14 September 2011

App Note 35 part 2

The circuits in this app note are applications of the LT1074 step-down regulator chip. Several of these applications are variations on themes that we've seen in previous app notes. Given the number of notes that Jim wrote on the topic of switching power conversion (App Notes 25, 29, 32, and now 35), these efforts were a clear priority of the "Captains of the corporation" (to steal a phrase from the introduction of App Note 25).

Most of the circuits here are straight-forward applications. Figure 3 shows the basic step-down topology, while Figure 8 adds a coupled inductor to provide a negative output voltage. Figures 9 and 11 show negative output regulators. Figure 12 provides current boosting with a tapped inductor. Figures 14 and 15 show switching converters with linear regulators (similar to the circuits in App Note 32). Figures 16 and 18 shows micropower converters (achieving quiescent currents of 150 uA with the shutdown pin) that are similar in technique to Figures 12 and 19A in App Note 29.

The best circuits in this app note are the high-voltage applications in Figures 26 and 31. Figure 26 provides a 100-watt variable output up to 500 volts. (Note that the LT1074 in this circuit controls the input voltage to the high-voltage transformer. The LT1074 does not (directly) provide the HV output.) The 500-volt step response into a 100-watt load shown in Figure 30 is an impressive achievement. (The footnote on page AN35-13 references App Notes 18 and 6 for this design.)

Figure 31 shows a sine-wave-output converter. The circuit takes 28 volts in and outputs a 400-hertz 115-volt AC waveform. (Wait... 28 volts and 400 hertz? Smells like military.) Again, the LT1074 is pressed into service on the input side on the HV transformer. The switching drive to the transformer is provided by the "timing and reference half-sine generator" and the 74C74 flip flop. Figures 33a and 33b show the details of the zero crossing, where the "half-sine" reference waveform gets "de-rectified" into a full size wave. Pretty. Figure 34 shows the purity of the final sine-wave output.

I'll discuss the appendices on Friday.


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