Why power sequencing is required?

          Now a days high speed processors used everywhere. Which require multiple power supplies that generate different core voltages and I/O voltages itself. So there is proper ordering of power supplies to up and down the device, this is mandatory for proper device operation & long term reliability.This note shows some of power sequencing requirements.

• Improper power sequencing to device may occur immediate fault in device or sacrificed it's long term reliability.
• When an active power supply & inactive power supply is fed to device, can stress electrostatic discharge protection and other interfaced which deal with different voltages.
• When designing with multi-power supply logic, timing interval between IO voltages & core voltages during its power ON/OFF condition should be given. During designing this kind of logic, each power supply is isolated by structure which may break (means become forward bias and conduct when it's not suppose to be!!) when supplies are not in sequence.
• Power sequencing is require "To avoid system level bus contention"
• Some time I/O power supplies and peripherals power supplies are connected a common points. So when core voltage & I/O voltages applied at same time, some of data from peripherals come to processor & processor data goes to peripherals on same bi-directional bus which may oppose each other.

• "To avoid Latch up problem"
• CMOS technology is used to fabricate most multi voltages devices & chips. 
• Latch up occur when output voltage of CMOS drops below ground level due to undesired noise spike or an improper circuit hookup.
• Due to this insufficient current flow inside CMOS, it'll create low resistance path and it'll conduct current.
• Only way to stop it is to reduce current below circuit level. This can be achieve by removing power from device only.
• To avoid latch up problem, provide proper power sequence is advisable.
• Most likely this problem occurs in pad drives when large voltage transients with large amount of current.
• Some processors support both sequence, IO power supplies before core supplies & Core supplies before IO power supplies. Power sequencing can be achieve using power sequencer ICs and power management ICs. 

Recommendation is 100 ms between one power supply rail valid and next power supply rail in sequence starting to ramp.

Every processor designer provide power sequence in its datasheet. Keep RESET signal low during power sequencing.

Types of Capacitors And Their Application

Capacitors comprise the largest variety of electronics components. There are many type of capacitors,great variations in their performance, many methods of packaging and making, dozens of major manufacturers, not to mention new types constantly being introduced with specific application and performance.

As a result, capacitors often cause lots of problems for home brewers. Hopefully this blog will take some of the mystery out of the myriad of capacitors available.

Principal capacitor type

• Disk Ceramic Capacitors

Disk capacitors consist of two metallic plates separated by a ceramic dielectric, whose area and spacing determines the capacitance.

These caps are low cost and suitable for many application. Their main disadvantage is high capacitance changes with temperature,except for the "NP0" varieties that are temperature stable.

 These caps are the most commonly used for general purpose circuit but the non-NP0 types should be avoided in frequency determining circuit.

• Monolithic Ceramic Capacitors
  
  
  
  
  

Alternating layers of electrodes and ceramic dielectric allow higher capacitance in physically smaller packages. Their characteristic are very similar to disk ceramic. They are encapsulated in epoxy to withstand insertion,soldering and solvent cleaning by automatic PCB assembly machines. Introduced for mass production, they are inexpensive and available from surplus dealers.

• Polyester Film Capacitors
  

Polyester Film use layers of metal and polyester dielectric to make a wide range of capacitance in relatively small packages at low voltages. These have become the standers caps for DC applications. The "rolled" film layers cause high dissipation and capacitance vs. temperature problems, and should be used carefully in high frequency or high current application.

• Polyethylene Film Capacitors
  

Polypropylene films use layers of metal and polypropylene dielectric films virtually identical to Polyester Film Caps. The polypropylene, however, is a dielectric offering a higher breakdown voltage than polyester, and thus more suitable for high voltage applications,such as switching power supplies. They also have low loss factors and good capacitance stability making them a good choice for high frequency applications, including oscillators and other frequency sensitive circuit. The main disadvantages are a slightly higher cost, and larger physical sizes over other film dielectric capacitors.

• Silver Mica Capacitors
  

This is a type of capacitor known as metalized film capacitor, in that the electrodes are a metal deposited by a spluttering process onto the dielectric film. Silver Mica's use a mica film dielectric with thin layer of deposited silver forming the electrodes. These are very stable capacitors for high frequency circuits and preferred choice for VFO and oscillator circuits. The main disadvantage are their higher cost, low operating voltages, and sometimes hard to find from hobby vendors.

• Polycarbonate Film Capacitors

These capacitors have become the standers for high stability MIL-SPEC film dielectrics. Their very low dissipation and extreme temperature stability make them almost the ideal capacitor --at a price! They are very expensive capacitors and not available from hobby vendors, but listed here in the event you have the opportunity to appropriate some!!!.

• Electrolytic Capacitors
  

Aluminum Electrolytic are the most common, inexpensive electrolytic available from hobby vendors. They are made similar to the polyester film, using aluminum foil electrodes and a dielectric material rolled into layers to increase the effective plate area to from high capacitance in small packages. The aluminum foil is "wetted" with a chemical   agent to assist in conduction and increase the dielectric property when a DC voltage is applied.           

 This wetting agent can dry out after long periods of no use, or exceeding the rated voltage, causing a breakdown of the dielectric and component failure(usually a short circuit between the terminals). This is why electrolytic are often found shorted in older equipment that has not been powered for years. This is seldom a problem with equipment that is periodically powered up. These inexpensive aluminum electrolytic caps are suitable in all QRP application.

• Tantalum Capacitors
  

Tantalum's are a most unusual process that yields a high reliable electrolytic with a long life. Tantalum pent-oxide powder is mixed with manganese dioxide electrolyte and formed into "pellet" forming both the dielectric and the positive electrode plate. Graphite or silver plating forms the negative plate. The "pellet" form a very large effective plate area, and thus very high capacitance in very small packages. Both wet and dry electrodes are used, and called wet or dry tantalum. There are few QRP applications where tantalum's would be must, but if you have them--use them! The chief disadvantages are higher cost due to complicated manufacturing process, and ensuring you never reverse the polarity. A small positive voltage on negative terminal can fuse the "pellet".

Why capacitors are used with crystal oscillator?

          There are two kinds of crystal oscillators. One operates at what is called the "series resonance" of the crystal. This resonance is the frequency at which the (AC) impedance between the pins of the crystal is almost zero. The frequency is independent of how much capacitance happens to be in parallel with the crystal - its inside the oscillator and part of the circuit board, etc. But, even frequency that the oscillator runs at.

          The other kind of oscillator oscillates at "parallel resonance"of the crystal. At this frequency, the impedance from pin to pin of the crystal is almost infinite. This frequency depends on how much capacitance is connected in parallel with the crystal. This parallel capacitance is called "load capacitance". Generic signal-inverter oscillator is this kind of oscillator.

          The common oscillator connection is for the crystal to be connected from the inverter output to the input. And, there is a capacitor at each end of the crystal to ground. The NET load capacitance is SERIES equivalent value of those two capacitors.PLUS stray capacitance from the circuit board and the guts of the oscillator. Suppose that the crystal is rated for 22pF load capacitance. The stray capacitance is about 7pF. So, that leave 15pF to be made up from discrete external capacitors. If the external capacitors are equal, then their equivalent is half of their individual value. Thus, in this case, we would want a pair of 30pF capacitors.

          It should be made clear that the same crystal exhibits BOTH series and parallel resonances. If a crystal is ground so that it oscillates at 8.000MHz in oscillator that runs in series resonance,this crystal is called an "8.000MHz series resonant" crystal. But this same crystal, with maybe 22pF of parallel capacitance, might have a parallel resonance of 8.1MHz. Same crystal, different oscillator circuit. This SAME crystal could, if the manufacturer wanted, be sold as an 8.1MHz/22pF parallel resonance crystal.

• Most microprocessor oscillators run in parallel resonance mode.

Why single ended trace require impedance of 50 Ohm?

Three factors which is highly influence PCB trace impedance calculation

1.crosstalk: dramatically various with nearest ref plane.Cutting height by half reduces the crosstalk by 4 times.

2.The height of the trace above the nearest ground plane: It should be minimum. Less height means less radiation.

3.Less height means less radiation..it 'll reduce the impedance: which is less susceptible to capacitive loading.

• As per this three consideration PCB need to be thin as possible as to reduce distance between reference plan and signal plan.So we can not compromise with height of trace.What stops you from pressing height of trace down to zero is the fact that most chips can not comfortably drive impedance less than about 50 Ohm.

  EXCEPTIONAL: The old National BTL family drives 17 Ohm 

                             Rambus which drives 27 Ohm  

• Now impedance is inversely proportional to cross sectional area of trace.We have another parameter width of trace to maintain impedance of trace which is also not possible because all high speed boards are highly dense board.

• It not wise to use always 50 Ohm trace because an application like NMOS 8080, which works on low operating frequency(ex. 100 kHz) doesnt have Electromagnetic Interference and that type of application can't drive 50 Ohm.For this kind of processor you should use thinest and heighest impedance line to minimize operating power. 

• As  per mechanical view increasing impedance require decreasing in width of trace.The tiny lithography that high impedance trace require becomes difficult to fabricate.Where as 50 Ohm trace gives much wider trace to be manufacture.

As per above conditions we should use 50 Ohm impedance of single ended trace. Other differential pair traces have impedance other than 50 Ohm like USB having impedance of 90 Ohm!!!. 

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