High speed Digital Design fundamentals - Power-Speed-Package

• Power dissipated in any device is not just related to Icc current rating given in the datasheet
• Quiescent power dissipation is the power dissipated while holding a logic state
• Active power dissipated in a device = (Cycle frequency) x (Excess energy used per cycle)
• Energy per cycle = (0.5 x C x Vcc x Vcc) x 2
• Therefore Power = F x C x Vcc x Vcc
• The cycle frequency is half of the operating frequency if the system is toggling between 1 and 0. It is 1/4th if the system toggles randomly
• Load of backplane capacitance is nearly 2 pF / inch
• The inputs pins of ICs still load the drivers in OFF condition. Hence extra care has to be taken when fanout is really huge, or power dissipation is a matter of importance
• Ground bounce is a problem which occurs due to inductance of individual leads
• Vgnd, ground bounce voltage = Lgnd x d/dt (Idischarge)
•  Everytime the capacitor discharges and charges, the inductance of the leads, develop voltage everytime the level toggles
• In DIP flipflops packages which have very fast drivers and if connected to huge capacitive loads see double clocking errors. The clock seen from external world looks fine. The Error is internal to the IC!
• Internal temperature of the logic device is also called junction temperature and is equal to sum of ambient temperature and a value proportional to internal power dissipation
• Tjunction = Tambient + Theta JA x P
• Theta JA is called thermal resistance from junction to ambient, of the package
• Most common method to divide thermal resistance is junction to case and case to ambient.
• Thermal resistance varies according to the die attachment method, package material, size etc
• The bigger the package smaller is the junction to case thermal resistance 
• Case to ambient thermal resistance can be reduced to certain extent using external heatsinks
• Standard manufactures state the thermal performance of device with heat sinks assuming airflow to be around 400 ft/min, typical PC fan is around 150 ft/min .
• 400 ft/min is a lot of air