20 July 2011

App Note 8

This app note includes useful circuits for battery-powered systems, including voltage doublers, voltage inverters, and rail splitters. The LTC1044 is used in several circuits, as is Bob Widlar's low-voltage op amp, the LM10 (Figures 3, 16, and 18).

Figure 6 is an interesting low-dropout regulator, using a voltage doubler to supply the op amp to drive the base on the NPN pass transistor, achieving a dropout voltage of Vce,sat. Figures 9 and 11 show low-power switching regulators (Figure 11 includes a linear-regulator stage with a JFET pass transistor). Figure 12 shows an "inductorless" switched-capacitor regulator (which is exactly the same as Figure 23 in App Note 3).

Germanium! Excellent! Figure 18 uses germanium diodes and transistors. How often do you get to use 2N1194 transistors and 1N100A diodes? Can you even buy 2N1194 transistors anymore? I bet Jim was hoarding them, waiting for an opportunity to use them. (At first glance, I thought that Figure 18 was awfully strange, given that it doesn't use any Linear Technology parts. After a little more digging, I found out that Linear did second-source the National LM10 for a while (along with some other parts). Interesting. Given the contentious (and litigious) relationship between Linear and National in the early days, I'm surprised there was that much technology transfer.)

(With my over-active imagination, I can imagine how this situation arose: Linear Tech asked Bob Widlar to design a low-voltage op amp for them, and Widlar replied, "Been there, done that, it's called the LM10." When Linear said, "No, that's a National part; we want one of our own to make and sell," Bob gruffly replied, "Then get it from National." Disclaimer: I totally made this scene up.)

The best circuit is the single-inductor, dual voltage flyback regulator in Figure 22. I just think it's a neat topology to get a bipolar 15-volt power supply from a 6-volt battery using a single inductor, although I'm surprised that the clock input on the 74C74 is just labeled "30kHz input". I'm shocked that he didn't design a Wien-bridge oscillator to provide the 30-kHz input. Shocked.

Best quote (page AN8-8): "This circuit [Figure 18] will supply a 5V, 150uA load (about 25 CMOS SSI ICs) for 3000 hours from a single 1.5V D battery." Impressive.

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