Why CPU has so many power supply pins (e.g. Vcc, Vss)?

We know that most of the electronic ICs have a maximum of two pins for supply, one is Vcc or Vdd and another is GND or Vss or Vee. But CPU has so many power supply pins. So, the question is arise, why CPU has so many power supply pins.

There some very very important reasons for using so many power supply pins in CPU which are given below.

Current Handling

CPU pins are generally made using Ball Grid Array(BGA) technology. So the BGA balls are not capable to conduct a very high amount of current. Suppose a CPU dissipates 100 watts of power. So if the Vdd is 1V, then the current will be (100/1) = 100 Ampere of current.

So a single BGA ball cannot handle 100A current. If 100A current is transferred using a single BGA ball then the following problems will occur.

1.      Very Hig Voltage drop which will reduce the input voltage.

2.      Large heat produces.

3.      High loss occurs.

So if a multiple no. of Vdd pins are used, then this problem will be solved. If we use 200 Vdd pins then the current per pins will be (100A/200) = 0.5A which is more reliable and efficient.

Different Voltage Levels

CPU has many circuits such as many digital modules, high-speed drivers inside it. So all the circuit are not working at the same voltage level. To provide the different voltage levels different pins are used. The arrangement of providing different voltage or power to the different circuit inside the CPU is called power integrity. So Power Integrity and Signal Integrity are very important factors for using multiple pins.

Inductance Effect

It is seen that after examination when a single pin is used for power supply, the pin act as an inductor. As the CPU operate in very high frequency, the pin produces more inductance effect. Inductance effect causes sudden changes in power drawn by the CPU. As a result, the CPU behaves incorrectly.

We know that when the inductors are connected in parallel, the total equivalent inductance is very smaller than the individual inductance. So when a multiple no. of pins are used for power supply the inductance effect decrease.

Impedance

The impedance effect is seen where the inductance and capacitance are present. So using so many pins for power supply reduces the Impedance of the power supply coming into the chip. This also helps to reduce the noise and other issues during the high impulsive transient loading.

Power Distribution

Using so many pins for power supply to CPU helps to power distribution to all circuits inside the CPU. If a single pin is used for power supply, then the power is to be distributed from that single pin to all the circuits, therefore the length of the wire will increase in order to route the power supply to different modules inside the CPU which makes the circuit more complex, bigger.

Read Also:

• [Explained] Main Components of CPU and their Functions
• Mobile CPU VS Desktop CPU | The Actual Difference
• Difference between Microprocessor and Microcontroller and CPU
• Concepts of Semiconductor Memory in Digital Circuit

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