why there are two pair of vcc and grond bases on some IC ?

Question

why there are two pair of vcc and grond bases on some IC ?
Why there are not just one pair +,-?

why there are 2 pair of Vcc and Ground bases on IC 8086?
thanks.

Answer 1

For some very fast digital circuits, there are actually many Vcc and Gnd leads.

The reason for this is:
When the outputs switch from low-to-high or vice-versa, the output current also switches. For a typical CMOS output driver the current may go from 0 to 20 mA in 500 picoseconds. This is a rate (di/dt) of 40 MegaAmps/second. The power supply must be able to produce this, but more importantly the IC must be able to accept the additional current.

Every pin on the IC has some inductance. Shorter, smaller pins make for smaller inductances, and surface mount leads make for the smallest inductances, but there will still be some inductance present.

From basic electical circuits and physics, the voltage across an inductor (the lead of the IC, etc.) is L * di/dt. With di/dt in the 10's of MegaAmp/sec. range it doesn't take much "L" before you get voltages that rival the logic switching levels. When you can't reduce the size of the lead any more the only other thing to do is add more power leads (and ground leads) to cut down the inductance.

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Answer 2

It all depends on the part. If the device is any type of amplifier or D/A, A/D converter then often the first pair (5V and gnd usually) handle any internal logic in the device. The second pair then usually handles the actual voltage used by the amplifier or converter itself. Another reason is if the device pulls a lot of current. In that case, there are multiple power signals to prevent the device from pulling to much current off of one pin which could either burn the pin up or traces going to the device itself.