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Design007-Dec2020

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80 DESIGN007 MAGAZINE I DECEMBER 2020 Why do we need capacitance between the power and ground planes? The power planes are just two different levels of voltages. Power is typically at five or three volts, which powers the chips, and ground is zero volts or the return path for the voltage. When you think of volt- age, remind yourself it must be a loop. What you send out to power the chip, you have to get back to ground to complete the path. Basic electrical law states that every power or signal line needs a ground or return. The reason we need capacitance on the power and ground plane is the planes are full of electrical noise, which interferes with the signals. Every time we make the voltage sig- nal turn on or off quickly, such as in a chip or power transistor, we get reflections and noise spikes in the power traces. The quickly rising voltages are driving signals into a low imped- ance. This causes an initial dip in the power line, then we get a spring-like voltage snap return, which causes a spike, and this electri- cal noise is embedded right in the power and ground plane signals. To get rid of this electri- cal noise, we can use the ability of a capaci- tor to absorb and fill in the electrical voltage variations. Capacitance acts like a small battery. The capacitor absorbs voltage increases and then releases the stored voltage during a decrease in the present voltage condition. Because the capacitor is like any battery, it can only absorb so much and so fast. The smaller the capacitance value, the faster it can absorb small spikes in voltage (i.e., a small battery is quicker to charge), but the smaller the capaci- Buried Capacitance Power Planes Consider This by John Talbot, TRAMONTO CIRCUITS

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