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 the CPU has so many power supply pins. So, the question arises, why the CPU has so many power supply pins.

There are some very very important reasons for using so many power supply pins in the 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 of conducting 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 high voltage drop which will reduce the input voltage.

2.      Large heat produced.

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 pin will be (100A/200) = 0.5A which is more reliable and efficient.

Different Voltage Levels

CPU has many circuits such as many digital modules, and high-speed drivers inside it. So all the circuits 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 circuits 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 the power supply, the pin acts as an inductor. As the CPU operates at a very high frequency, the pin produces more inductance effects. The 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 much smaller than the individual inductance. So when a multiple no. of pins is used for the power supply, the inductance effect decreases.

Impedance

The impedance effect is seen where the inductance and capacitance are present. So using so many pins for the 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 highly impulsive transient loading.

Power Distribution

Using so many pins for power supply to the CPU helps to power distribution to all circuits inside the CPU. If a single pin is used for the 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
• Comparison between RTL, DTL, and TTL Logic Families

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