Switching Regulators, Inductance selection and TI WebBench? LM-2574-ADJ
rwgast_logicdesign
Posts: 1,464
Ok so I ordered some LM-2574-ADJ adjustable regulators the other day based on Dr_Aculas recommendation. I skimmed through the data sheet real quick to see what all passive parts I needed to layout the reference circuit and I also found this helpful site based on the lm576 verifying what I read in the data sheet. It turns out I need a 330uH inductor for the main circuit and a 20uH inductor to filter the source. I tried to use the WebBench thing at TI/National but it wouldnt let me pick my chip, so I got the downloadable version which would and for some reason it did not list this particular chip..
Anyways looking around at samples it seems that vishay samples inductors so searching I came across this one and its Data Sheet. Unfourtantely I dont know much about inductors, just the basics on how they work, this will be the first circuit ive designed using inductors. Im confused on two issues though first one is they say the inductance is 300uH at 1khz well my switching regulator is 52khz, im not sure if these numbers somehow correlate with each other so I guess im asking is using this inductor fine? The second issue is Vishay doesnt have a 20uH version of this inductor but they have a 18uH (ithink) and a 22 uH is it ok to use one of those values in the filter schematic instead of a 20uH. Usually you dont have to be dead on with resistors or caps in most applications as long as your relatively close, is this the same with inductors?
Anyways looking around at samples it seems that vishay samples inductors so searching I came across this one and its Data Sheet. Unfourtantely I dont know much about inductors, just the basics on how they work, this will be the first circuit ive designed using inductors. Im confused on two issues though first one is they say the inductance is 300uH at 1khz well my switching regulator is 52khz, im not sure if these numbers somehow correlate with each other so I guess im asking is using this inductor fine? The second issue is Vishay doesnt have a 20uH version of this inductor but they have a 18uH (ithink) and a 22 uH is it ok to use one of those values in the filter schematic instead of a 20uH. Usually you dont have to be dead on with resistors or caps in most applications as long as your relatively close, is this the same with inductors?
Comments
The 1KHz is just where they do the standard measurement. Should be OK at 52KHz.
(Now if you start to get up past 250KHz things get a bit more complicated.)
Make sure the RATED DC CURRENT is sufficient plus a bit of margin, maybe at least 25% or so.
The filter inductor is not to critical, so I would recommend the 22uH. Bigger is generally better in filters.
Make sure it also has sufficient RATED DC CURRENT.
Duane J
I'll second Duane's comment about rated DC current. 1A or thereabouts, which rules out a lot of "RF" inductors.
Futurlec call them 'power inductors' and sell this one http://www.futurlec.com/Components_Others/PIND330.shtml for 45c. They also sell the switching diodes and the switchers.