1.8 Power management  (Page 6/7)

Table 1 summarizes some of the other key parameters for both devices.

Table 1. summary of other key parameters for both devices.

Each device has strengths and weaknesses in different areas – so you will need to consider which parameters are most important for your specific use case (end application) in order to decide which device to use. If the application is battery-operated or needs the smallest possible physical space for the design, the TPS62160 would be the best choice. If on the other hand, the application requires higher load current or input voltage tolerance, choose the TPS54140A.

External components – inductors and capacitors

In addition to the silicon components, the proper external passive elements (resistors, capacitors and inductors) are essential to the operation of a power-supply circuit. A rudimentary understanding of the non-ideal aspects of these passive components will help you properly select the right type of external components for your design. For example, an ideal inductor has zero resistance at DC, while an ideal capacitor has zero resistance at infinitely high frequencies. Furthermore, these ideal components retain their characteristic inductance (or capacitance) across the entire frequency spectrum.

Consider, for example, that the wires used to wind around the magnetic core of an inductor are made of good but imperfect conductors (like copper) and will have a finite, nonzero DC resistance (DCR). Thicker wires will have lower DCR, but that will result in a physically larger and more expensive component. So you will need to choose a component that optimizes low-enough DCR but still meets the cost and size requirements for a particular design.

As another example, the behavior of a real capacitor under different operating conditions is a function of its physical construction and the type of dielectric materials used in the component. The data sheets for these components should illustrate the variation in effective capacitance versus DC bias (operating voltage) (see Figure 8), as well as the chance in capacitance vs. operating frequency. See Reference 4 for a more complete discussion on external component selection procedures for power-supply circuits.

WEBENCH® design example

Most voltage regulator/DC/DC converter data sheets offer some amount of information on designing the overall power circuit, including example schematics and layouts. TI also provides the WEBENCH® free online power-supply design tool to assist with component selection and to provide an initial analysis of the circuit’s operating conditions. You can access the WEBENCH tool, as well as other design tools, from ( www.ti.com ) or at webench.ti.com . Figure 9 is an example of a design schematic for a (nominal) 12-V input, 3.3-V output converter with a load-current rating of 2-A output. From the data entry screen, you simply enter the tolerance of the input supply (in this case, 12 V +/-2 V = 10-V min, 14-V max input) and the desired output specification (3.3 V, 2 A). Several choices using different ICs will be offered, and you can select from these based on whether you want to optimize cost, size or efficiency.