Introduction

Current increase with integration

The 16 bit micro-controller with a 16MHz clock has a typical supply current of 100mA, with 62.5ns delay between current peaks. For a 32 bit micro-controller, the current reaches 3 Amperes. The current peaks reflect the phases of the internal sequences. The delay between two peaks is reduced to 2ns.

In the case of the last generation micro-processors, up to 20 amperes flow within the integrated circuit.

This oscillation is generated by the rapid variation of current flowing within the package. This slide shows the power supply fluctuations on a 0.7µm CMOS chip mounted on a Dual-In-Line (DIL) 28 pin package. The variations of current through the bonding and package inductance induce a 1Volt voltage drop (5V supply) that generates a severe ground-bounce. Notice the 150MHz oscillation, close from the FM band.

Noise linked to package

We illustrate here the common mechanism that transforms the internal current switching into a parasitic emission. Let us consider a quad-flat-packaging (QFP) with 44 pins, with a chip placed in the center of the package. The die occupies only a reduced surface of the package.


What is the noise amplitude ?

The die is connected to the package using bonding wires. The supply wire length from the die to the external printed circuit board is approximately 10 mm. The usual rule of thumb considers 1nH/mm.

What is the consequence of a 100mA peak with a rise/fall time of 1ns? The answer may be found using the approximation of the voltage drop in each inductance (Ldi/dt). We find around 1V, that is confirmed by measurement.