Inductorless Switching Regulators eliminate costly external component

Inductor-based Switching Regulators (SRs) have historically represented the preferred architecture for power supplies. Nowadays, for low-power and highly integrated electronic systems, embedded inductor-based SRs show several limitations that can be overcome by the use of inductorless SR architectures. This paper provides a qualitative and quantitative comparison between both types of SR in terms of implementation cost (Bill of Material, and pin count), and performance (efficiency, noise, and reliability).

Implementation Cost and Bill of material (BOM)

The first and foremost important limitations of induction-based SRs (Figure 1) are the cost and the size of inductors. In most cases, the inductor dominates the cost and size of integrated SR solutions. To address this, consumer product engineers dealing with cost constraints are tempted to choose cheaper inductors that dissipate more power, and result in power efficiency degradation. As an example, an inductor-based SR may loose up to 10% efficiency if used with a cheap inductor instead of a low ESR (Equivalent Serial Resistance) inductor. Consequently, the main advantage of inductor-based SRs, simply vanishes.

Figure 1: Generic boost (a) and buck (b) Inductor-based Switching Regulators.
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On the other hand, inductorless SRs (Figure 2) exploit only one or two small and cheap ceramic capacitors (flying capacitor C_f1 and C_f2) instead of an expensive inductor. In fact, for a given output capacity, the inductor of an inductor-based SR will roughly be 5 to 10 times more expensive than the flying capacitors of an inductorless SR, and will also occupy 5 to 10 times more volume.

Figure 2: Generic Inductorless Switching Regulator (a), Generic Inductorless Switching Regulator with integrated flying capacitors (b).
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For small current (<25mA), it is even possible to integrate those flying capacitors within a reasonable silicon area, providing an ultra integrated solution for efficient voltage conversion. The regulator then only needs one external output capacitor (C).

Studies published about integrated inductor-based SR using embedded inductors clearly show that this type of SR requires very high switching frequencies and finally turns out to be unattractive because of poor efficiency, huge silicon size, and RF process requirements.

Note also that external passive components are typically needed to compensate the conjugate pole formed by the LC filter (inductor and output capacitor) of an inductor-based SR, ensuring regulation loop stability and optimal performance. There is no such compensation needed for inductorless regulators, reducing even more the passive component cost.