Semiconductor device

1. A semiconductor device comprising:

a power supply pad;

a prescribed circuit including a power supply interconnect;

a first interconnect interconnecting said power supply pad and said power supply interconnect;

a second interconnect being set to a prescribed potential;

a first electrostatic protective element providing a current path from said first interconnect to said second interconnect when potential on said first interconnect reaches a first threshold value; and

a second electrostatic protective element disposed between said power supply interconnect and said second interconnect, said second electrostatic protective element providing a current path from said power supply interconnect to said second interconnect when the potential on said first interconnect reaches a second threshold value.

2. The semiconductor device according to claim 1 , wherein said second electrostatic protective element provides a current path between said power supply interconnect and said second interconnect when said first electrostatic protective element provides the current path from said first interconnect to said second interconnect.

3. The semiconductor device according to claim 1 , further comprising:

a different power supply pad that is different from said power supply pad;

a third electrostatic protective element connected to said different power supply pad; and

a circuit area in which a different circuit which is different from said prescribed circuit is provided, said circuit area including a different power supply interconnect supplying electric power from said different power supply pad to the different circuit;

wherein the semiconductor device is devoid of an electrostatic protective element other than said third electrostatic protective element that is connected to the different power supply interconnect in said circuit area.

4. The semiconductor device according to claim 1 , further comprising:

a substrate in which said prescribed circuit is disposed;

an isolating layer which electrically separates said substrate into a prescribed circuit area including said prescribed circuit therein and into a different area which is different from said prescribed circuit area; and

an electrode, which is disposed in the different area, supplying said prescribed potential to said second interconnects.

5. A semiconductor device comprising:

first and second power supply pads supplied with first and second power voltages, respectively;

a first protection circuit coupled between the first and second power supply pads;

an internal circuit including a first power line and a plurality of transistors electrically coupled to the first power line, the first power line including first and second portions, the first portion being electrically connected to the first power supply pad; and

a second protection circuit coupled between the second portion of the first power line and the second power supply pad.

6. The device according to claim 5 , wherein the device further comprises a third power supply pad supplied with a third power voltage, and the internal circuit further includes a second power line connected to the third power supply pad.

7. The device according to claim 6 , wherein the second and third power voltages are substantially equal to each other.

8. The device according to claim 6 , wherein the device further comprises a third protection circuit coupled between the first and third power supply pads.

9. The device according to claim 8 , wherein the device further comprises a fourth protection circuit coupled between the second and third power supply pads.

10. A semiconductor device comprising:

first and second power supply pads supplied with first and second power voltages, respectively;

third and fourth power supply pads supplied with third and fourth power voltages, respectively;

a first protection circuit coupled between the first and second power supply pads;

a second protection circuit coupled between the third and fourth power supply pads;

a third protection circuit coupled between the first and fourth power supply pads;

a first internal circuit including first and second power lines and a plurality of first transistors electrically coupled between the first and second power lines, the first power line extending to be connected at a first end portion thereof to the first power supply pad, the second power line extending to be connected at a second end portion thereof to the second power supply pad, the first power line including a first part that is distant from the first end portion;

a second internal circuit including third and fourth power lines and a plurality of second transistors electrically coupled between the third and fourth power lines, the third power line extending to be connected at a third end portion thereof to the third power supply pad, the fourth power line extending to be connected at a fourth end portion thereof to the fourth power supply pad; and

a fourth protection circuit coupled between the first part of the first power line and the fourth power supply pad.

11. The device according to claim 10 , wherein each of the first, second, third and fourth protection circuits includes a diode-connected transistor.

12. The device according to claim 10 , wherein the second and fourth power voltages are substantially equal to each other.

13. The device according to claim 12 , further comprising a fifth protection circuit coupled between the second and fourth power supply pads.

14. The device according to claim 10 , wherein:

the second and fourth power voltages are substantially equal to each other;

wherein the first and third power voltages are substantially equal to each other;

the device further comprises a fifth protection circuit coupled between the second and fourth power supply pads; and

each of the first, second, third, fourth and fifth protection circuits includes a transistor of a diode-connected type.

15. The device according to claim 10 , wherein:

the first power line further includes a second part between the first end portion and the first part; and

the device further comprises a fifth protection circuit coupled between the second part of the first power line and the fourth power supply pad.

16. The device according to claim 15 , further comprising a sixth protection circuit coupled between the second and fourth power supply pads.

17. The device according to claim 10 , wherein:

the first power line further includes a second part between the first end portion and the first part and a third part between the first and second parts; and

the device further comprises a fifth protection circuit coupled between the second part of the first power line and the fourth power supply pad and a sixth protection circuit coupled between the third part of the first power line and the fourth power supply pad.

18. The device according to claim 17 , further comprising a seventh protection circuit coupled between the second and fourth power supply pads.

19. The device according to claim 10 , wherein the first internal circuit comprises one of a delay-locked loop (DLL) circuit and a phase-locked loop (PLL) circuit.

20. The device according to claim 10 , wherein the first internal circuit is formed in an area that is surrounded by a deep well region to be electrically isolated from a semiconductor substrate in which the second internal circuit is formed.