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Coupling/decoupling capacitors

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Nora

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Hello-
I'd always thought that the bypass caps that are placed next to an IC on a PCB were for noise filtering.
Now I'm reading that they are there for current supply during switching.
Any insight is welcome!
Thanks in advance-
Nora
 

Nora,
Some devices draw large amounts of transient current while they are switching. These currents can have very short rise times. Because of the inductance in the PWB tracks, this causes a momentary drop in voltage at the device power terminals. A capacitor placed close to the device provides a low inductance voltage supply, thus preventing the drop in voltage. The size of the capacitor is determined by the amplitude and duration of the transient vcurrent.
Regards,
Kral
 

Thank you for the great explanation.
What is PWB? Printed W Board?
Also did you mean transient current or voltage to determine the size of the capacitor?
Nora
 

Nora,
PWB means printed wiring board or printed circuit board. It is the transient current that determines the amount of drop, or sag, in the voltage applied to the device. The voltage drop across an inductance is equal to L dv/dt, where L is the inductance of the PWB track, and dv/dt is the rate of change of the current. Theoretically, if you know the inductance and the dv/dt, you could calculate the voltage drop across the inductance. In the real world, these parameters are not known exactly. As a result, designers use "rules of thumb" based on experience. A common rule is to use a .1uF capacitor for every other device, plus a 10uf capacitor at the point on the PWB where power enters.
Regards,
Kral
 

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