Method and apparatus for reducing crowbar current and controlling slew rate

1. An output driver having a reduced crowbar current and a controlled slew rate, the output driver comprising:

a pull-up network comprising a first plurality of resistive branches forming a first R-2R resistive ladder structure, wherein each of the first plurality of resistive branches of the pull-up network is coupled to a high voltage supply through one of a plurality of pull-up switching transistors;

a pull-down network comprising a second plurality of resistive branches forming a second R-2R resistive ladder structure, wherein each of the second plurality of resistive branches of the pull-down network is coupled to a low voltage supply through one of a plurality of pull-down switching transistors; and

a control circuit coupled to a gate of each of the plurality of pull-up switching transistors and pull-down switching transistors, the control circuit to selectively activate or deactivate each of the first plurality of resistive branches through the pull-up switching transistors and to selectively activate or deactivate each of the second plurality of resistive branches through the pull-down switching transistors to reduce the crowbar current of the output driver when switching between complementary logic states.

2. The output driver of claim 1 , wherein the pull-up network further comprises:

a first pull-up transistor having a source coupled to the high voltage supply, a drain coupled to a first terminal of a first resistor and a gate coupled to a first control signal of the control circuit;

a second pull-up transistor having a source coupled to the high voltage supply, a drain coupled to a first terminal of a second resistor and a gate coupled to a second control signal of the control circuit;

a third pull-up transistor having a source coupled to the high voltage supply, a drain coupled to a first terminal of a third resistor and a gate coupled to a third control signal of the control circuit;

a fourth resistor having a first terminal coupled to a second terminal of the second resistor and a second terminal coupled to a second terminal of the third resistor; and

a fifth resistor having a first terminal coupled to the second terminal of the fourth resistor and a second terminal coupled to an output load.

3. The output driver of claim 2 , wherein a resistance of the first resistor is equal to a resistance of the second resistor and a resistance of the third resistor.

4. The output driver of claim 2 , wherein a resistance of the fourth resistor is equal to a resistance of the fifth resistor.

5. The output driver of claim 1 , wherein the pull-down network further comprises:

a first pull-down transistor having a source coupled to the low voltage supply, a drain coupled to a first terminal of a sixth resistor and a gate coupled to a fourth control signal of the control circuit;

a second pull-down transistor having a source coupled to the low voltage supply, a drain coupled to a first terminal of a seventh resistor and a gate coupled to a fifth control signal of the control circuit;

a third pull-down transistor having a source coupled to the low voltage supply, a drain coupled to a first terminal of an eighth resistor and a gate coupled to a sixth control signal of the control circuit;

a ninth resistor having a first terminal coupled to a second terminal of the seventh resistor and a second terminal coupled to a second terminal of the eighth resistor; and

a tenth resistor having a first terminal coupled to the second terminal of the eighth resistor and a second terminal coupled to an output load.

6. The output driver of claim 5 , wherein a resistance of the sixth resistor is equal to a resistance of the seventh resistor and a resistance of the eighth resistor.

7. The output driver of claim 5 , wherein a resistance of the ninth resistor is equal to a resistance of the tenth resistor.

8. The output driver of claim 1 , wherein the output driver is a double data rate (DDR) buffer output driver.

9. An output driver having a reduced crowbar current and a controlled slew rate, the output driver comprising:

a first pull-up transistor having a source coupled to the high voltage supply, a drain coupled to a first terminal of a first resistor and a gate coupled to a first control signal of the control circuit;

a second pull-up transistor having a source coupled to the high voltage supply, a drain coupled to a first terminal of a second resistor and a gate coupled to a second control signal of the control circuit, a second terminal of the second resistor coupled to a second terminal of the first resistor;

a third pull-up transistor having a source coupled to the high voltage supply, a drain coupled to a first terminal of a third resistor and a gate coupled to a third control signal of the control circuit;

a fourth resistor having a first terminal coupled to the second terminal of the second resistor and a second terminal coupled to a second terminal of the third resistor;

a fifth resistor having a first terminal coupled to the second terminal of the fourth resistor and a second terminal coupled to an output load;

a first pull-down transistor having a source coupled to the low voltage supply, a drain coupled to a first terminal of a sixth resistor and a gate coupled to a fourth control signal of the control circuit;

a second pull-down transistor having a source coupled to the low voltage supply, a drain coupled to a first terminal of a seventh resistor and a gate coupled to a fifth control signal of the control circuit, a second terminal of the seventh resistor coupled to a second terminal of the sixth resistor;

a third pull-down transistor having a source coupled to the low voltage supply, a drain coupled to a first terminal of an eighth resistor and a gate coupled to a sixth control signal of the control circuit;

a ninth resistor having a first terminal coupled to the second terminal of the seventh resistor and a second terminal coupled to a second terminal of the eighth resistor; and

a tenth resistor having a first terminal coupled to the second terminal of the eighth resistor and a second terminal coupled to an output load.

10. The output driver of claim 9 , wherein a resistance of the first resistor is equal to a resistance of the second resistor and a resistance of the third resistor.

11. The output driver of claim 9 , wherein a resistance of the fourth resistor is equal to a resistance of the fifth resistor.

12. The output driver of claim 9 , wherein a resistance of the sixth resistor is equal to a resistance of the seventh resistor and a resistance of the eighth resistor.

13. The output driver of claim 9 , wherein a resistance of the ninth resistor is equal to a resistance of the tenth resistor.

14. The output driver of claim 9 , wherein the output driver is a double data rate (DDR) output driver.

15. A method for reducing a crowbar current and for controlling a slew rate of an output driver during a transition of the output driver between complimentary logic states, the method comprising:

selectively activating or deactivating each of a first plurality of resistive branches which form a first R-2R resistive ladder structure through one of a plurality of pull-up switching transistors, each of the plurality of pull-up switching transistors coupled to a high voltage supply; and

selectively activating or deactivating each of a second plurality of resistive branches which form a second R-2R resistive ladder structure through one of a plurality of pull-down switching transistors, each of the plurality of pull-down switching transistors coupled to a low voltage supply.

16. The method of claim 15 , wherein selectively activating or deactivating each of a first plurality of resistive branches which form a first R-2R resistive ladder structure through one of a plurality of pull-up switching transistors, further comprises:

identifying the largest current carrying branch of the first plurality of resistive branches which for the first R-2R resistive ladder structure;

identifying the smallest current carrying branch of the first plurality of resistive branches which form the first R-2R resistive ladder structure; and

sequentially deactivating each of the first plurality of resistive branches, beginning with the largest current carrying branch and ending with the smallest current carrying branch.

17. The method of claim 15 , wherein selectively activating or deactivating each of a first plurality of resistive branches which form a first R-2R resistive ladder structure through one of a plurality of pull-up switching transistors, further comprises:

identifying the largest current carrying branch of the first plurality of resistive branches which form the first R-2R resistive ladder structure;

identifying the smallest current carrying branch of the first plurality of resistive branches which form the first R-2R resistive ladder structure; and

sequentially activating each of the first plurality of resistive branches, beginning with the smallest current carrying branch and ending with the largest current carrying branch.

18. The method of claim 15 , wherein selectively activating or deactivating each of a second plurality of resistive branches which form a second R-2R resistive ladder structure through one of a plurality of pull-down switching transistors, further comprises:

identifying the largest current carrying branch of the second plurality of resistive branches which for the second R-2R resistive ladder structure;

identifying the smallest current carrying branch of the second plurality of resistive branches which form the second R-2R resistive ladder structure; and

sequentially deactivating each of the second plurality of resistive branches, beginning with the largest current carrying branch and ending with the smallest current carrying branch.

19. The method of claim 15 , wherein selectively activating or deactivating each of a second plurality of resistive branches which form a second R-2R resistive ladder structure through one of a plurality of pull-down switching transistors, further comprises:

identifying the largest current carrying branch of the second plurality of resistive branches which for the second R-2R resistive ladder structure;

identifying the smallest current carrying branch of the second plurality of resistive branches which form the second R-2R resistive ladder structure; and

sequentially activating each of the second plurality of resistive branches, beginning with the smallest current carrying branch and ending with the largest current carrying branch.

20. The method of claim 15 , further comprising controlling a time delay for the selectively activating and deactivating of the first plurality of resistive branches and the second plurality of resistive branches to control a slew rate of the output driver.