 Base-TX and 1,000 Base-T modes.

4. The line driver as claimed in claim 1 , further comprising a load resistor disposed between a pair of output nodes and coupled in parallel with the current source.

5. The line driver as claimed in claim 4 , wherein the sum of resistance of the first and second source resistors is equal to a resistance of the load resistor.

6. The line driver as claimed in claim 4 , wherein the driving current flows through a transmission line to the load resistor and returns to the current source.

7. The line driver as claimed in claim 1 , wherein no significant amount of the driving current flows through either of the first source resistor or the second source resistor.

8. The line driver as claimed in claim 1 , wherein a resistance of the first source resistor is the same as a resistance of the second source resistor.

9. The line driver as claimed in claim 1 , wherein at least one of the driving voltage or the driving current can be at any level between certain limits of a power supply.

10. The line driver as claimed in claim 4 , wherein an output voltage between the pair of output nodes is at least partially determined by values of the driving voltage and the driving current, and a resistance of the load resistor.

11. A line driver, comprising:

a current source having a first terminal and a second terminal;

a first source resistor coupled to the first terminal of the current source; and

a voltage source that, in series combination with the first source resistor, is coupled in parallel with the current source,

wherein the current source provides a driving current and the voltage source provides a driving voltage at the same time, and wherein at least one of the driving voltage or the driving current is at a variable level between certain limits of a power supply.

12. The line driver as claimed in claim 11 , further comprising a second source resistor coupled to the second terminal of the current source.

13. The line driver as claimed in claim 12 , wherein the second source resistor is further coupled to the voltage source.

14. The line driver as claimed in claim 11 , further comprising a load resistor disposed between a pair of output nodes and coupled in parallel with the current source.

15. The line driver as claimed in claim 11 , wherein the driving current flows through a transmission line to the load resistor and returns to the current source, and wherein no driving current flows through the first source resistor.

16. The line driver as claimed in claim 14 , wherein an output voltage between the pair of output nodes is at least partially determined by values of the driving voltage and the driving current, and a resistance of the load resistor.

17. A method for driving a system requiring multiple power sources, comprising:

providing a current source, the current source having a first terminal and a second terminal providing a driving current;

providing a first source resistor;

coupling the first source resistor to the first terminal of the current source;

providing a voltage source;

coupling the voltage source to the current source such that the voltage source, in combination with the first source resistor, is coupled in parallel with the current source;

providing a driving voltage; and

providing both the driving voltage and the driving current during operation of the system, wherein at least one of the driving voltage or the driving current can be at any level between certain limits of a power supply.

18. The method as claimed in claim 17 , further comprising providing a second source resistor and coupling the second source resistor to the second terminal of the current source.

19. The method as claimed in claim 18 , further comprising providing a load resistor coupled in parallel with the current source, wherein the sum of resistance of the first and second source resistors is equal to a resistance of the load resistor.

20. The method as claimed in claim 19 , wherein the driving current flows through a transmission line, to the load resistor and returns to the current source, and wherein no significant driving current flows through the first source resistor.