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When I asked the folks there how they mea- sured phase, they told me that they used an LCR bridge, and the response was sharp enough to warrant my stand- ing around for a while to see how they did it- The woman at the inspection station was mumbling something to the effect of "Looks like we gotta train the eng-HUH-neers again! The first test was magnetizing in- ductance at the primary. OK, I got that. Dallas MAX6971

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Dallas MAX6971 second test was shorting secondar- ies with the same primary connections. Yup, that's leakage inductance; I was still in the game.

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All right, I was still keeping up. The third test connected Dallas MAX6971 secondary in series with the primary. She looked at the measurement and switched the phase on the secondary for another reading. It was a pass-fail test, no data logged.

This test and its results left me puzzled. The front end of my brain was in total compression. Yet, I did have some fun- damental knowledge, and I thought I knew something about flux and MMF magnetomotive forceas I was the guy that designed Dallas MAX6971 darn structure.


She ran one last test to measure the dc resistance of each winding, which was reassuring in that it made sense to me. A given mean- length turn of a Dallas MAX6971 wire over given turns should have con- sistent dc resistance. Maybe I should have specified that fact in the drawing.

I thanked the Dallas MAX6971 assurance woman and went back to my "eng-HUH-neer- ing" office to ponder. If inductance is proportional to turns squared and I have solenoidal windings around a common low re- luctance path, a winding of five turns may measure an inductance of jlH, and another winding of 10 turns might measure an inductance of fxH.

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OK, that idea makes sense. What, then, of combinations and phase? Well, if I con- nect the same two windings together in series, there may be two outcomes. One outcome places the windings in phase and should yield a total of 10 plus five turns, or fxH. The other outcome places the windings out of phase and yields a total of 10 minus five turns, or jjlH. Therein lies the solution: It is possible to arbitrarily connect the windings Dallas MAX6971 determine phase by flip- ping the polarities of Dallas MAX6971 of the wind- ings in the connection and observing the inductances associated with each phase arrangement.


Note that for this experiment to work, the windings must share the same Dallas MAX6971 flux paths. Given this knowledge, it would also then be possible to take an arbitrary structure and determine the turns simply by unwinding a few turns and measuring the inductance before and after, or by sneaking in a few turns, again measuring the inductance before and after and noting the phase.

Contact Maury Wright at mgwright edn. These new devices are not just high brightness, but are high power as well. Single die with dissipations of 5W and multi-die modules with power in excess of 25 W are now available. The requirements of high efficiency and low dissipation dictate a switching power supply for this new generation of High-Brightness HBHigh-Power HP LEDs, as a voltage regulator and a current limiting resistor are no longer appropriate. High-brightness, high-power LEDs require a constant-current source to take full advantage of their ever-increasing luminous efficiency and vibrant, pure color. The topology of choice for this new breed of switching constant current sources is the basic buck converter.

The most convincing argument for using a buck converter is the ease with which this simple DC-DC converter can be turned into a Dallas MAX6971 source. This article will explain the selection of, or possible exclusion of, an output capacitor when designing a buck regulator for constant-current drive of HB LEDs.

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Figure la. Traditional Buck Voltage Regulator Figure lb. Regardless of whether a buck regulator is used as a voltage source or a current source, selection of the inductor forms the cornerstone of the system design. With an inductor in series with the output, the average inductor current is always equal to the average out- put current, and the buck converter naturally maintains control of the AC-current ripple. By definition, the LED drive is a constant load system; hence a large amount of output capacitance is not necessary to maintain V Q during load transients. No Output Cap Yields High Output Impedance In theory, a perfect current source has infinite output impedance, allowing the voltage to slew infinitely fast in order to maintain a constant current.

For switching regulator designers who have concentrated on voltage Dallas MAX6971, this concept may take a moment to sink Dallas MAX6971. Completely removing the output capacitor from a buck regulator forces the output impedance to depend on the inductor.


Over the typical switching regulator frequency range of 50 kHz Dallas MAX6971 2 MHz the ripple itself is not visible to the human eye. These limits come from increasing thermal losses at higher ripple current a property of the LED semiconduc- tor PN junction itself and a practical limit to the inductance used.

The MAX serial-interfaced LED driver provides 16 open-drain For pricing, delivery, and ordering information, please contact Maxim/Dallas Direct! at. The MAX operates from a 3V to V. The MAX serial-interfaced For pricing, delivery, and Dallas MAX6971 information, please contact Maxim/Dallas Direct!

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