25 March 2012

Scope Sunday 26

I finally(*) got a chance to fire up the Tektronix 566 that I purchased before Christmas. Here it is with eight independent traces on the screen (using two 1A4 four-channel plug-ins and independent time bases). It works like a champ. Now, as soon as I can get my 1L5 repaired, I'll be proudly replicating Figure 34 in App Note 35.

(*) Actually, I fired it up several weeks ago, but I didn't get a very good picture of it then.

09 March 2012

App Note 104

Load transient response testing for voltage regulators: Practical considerations for testing and evaluating results. 16 pages.

This app note discusses testing linear regulators under varying loads. Most of the circuitry in this app note is geared toward producing fast and large current pulses, like those shown in Figure 4. The main idea is shown in Figure 2; by turning the transistor Q1 on and off, the "regulator under test" can be put through its paces.

The next circuits allow for a continuously varying load level (instead of the discrete levels produced by the circuit in Figure 2). Figure 6 is implemented with a MOSFET device, while Figure 8 uses a BJT. The main trade-off is in the number of circuit trims; due to the large gate capacitance of the IRLZ24, the loop dynamics are (apparently) harder to tame.

The rest of the app note shows a number of measurements made with this test circuit. Figure 15 is particularly pretty, showing a swept-sine input from DC to 5MHz, which clearly shows the frequency range of the LT1963A's load rejection. Figure 16 starts a discussion on the effects of parasitics in the output capacitor. Sage advice appears in footnote 4, "Always specify components according to observed performance, never to salesman’s claims." This point is re-enforced by the presence of the 100-mV spike in Figure 30, due to a "poor grade 10uF" capacitor.

Appendices A (capacitor parasitics) and B (load regulation from the LT1963A datasheet) were guest-written by Tony Bonte and Dennis O'Neill, respectively.

Appendix C discusses proper probing and cabling techniques for these measurement. Due to the wide-band nature of the transients, attention to detail is required.
The termination at the oscilloscope end is not negotiable.
Appendix D shows an improvement to Figure 8, which doesn't require as many manual trims.

The app note ends with a cartoon. "Methinks the load doth protest too much (with apologies to Wm. Shakespeare)."

07 March 2012

App Note 101

"Minimizing switching regulator residue in linear regulator outputs: Banishing those accursed spikes." 12 pages.

This short app note discusses the use of linear regulators to "clean up" the unavoidable ripple and spikes that exist in the outputs from switching regulators (see Figure 2). As Jim describes,
In practice, all linear regulators encounter some difficulty with ripple and spikes, particularly as frequency rises... Large numbers of capacitors and aspirin have been expended in attempts to eliminate these undesired signals and their resultant effects.
Much of the app note addresses the measurement of ripple-and-spike rejection of various linear regulators (of course), so Figure 5 shows a ripple-and-spike simulator. This circuit produces controllable input ripple and spikes to the regulator under test. Figures 7 through 13 show the results of such measurements.

Appendix A sings the praises of ferrite beads and Appendix B discusses the limitations of using inductors instead of ferrite beads. Appendix C discusses wide-band submillivolt measurements.

Best quote (from footnote 2, discussing the reduction in spike amplitude achieved with an appropriately placed ferrite bead): "Dramatic" is perhaps a theatrical descriptive, but certain types find drama in these things.

The app note ends with a cartoon that praises ferrite beads. "Megahurts to minihurts converter."

02 March 2012

App Note 98 part 2

Figure 37 is a highly linear voltage-to-frequency converter, based on a quartz-stabilized switched-current charge loop. Q=IT (see App Note 14). The 0 to 10-kHz range exhibits 0.0015% linearity. I surprised that the LTC1043 is that good as a current switch (since it is designed as a good voltage switch).

The rest of the circuits are power converters, covering a wide range of applications, most of them high-voltage regulators based on circuits from previous app notes.

Figure 40 is an LT3468 flashlamp circuit, simplified by removing the red-eye-reduction capability (see App Note 95). Figure 44 reuses the LT3468 flashlamp controller to implement a 0 to 300-V variable supply. Figure 46 improves the output-ripple voltage by including a discrete-linear-regulator design on the output (see App Note 32). Figure 48 is a 5V-to-200V switching regulator for APD biasing (see App Note 92). Figure 50 is a wide-range regulator that provides 0 to 500 volts at 100 watts of output power (see App Note 35).

The final circuit, in Figure 55, isn't a high-voltage circuit, but instead is a load-sharing scheme between two low-voltage linear regulators.

Appendix A briefly discusses bandwidth and rise time for high-speed measurements, and Appendix B discusses connectors, cables, adapters, attenuators, and probes for high-speed measurements.

The app note concludes with a cartoon. "It's dark in there. You can think about circuits."