RIPPLE declaration

The overall ability of a power supply to attenuate disturbances at its input is expressed as its PSRR (power supply rejection ratio). In graphs, PSRR is usually plotted as a function of frequency. We will invariably find that the rejection ratio is very low at higher frequencies. One reason for this is that the Bode plot cannot really help because the open-loop gain is very small at these frequencies. The other reason is, even a tiny stray parasitic capacitance (e.g., across the power switch and inductor) presents such a low impedance to noise frequencies (whatever their origin) that almost all the noise present at the input migrates to the output unimpeded. In other words, the power stage attenuation (which we had earlier declared to be Vo/Rin) is also nonexistent for noise (and maybe even ripple) frequencies. The only noise attenuation comes from the LC filter (hopefully). 

The most important datasheet parameter is the ripple current rating. This is typically stated in Amperes RMS at 120 Hz and 105°C. It essentially means that if the ambient temperature is at the maximum rating of 105°C, we can pass a (low frequency) current waveform with the stated RMS, and in doing so we will get the stated life. The declared life figure is typically 2000 hours to 10,000 hours under these conditions. Yes there are lower grade 85°C capacitors available, but they are rarely used, as they can hardly meet typical life requirements at high ambients. 

The declared phases, frequency and voltage and permitted frequency and voltage variation are in Regulation 30, but there is no requirement to provide a sine wave supply for a.c. or any limitation on ripple for d.c. supplies. 

A function is flexible. As the package body of ARITH TYPES shows, the parameters passed and returned from a function do not have to be constrained. If die function is designed without constraints it can be used in many different designs or several times in the same one, performing operations on n-bit objects. To achieve this flexibility, attributes are used to provide information about each object passed to the function. The RIPPLE functions use the Left and Reverse.ramge attributes discussed further in Box 6.3. A component, on the other hand, is constructed from an entity declaration and an architectural body. This means that the size of each I/O path must be determined at tfie component s compile time, reducing its flexibility. 

A function is very similar to a Process statement in its operation except that it cannot contain signal assignment or Wait statements. This means that a function caimot be used to create sequential logic. As the RIPPLE functions illustrate, variables that are local to the function can be declared in the declarative part and its execution is sequential. All objects declared locally are discarded when the final result is returned. The s)mtax of a function and other important details are given in Box 6.4. 

The third architecture RIPPLES takes this direct access a step further and dispenses with the Use clause altogether. Both the SIGNED type declaration and the RIPPLE function are accessed directly. This architecture accesses the second RIPPLE function, overloading the first by the number of parameters that are supplied. This is only one way of achieving function overloading Box 6.8 discusses these methods further. Another approach selects the appropriate function by the type of the parameters that are supplied. 

There are four fundamental design abstractions in HardwareC block, process, procedure, and function models. At the top level, a design is described as a block. A block describes the structural relationship and physical connectivity between diff(a ent components of a design. It has a declarative semantic and describes an interconnection of logic and instances of other blocks and processes. For example, a block model that describes a ripple chain of adders is shown in Figure 2.1. 

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