Even the smallest modern electronics systems now require more than one power-supply voltage. The challenge is producing the required voltages in as little board space as possible. Designers can produce three output voltages using a single controller IC, thereby reducing the supply's overall cost and size.
Figure 1 shows a circuit that generates 1.2-, 3.3-, and 5-V outputs. U1 (LTC3727-1) is a two-phase, two-output synchronous step-down controller with an input voltage range of 4 to 36 V and an output range of 0.8 to 14 V. These specifications make it suitable for a wide variety of applications, from computers to automotive. This particular circuit takes advantage of ceramic input and output capacitors to minimize ripple, cost, and size.
The third output is created using a MOSFET and a transformer. Regulation of the 5-V third output isn't as tight as the other two outputs, but it's good enough for most applications. Figure 2 shows that load regulation for the 5-V output remains within a ±3% window over a wide range of output conditions. Figure 3 depicts the output ripple of each of the three supplies.
Because the controller IC has such a wide output range, the first output in Figure 1 can be modified to generate the 8-V supply commonly required by DVD players, digital tuners, and car-navigation systems. External components limit load currents, which can be adjusted for higher output currents if necessary.
This particular controller IC (U1) features a 20-µA shutdown current, which is important for increasing battery run time in applications that are idle for significant lengths of time.
For some applications, the operating frequency must be carefully selected to avoid certain problem frequencies. There are two ways to set the frequency over a 250- to 550-kHz range. A dc voltage from 0 to 2.4 V can be applied at the PLLFLTR to set the frequency, or the PLLIN pin can be synchronized to an external oscillator.