- IC By-Pass Design Information -

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Dielectric Material

The different capacitor types only come in certain values; How ever the application will push the design toward a particular type.

Dielectric Types

• PAPER Dielectric Capacitors are used in High voltage applications. Paper devices should not be used in a high moisture environment unless the device is in a hermetic case, the capacitance will increase, the power factor - insulation resistance - dielectric strength - and life will degrade. Paper capacitors have near 10% tolerances.
• Plastic FILM
  • Mylar: has a wide temperature change of 20% over -55 to +125C. Moisture absorption is half that of paper. Mylar is the same as polyester. The term Mylar is a trade name.
  • Polycarbonate: Out performs Mylar in all areas. Low Power factor, works best with AC. Polycarbonate has a low temperature drift (graph below), dissipation factor, and dielectric absorption. Use polyphenylene sulfide as a possible replacement.
  • Polyester: Replaces paper for many applications, and smaller in size. Polyester does not have the moisture problem that paper does, but does have a worse tolerance, at 20% ~ over -55 to +125C. May also be called PETE, or PETP. The Polyester Dielectric has an Absorption of around 0.20%.
  • Polypropylene: It has negative temperature coefficient.
  • Polysulfone: Has a very high operating temperature of +170C.
  • Polystyrene: Will only work to +65C. It has a flat to negative temperature coefficient.
  • Teflon: Has a very high operating temperature of +170C.
• MICA High Precession. The material is inert it will not change over time or temperature.
• CERAMIC
  The standard By-pass capacitor resides with in two main classes, Type I and II; However the major distinction resides with the temperature and frequency characteristics. Normally the frequency characteristics come first and then the temperature {It depends on the design application}.
  • NPO (Type I) Stable over temperature [[CGO] EIA temperature Coefficient Designation]
  • X7R (Type II) 15% variation over temperature
  • Z5U General Purpose [EIA temperature characteristic code]
  • Y5V General Purpose [EIA temperature characteristic code]

Ceramic and Electrolytic Capacitor info NIC Components

{Back to Electronic Capacitor Data Index}

Value x Temperature

Normally capacitors have a stated value at some frequency [1KHz], and some temperature [room temp]. However that capacitance value will change by some percentage [depending on dielectric] over temperature. A chart depicting the percentage change in capacitance depending on dielectric type versus temperature change is listed on; Cap Value Change Over Temperature. The chart provides data for Plastic Film capacitor dielectrics.

Derating guide lines for capacitors is listed on the Component De-Rating page.

{Electronic Capacitor Data Index}

Board Placement

Capacitor placement: to Integrated Circuits [ICs]

The Decoupling capacitor is used to decouple the IC from the power source. The By-Pass capacitor reduces the trace length from the power source to a trace length from the IC to the decoupling capacitor ~ a reduction from many inches to less then an inch. A reduction of 20nH an inch in trace inductance is achieved by introducing a decoupling capacitor in between the IC and its power source. Assuming the power source is a regulator 12 inches away, and the by-pass capacitor is placed within an inch of the IC, the trace inductance is reduced from 240nH to 20nH. The 20nH vaule is just an example.

For a PWB with out a Ground plane, place the capacitor equal distance between the Voltage and Ground pins. Or place the capacitor near the Voltage pin. Run the remaining line directly to the ground trace. Try not to pass into another layer with a via. Limit the number of vias (to zero), each via acts like a low pass filter. Running a long trace between the by-pass capacitor and the IC defeats the purpose of the by-pass cap. As the trace length increases, so does the trace inductance.

Things get better when you have a Ground plane. Place the via on the far side of the connection between the capacitor and chip. This insures the capacitor is not effected by the via [increased inductance]. Make the traces as wide as possible. If you have a near-by island be sure it's stitched with as many vias as possible so it represents a good power or ground connection.

Capacitor placement: on the PWB

Place the capacitors near the body of the device, leaving the shortest [and widest] possible trace. Additional larger tantalum capacitors may be placed around the board. The tantalum help to reduce the over-all impedance of the power plane. The larger value near-by tantalum also helps to re-charge the ceramic, instead of taking the charge from the power supply source. Placing Power planes adjacent to ground planes in the board stack-up also helps. It doesn't hurt to embed signal layers between a ground layer in the stack-up either, assuming you can add board layers. Refer to this page for a graph showing the frequency response developed from placing two capacitors in parallel Compound Capacitor By-Passing

Capacitor placement: Cap to Cap

Equivalent PWB Trace

A capacitor only has an effect with in a certain distance from the device. The more you move away from the capacitor, the less it does. The Inductance, and Resistance (of the Trace) tend to increase. This is the reason the App notes recommend placing capacitors on the reverse side off the board - over the (power) pin. It's a trade-off between the (low Pass) filter effects of passing through a via or an elongated trace. How ever these capacitors placed out across the board do have an effect in reducing the impedance of the board ~ the power and ground planes. Assuming most of the capacitors on the board are 0.1uF by-pass caps, then placing 2uF or 4uF around the board helps the impedance.

By-Pass Capacitors

The Classic approach. The capacitor is equal distance between both the positive and negative supply.

-- Placement: Dont ask for it or, plan for it, or depend on it - it just costs to much {hand routing}.
The point is, know if the design can handle another placement. This particular placement helps in noise suppression,
but hurts with critical signal routing. If this particular device needs noise suppression more then signal routing, then pick the cap placement first, otherwise let the computer pick the placement. The by-pass capacitor stores an electrical charge that is released to the power line [to the IC], whenever a transient voltage spike occurs. So by-pass capacitor provides a low impedance supply, thereby minimizing the noise generated by the switching outputs of the device.
Because the capacitor is used to supply current to the IC it wants to be close to the Vcc pin. The more current the device needs to supply - the larger the cap [or increase the number of same value capacitors].
If this a clock driver - watch the length of the trace {another page}, then give the Cap priority.

{Electronic Capacitor Index}

Decoupling Capacitor Value

The value of the Decoupling Capacitor that should be used depends on your load the IC has to drive. From the graphic above the load of device A [the driver] is device B [the receiver]. If device A has to drive one input at 3.3 volts then the load depends on the load of both inputs, taking into consideration the rise time of the signal. The input capacitance for a given device is parameter C_i in the device data sheet. If the receiver [B] has a load of 12pF [C_i] and the output driver [A] has a rise time of 1nS then the current required is: I = dV / dt or I = 12pF*(3.3v)/1nS. The current required = 39.6mA. If we keep the voltage droop to 3.0 volts, or a reduction of 300mV.
The capacitor then equals: C = I * dt / dV. C = 39.6mA * 1nS/300mV = 22pF
Choose a capacitor whose resonant frequency is at least as high as the corresponding edge rates of the switching signals,
where frequency response @ 1 / (3.5 × T_{rise, or fall}).

Equivalent Capacitor Circuit, was moved to Equivalent Capacitor Circuit

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