This app note discusses thermoelectric coolers (TECs) for laser-diode temperature control. The Figure 3 shows the main circuit, built around the LT1923 TEC-controller chip. Most of the app note (pages 3 to 7) discusses compensating the temperature-control feedback loop. As Jim says, "The unfortunate relationship between servo systems and oscillators is very apparent in thermal control systems." Figures 6 through 9 show a trial-and-error approach to setting the loop gain.
Best quote (with footnote):
Figure 6 shows large-signal oscillation due to thermal lag dominating the loop. A great deal of valuable information is contained in this presentation. (When a circuit "doesn’t work" because "it oscillates," whether at millihertz or gigahertz, four burning questions should immediately dominate the pending investigation. What frequency does it oscillate at, what is the amplitude, duty cycle and waveshape? The solution invariably resides in the answers to these queries. Just stare thoughtfully at the waveform and the truth will bloom.)Wise words.
Once the loop is "optimized", as in Figure 9, the temperature "stability" is measured (Figures 10 through 12). The short-term stability is good (Figure 10), but disturbance rejection (the response to changes in ambient temperature) in Figure 11 could be better. In this application, proportional control is good, but proportional-plus-integral control would be better. Jim likes to mock "control aficionados" (see footnote 7), but really, we can help!
The last section of the app note discusses switching-regulator "noise" (ripples and spikes) in the voltage to the laser diode. Like the LT1533 discussed in App Note 70, this controller also uses slew-rate control on the switching edges to minimize the high-frequency harmonic content. Figures 13 to 15 and Figure 17 to 19 show the advantage.
The app note ends with a cartoon. "That kind of talk makes me feel so coherent."