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I'm wondering how to select a correct zener diode value for this led driver I am using RT4533AGJ6 (link to datasheet). My understanding of boost converters is limited, although I understand that there is an internal switch that charges up the inductor and when switched off, the inductor discharges onto the load...through the zener diode, which is to my point. The datasheet does not give any values for breakdown voltage, let alone why its required.. i did some further searching on other led driver datasheets and ran into an additional issue, the reaction/reset speed of the zener diode, which is of course dependent on the switching speed.

To make matters worse, I had contacted a customer service representative from a electronics warehouse, he said that it didn't matter what value diode I choose....which just seems silly and must be wrong in some way.

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This is a boost converter topology. The diode should be Schottky, not Zener. The reference design for RT4533 has a bill of materials, where D1 is a Schottky.

The symbol in the datasheet is for a Zener. I blame lack of proofreading.

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  • \$\begingroup\$ Thank you very much, if I had known more about Schottkys and had looked at the general boost topology I might have made the connection that the regular diode portrayed in pictures is more efficient as a Schottky. Basically I should have known better \$\endgroup\$ – Aaron Aug 9 '16 at 2:10
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That is not a zener diode, that is a schottkey diode. A Schottkey diode is basically just a fast diode -- honestly, in my experience almost all "diodes" are shottkey diodes now.

So when the guy said "it doesn't matter what value diode you use", he meant it: just use any old diode that can handle the current you need it to handle, and is called a schottkey. Just don't use a zener: they generally can't handle the amount of current that might be necessary for a LED driver.

Schottkey diodes are faster than "bog standard diodes", and should be used instead of something old and useless like the 1Nn00x series. Most diodes that you find someplace like digikey will be shottkey, and will be able to switch fast enough. Older diodes (like the 1N400x series), struggle to keep up.

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  • \$\begingroup\$ that would explain a lot, but what about learn.sparkfun.com/tutorials/how-to-read-a-schematic/… thats what I was basing it on, how am I supposed to know which is which now.. \$\endgroup\$ – Aaron Aug 9 '16 at 0:49
  • \$\begingroup\$ Honestly, I don't think the symbols are standard enough that you can rely on that. The diode is essentially a reverse protection diode, and zeners should not be used that way. Look up boost converter circuits, and then examine the functional diagram on page three of your datasheet. That should convince you it is not a zener. \$\endgroup\$ – Andrew Spott Aug 9 '16 at 0:57
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    \$\begingroup\$ @AaronSchulte Context is key here. That's a pretty generic boost circuit, and a Zener would not make sense there. \$\endgroup\$ – Matt Young Aug 9 '16 at 0:58
  • \$\begingroup\$ Ah should have known better, all it would have taken was a glance and the general boost topology and associating that diode with a schottky. Thanks all! \$\endgroup\$ – Aaron Aug 9 '16 at 2:11
  • \$\begingroup\$ @ Andrew Spott: what you said about any old diode working in a high-frequency switch-mode converter is just plain wrong! Try using a bog-standard 1N4005 or similar and watch the smoke rise! \$\endgroup\$ – Dwayne Reid Aug 9 '16 at 2:16
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As others have mentioned, the diode is a Schottky not a zener.

You can select a 1N5819 (there are SMT versions) which is cheap and will handle 1A/40V, adequate for the maximum voltage of this chip.

You should not pick Schottky diodes with excessive PIV voltage capability because the forward drop goes up with voltage capability until they are no longer much better than conventional diodes. For example, for a lower voltage circuit you could use a 1N5817.

For higher voltage (more like mains) an ultrafast conventional diode is called for such as a UF4005. The additional forward drop even in your particular circuit will not be that important because the voltage of those series LEDs is relatively high.

Do not use a regular 1N400x diode- the switching frequency of this converter is 1MHz and that is about two or three orders of magnitude higher than is suitable for a diode with several microseconds of reverse recovery time.

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