07 September 2011

App Note 31

"Linear circuits for digital systems: Some affable analogs for digital devotees." 16 pages.

This app note is a time capsule from 1989, a window of time when "Linear Circuits for Digital Systems" didn't refer to high-fidelity audio interfaces, high-resolution video, or RF systems for networking. Instead, this note discusses unique power supplies for memories, voltage-dropout detectors, and clock circuits.

Several of the circuits are additional applications for the LT1070 family, particularly targeted to the stringent requirements of in-situ flash memory and EEPROM reprogramming. Various buffer circuits are used to present clean and well-controlled (read: free from overshoot) voltage transitions to the memory-programming pins to avoid damage. Some of these buffer circuits use the LT1010 (Figures 4 and 9), and one uses a high-current discrete buffer design (Figure 7).

Other circuits included here are voltage-dropout detectors, both for the AC line (Figure 14) and the DC supply (Figure 16), a crowbar protection circuit (Figure 18), and a power-on reset generator (Figure 20). I think the best circuit (the circuit that is still the most useful today) is the SCR-crowbar circuit in Figure 18. The main section concludes with a watchdog timer (Figure 22), and some crystal-oscillator circuits repeated from App Note 12 (Figure 24).

Appendix A is a primer on flash memory, guest written by Saul Zales from Intel, extolling the virtues of flash memory. This appendix is another interesting time capsule, especially the last line in Figure A2: Remember when updating code in flash memory was a novel concept? Remember when the only option to download new code was a floppy disk or a serial link? Good times, good times. Flash memory seemed like a revolution after having to crack open machines to pull and replace standard EPROMs.

Appendix B discusses transmission-line effects, and how voltage overshoot can destroy memory chips. Appendix C again discusses the physiology of the LT1070 family.

The best quote is on page AN31-1, with the footnote: "While I certainly wouldn't wish lifetime employment on a digital circuit board to anyone, the reality is that the need exists. (Footnote: I suppose it's not all that bad. Some of my best friends are digital circuits. If I had a daughter, I'd even consider letting her go out with one.)" I suppose it's a good thing that Jim only had a son, so this footnote confession never had to be put to the test.

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