03 February 2012

App Note 90

Current sources for fiber optic lasers: A compendium of pleasant current events. 24 pages.

This app note continues the fiber-optic-laser-diode theme from App Note 89. Now, attention is paid to the power supply of the laser diode itself, a precision current source. The app note begins with a discussion of performance requirements and protection issues for the laser. "The delicate, expensive load, combined with the uncertainties noted, should promote an aura of thoughtful caution."

Figure 2 shows a simple current-source topology, which is refined in various ways throughout the app note. Figure 3 uses a switching regulator for improved efficiency. I like his intuitive explanation that "It is useful to liken the switching regulator’s input (VCC), feedback (FB) and output (VSW) to the transistor’s collector, base and emitter."

Figure 4 uses an LT1970 "power op amp" to allow a grounded cathode on the laser diode. I have to laugh whenever I see a "power op amp" that comes in a TSSOP body. A real power op amp, like Bob Widlar's LM12, comes in a TO-3 package! (It also drinks scotch.)

The best circuit is Figure 6, which includes a full array of protection circuits: low-voltage drop out, a self-enable regulator, an input current clamp, and an open-circuit lock out. The oscilloscope traces in Figures 7 to 11 demonstrate the performance and operation of these features.

The rest of the circuits in the app note implement low-"noise" switching regulators to provide higher-power output (I heartily endorse footnote 2 on page 8). Figure 15 uses the LT1683, while Figures 17 and 19 use the LT1533 (discussed in App Note 70).

Appendix A discusses using regular diodes and a simple circuit (Figure A2) for simulating a laser diode. Appendix B discusses switching regulator "noise" measurement, using excerpts from App Note 70. Appendix C discusses current transformers for low-level measurements. The Tektronix CT-1 is very nice. The best quote is the footnote in Appendix C,
Diehard curmudgeons still using high quality analog oscillscopes routinely discern noise presence due to trace thickening. Those stuck with modern instruments routinely view thick, noisy traces.

The app note ends with a cartoon, ghost-written by Jim's wife.

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