Application-specific ICs (ASICs) have the benefit of integrating a maximum amount of circuitry for a particular function on a single IC. This saves board space and cost. However, ASICs also have disadvantages for low- to medium-volume products that should be remembered when making design decisions among integration alternatives.
With obvious size and cost benefits, ASICs seem to be the way to go if the function you need has already been integrated. System-level design then becomes a simplified task of following the "typical circuit" in the data sheet application examples, making minor adjustments for system specs, and voila! Board-level design is trivialized and the best designers will eventually all work for semiconductor companies.
This scenario is fictional or at least highly idealized. A just-right ASIC can be adjusted parametrically to the application while maintaining space and cost advantages. Yet the goal of commercial ASIC design is to make it as general as possible in its specificity. If the ASIC is optimized for one particular design, it is a custom IC, of lesser use for other applications. If it is too general, it is likely to be suboptimal for any particular application.
In reducing package size and cost by reducing pin count, ASICs also reduce observability. In practice, a functionally-rich ASIC will lure the unwary designer to adapt it to a different application than was intended. For instance, most power-factor controller (PFC) ASICs are based on a boost (common-active) converter topology. But what if you want to use a SEPIC instead? It has the advantage of eliminating power-on inrush current, and the current-limit thermistor -- a somewhat large and expensive part.
Suppose you try to adapt a boost-converter ASIC to a SEPIC topology. What happens? First, the incremental (small-signal) transfer function is different. Consequently, the ranges of voltages and currents are different, as are parts values that are found in control loops. These value changes affect loop behavior, including stability. Additionally, ASIC control loops depend on a certain polarity of phase input at their feedback pins. Inversion requires additional circuitry. More subtle difficulties involve layout effects because your circuit is different.
Another problem with ASICs is that many come and go. Because they are specific, they tend to have a short market life. While I was designing a PFC (power factor correction) circuit years ago, I chose to use a Unitrode UCC3858. While I was making progress on the design, TI acquired Unitrode and cancelled the part. Perhaps it was a good marketing decision for TI but not for my project. If I had used well-established, multiple-sourced ICs, I would have avoided this problem, but no legacy ICs for PFC were available.
A semi-discrete design uses more components and board space than an ASIC but is based on the principle of using only well-established parts that are multiple-sourced in high volume, are consequently low in price, and continue to be available. In the PFC case, examples are the UC3843 PWM current controller, commodity op-amps and comparators, and a legacy part such as the CA3080 for the PFC multiplier. These parts are produced in high volume and have reached commodity price levels. The sum of their costs can be less than an ASIC for lower-volume designs.
The disadvantage of additional board space is offset by more accessible nodes to probe in test, thus reduced production test cost. For design, such access is obviously desirable. For long product lifetimes, part obsolescence problems are minimized or avoided. Furthermore, semi-discrete designs become an "elastic" technology base from which larger adaptations can be made, saving redesign costs and reducing time-to-break even.
On the other hand, for high-volume designs, the tradeoffs are just the opposite. Few if any of the benefits of semi-discrete design apply. High-volume purchase of an ASIC over the product lifetime will sustain the interest of the supplier. The lack of circuit observability is overcome with accurate simulation and automated testing. It is the supplier's responsibility to insure that the IC meets its specifications. ASIC integration has the expected benefits.
Sometimes design trends should not be taken for granted. They are not always best to follow for every project. While integration is generally a good trend, it can also produce undesirable side-effects for lower-volume products. Happily, all major semiconductor suppliers support parts suitable for semi-discrete design. In considering ASICs, it is sometimes best for low-volume designs to use only those that have proven their ability to endure.