Definition of Decoupling Capacitors and its uses

In this article we are going to discuss about what is Decoupling Capacitors and what is its use?

 

Definition of Decoupling Capacitors and its uses


A decoupling capacitor is a capacitor, which is used decouple the critical cells from main power supply, in order to protect the cells from the disturbance occurring in the power distribution lines and source. The purpose of using decoupling capacitors is to deliver current to the gates during switching.

Decoupling provides a low impedance path from the power supply to the ground. Therefore, choosing a low-inductance but high-value capacitor (low impedance) is very important.


Decoupling capacitors serve two purposes:

First, they are sources of charge to devices that are sinking/sourcing HF currents. The caps act like charge buckets, quickly supplying/accepting current, as required by the devices located in the immediate vicinity. Decoupling caps reduce the voltage sags & GND shifts.

Secondly, the capacitors provide a path for the HF return currents on the PWR plane to reach GND. If the capacitors are not available, these currents return to GND through I/O signals or PWR connectors, creating large loops and increasing radiation.

 

Bypass caps self-resonate at a specific frequency & this phenomenon must be considered. For noise signals a few sometimes above the self-resonant frequency, the bypass capacitor becomes inductive & ineffective in filtering these signals. More capacitance is not always better. Leads on components & in IC packages typically add about 8 nH/inch. This is a series LC circuit with the capacitor, causing it to appear as an inductor above the resonant frequency. Generally SMT capacitor installation has about 3 nH of series inductance from all sources.

 

Thus, 100 pF caps typically are adequate for RF bypassing on many current products. With a total inductance of 3 nH, these caps resonate at about 290 MHz. However, 500 pF caps with a total inductance of 3 nH will resonate at about 130 MHz & must be used with care in high-speed systems.

 

·         Keep the trace lengths less than 0.2 inches.

·         Adhere to the recommended bypass requirements specified by the component mfrs.

·    It is best to try to use for bypass capacitor at each component. However, If it is not possible to bypass every active component, skip the slower devices in the interest of the HF devices.

·       Alternate 2 values of bypass capacitors. Always be sure the values are different by at least 2 orders of magnitude. Like that 0.1 uF and 0.001 uF capacitors could be used.

·         Distribute the bypass capacitors in a checkerboard pattern.

·      Carefully match the type & value of capacitor to the range of frequencies it must bypass (i.e., tantalum caps are more effective at HFs than aluminum caps, & caps of different values are effective at different frequency).

·     Use SM bypass caps. They are more effective than TH caps because they eliminate inductive leads.

·     Each bypass capacitor should have its own via to GND & PWR. The lead inductance can’t be totally place leads inside the IC, but it can be minimized.

·      Bypass all Power leads with large capacitor  (for example 22 uF) where the Power first enters the board. • Don’t use feed through holes between the bypass capacitor & the circuitry, its intended to decouple. This is generally applicable to high-speed digital circuitry such as Schottky or ECL, or low level analog circuit.

 

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