29 August 2011

App Note 29 part 2

The main section of this application note contains six major sections of converter circuits, from micropower supplies to high-voltage sources. There's a lot of good stuff here.

The first major section discusses several 5V to +/-15V converters (Appendix A also covers a 5V-to-15V converter, but a special case). Figure 1 is a "low-noise" converter that limits the slew rate of the transformer drive to limit the high-frequency harmonics at the output. This trick is a good one, although the circuit here is complicated by the overdrive (and underdrive) that the MOSFET gates require to fully turn on and off. Figure 4 goes for even lower noise by using sine wave drive (amplifier A1 is a 16-kHz Wien bridge oscillator!), but efficiency is only 30%. Figure 6 uses a single inductor (similar to the battery-powered design in Figure 22 of App Note 8). Figure 8 has low quiescent current.

The second major section discusses micropower converters. Figure 12 is a tricky micropower boost converter, using the VC pin on the LT1070 to duty-cycle the part into a low-power state when the output is lightly loaded. (Am being I paranoid? I think Jim is mocking me in footnotes 2 and 3 on page AN29-11). Figure 19A is a micropower buck converter using the same trick, and Figure 19B has multiple outputs. Figures 20 and 23 are single-cell 5V converters, using the LT1017 and LT1018 low-voltage comparators (these parts have been previously discussed: see App Note 15). Figure 23 uses an LT1070 at 1.5V, but it requires a start-up circuit to get at least 3V on the supply pin.

The third major section discusses high-efficiency conversion (other trick are discussed in Appendix E). Figure 32 uses synchronous rectification to reach 90%. Figures 35 and 37 show isolated and nonisolated designs that are 75% efficient.

The fourth major section discusses wide-input-range converters, starting with a 40V-to-60V-input telco converter with 5V output in Figure 38 (almost the same as Figure 4 in App Note 25). Figure 40 is a flyback converter that produces a -5V output from an input between 3.5V and 35V. Figure 42 is a buck converter and Figure 44 is a buck-boost converter, similar to the flyback in Figure 40. Figure 46 is another example of a linear regulator with a switching loop that controls the voltage across the linear regular (we've seen this trick before, too; see App Note 2 Figure 5).

The fifth major section discusses high-voltage and high-potential supplies. Figure 49 produces 1000V out (make note of the damper network), while Figure 50 produces a floating 1000V output. Figure 51 uses a piezoceramic transformer (foreshadowing of future topics, here) to achieve an isolation voltage up to 20 kV (the output voltage is only 10V, but the common-mode voltage can be huge).

The final major section discusses switched-capacitor converters, using the LT1054 and the LT1026, (similar to circuits we saw in App Note 11). He also includes some new tricks, like the diode-capacitor voltage multipliers in Figure 60, and the LT1020 duty-cycle trick in Figures 58 and 59. The main section of the text concludes with Figure 61, the high-power switched capacitor converter that we previously seen a couple times before: Figure 23 in App Note 3 and Figure 12 in App Note 8.

Best quote (so far) is on page 22: "These 'boingies' can be seen in trace B on Figure 43B." That deserves to be in the technical lexicon. Then again, I adore jargon.


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