When designers think of passive components, they think of manufacturing tolerances for inductors and capacitors that are typically ± 20% or ± 10%. That's okay in theory, but not when these components are used in an actual application. Applying dc bias to a ceramic capacitor or current to an inductor, at a particular frequency, changes the characteristics of these components. Hence, the name "active passives." For example, a 10- µF , 0603, 6.3-V capacitor can measure as low as 4 µF with 1.8-V dc bias at -30º C. A 3.3- µH inductor can measure as low as 0.8 µH when used in the actual application at 85º C.
Also, component manufacturers are becoming aggressive and perhaps releasing parts that are just good enough to keep up with competitors in the size-versus-value war. This is analogous to various practical situations — for example, a car manufacturer specifying 30 mpg in EPA tests, while real-life driving conditions bring 20 mpg. The consequence is more frequent trips to the gas station than you expect.
This example can be extended to portable power systems. Every component used in the various blocks within the system directly influences system performance. Key performance metrics in portable power systems include battery life, solution size, system resource friendliness, etc. For example, in portable power systems, more-frequent device charging negates the "portable" usefulness.
The system designer already took the first step toward achieving the key performance metrics by choosing a switching regulator to power various system blocks. The next step is to ensure the selected switching regulator works at peak efficiency. Key performance metrics for a switching regulator are efficiency, accuracy, and output-voltage tolerance (including transients, voltage ripple, solution size, etc.). To meet these performance metrics, the switcher IC must be a team player along with the external components.
External components for a switching regulator are typically an inductor, an input capacitor, and an output capacitor. Just like any game that requires team coordination for victory, the external components and the switcher must be matched and coordinated to meet the performance metrics expected of a dc-dc converter solution.
When designing switching regulators, compensation is optimized for a range of values for the inductor along with the input and output capacitors. The output-current capability of the part also depends on many factors, one of which is the inductance value.
This article addresses the key parameters that are affected, and what the system designer must know, when selecting external components for the smallest and most-efficient solution in a portable power system.
Capacitors
Let's focus on ceramic capacitors. They're ideal for portable applications when it comes to size, cost, and performance. And they're well-suited for high-frequency applications due to their low equivalent series resistance (ESR) and impedance at the switching frequency. Low ESR minimizes output voltage ripple, and the low impedance yields excellent filtering characteristics. Capacitors using Y5V-type dielectric have a poor temperature coefficient and can drop in value by 80% at 85 ºC . They're not recommended for portable applications, so this section will concentrate on X5R/X7R capacitors.
Figure 1 shows the history of case sizes for a 10- µF , 6.3-V, X5R ceramic capacitor. The key advantage of using a smaller case size is the savings in the board area for a switcher, and the height of the total solution. At present, major mobile-phone manufacturers have a maximum height limit of 1.2 mm for components used in the phone. As phone models slim down further, this limit can be expected to drop. Today's ceramic capacitors are well poised to meet this requirement.
So far so good, but does the system designer need to know something more about ceramic capacitors? Absolutely! For instance, dc-bias effects in ceramic capacitors must be considered when choosing capacitor values and case sizes. An improperly selected capacitor can wreak havoc in the system design from a stability viewpoint. Typically, dc bias occurs in ferroelectric dielectrics (Class 2), such as X5R, X7R, and Y5V capacitor types.
The basic formula for a ceramic capacitor is:
C = K × [(S × n)/t]
where C = capacitance, K = a constant, n = number of layers, S = overlapping area, and t = layer thickness.
The factors affecting dc bias are K, layer thickness, percentage of rated voltage, and grain size of material. An electrical field across the capacitor "polarizes" the inner molecular structure, which causes a temporary change in the K constant and, unfortunately, only lowers it. Smaller-case-size capacitors have a bigger percentage drop in capacitance with dc bias. The higher the dc bias voltage, the greater the percentage drop in capacitance for a particular case size. System designers must be careful when replacing a 0805 capacitor with a 0603 capacitor for space savings — unless the converter is tested with the intended type of capacitor. Or, the 0603 capacitor is recommended in the datasheet.
Figure 2 shows the effect of dc bias on several different capacitors over the ambient temperature range for a typical portable application. Looking at the dc bias characteristics, you can see that a 10- µF , 6.3-V, 0603 capacitor from manufacturer A has a capacitance value of 5.75 µF at 1.8-V dc bias and -30º C. Notice the distinction between capacitor and capacitance. Capacitance is the actual value of the capacitor as seen by the application. The same capacitor from manufacturer C has a capacitance value of 3.5 µF under the same conditions. In fact, the 4.7- µF capacitor from manufacturer A is almost as good as the 10- µF unit from manufacturer C.