03 August 2011

App Note 15

This app note suffers from a curious self-contradiction, to which Jim almost admits in the last section: "No commercially available logic, processor or memory family will operate from 1.5V. Many of the circuits described [herein] normally work in logic-driven systems." In other words, there are some great circuit designs here that work within 1.5-volt power supply, but what circuits interface to the ADC, VFC, and S&H? The A-to-D converter and the V-to-F converter have "logic" level outputs, and the sample-and-holds have "logic" level inputs (where "logic" means high (1.5V) and low (0V) voltages), but there is no logic family that works at 1.5V (or at least there wasn't in 1985; ironically, these circuits are probably more useful today than they were then).

But, if you go ahead and use Figure 13's flyback regulator (based on a Bob Widlar design) to produce a five-volt supply to power your logic, why not run the whole system from five volts? In fact, App Note 11 has already covered clever tricks for five-volt circuits, and App Note 8 has already covered power-supply design techniques for battery-powered systems (especially App Note 8 Figure 18). So what are we doing here?

Why do people climb Mt. Everest? Because it's there. Why does Jim design circuits for 1.5V operation? Because he can. Getting active circuits to work at 1.5V is deep magic, and Jim has it. Part of the magic is the LT1017/1018 comparator, part of the magic is Widlar's LM10 op amp, and part of magic is clever circuit design. Figures 1 and 3 show how to use capacitor charging and the low-voltage comparators to implement a V-to-F converter and a single-slope A-to-D converter, respectively. Figure 6 is a ramp-and-hold circuit using the LM10 op amp.

The best circuit is Figure 7, a more traditional sample-and-hold with a JFET switch and a charge pump to get enough negative voltage to turn the switch off. (Is a charge pump cheating? Maybe.) This S&H requires careful hand selection of the JFETs to get a below-500-millivolt pinch-off transistor for Q1, and 500-microvolt matching for Q2 and Q3. Do you want low voltage operation? You pay for it!

Figure 9 is another temperature-compensated crystal oscillator (see App Note 12 Figure 6), using a voltage-boost stage that is shown in Figure 11 here (and also App Note 8 Figure 18 and similar to App Note 6 Figure 7; he must have had a box of Triad transformers).

The App Note concludes with a box section on "Components for 1.5V operation" which basically just says "LT1017, LT1018, and LM10; good luck, sparky!" (He also mentions the LT1004 and LT1034 voltage references.) He discusses the pros and cons of silicon and germanium diodes and transistors (if you have a stash of germanium diodes and transistors, that is). There is also a graph of battery voltage versus life for carbon-zinc and mercury, which is quite interesting.

Best quote (for the specifications, from page AN15-8): "The LM10 op amp-reference runs as low as 1.1V; the LT1017/LT1018 comparator goes down to 1.2V. The LM10 provides good DC input characteristics, although speed is limited to 0.1V/us. The LT1017/LT1018 comparator series features microsecond range response time, high gain and good DC characteristics." Amazing.

No comments: