need help in designing adjustable dc to dc converter using lm2596

[khankll]

hi
i want to build a dc to dc converter that should be able to give me output volts ranging from 1.5 to 28 volts .. my input will always be 31 volts from a smps adapter..

i earlier used lm317 but that was wasting too much power..so need one using buck converter..

i have chosen LM2596 ... so her are my questions
1: is there anything better then this ? more efficient and not more complex then it..
2: i have found a circuit for it in the TI DS for this ic which is using the -ADJ or adjustable version to make adjustable buck converter..
but that ckt has no pot or any mean to adjust the output voltage at whim.. so by the adjustable feature of that ic as opposed to fixed output ic i can get that it needs to be designed for only on output.. ? for another output i will have to redesign it..?am i right ?

this is my main concern.. how do i overcome it

my max current will be 1.5 amp..

i want to make something like this
http://www.suntekstore.com/goods-14...tep-down_adjustable_power_supply_module_.html

thanks

 

[CocaCola]

This is one of those instances when I have to ask why don't you purchase that one and see how it works? I'm going to bet it's the working example right out of the datasheet as that is pretty much what every one of these Asian modules are... If it works, then rebuild a new one with tweaks as necessary like a permanent heat sink and better components...

2: i have found a circuit for it in the TI DS for this ic which is using the -ADJ or adjustable version to make adjustable buck converter..
but that ckt has no pot or any mean to adjust the output voltage at whim.. so by the adjustable feature of that ic as opposed to fixed output ic i can get that it needs to be designed for only on output.. ? for another output i will have to redesign it..?am i right ?

The module you linked clearly has a pot on it and claims adjustable, as I said pick it up test it, if it works either use it as is or reverse engineer a new one based on that circuit as I doubt it's a proprietary design...

 

[BobK]

In the datasheet:

http://www.ti.com/lit/ds/symlink/lm2596.pdf

Look at the section called "Test Circuit and Layout Guidelines" . Go to the second figure. "Adjustable Output Versions." Replace R1 and R2 with a POT and you are done. You probably want a fixed resistance in series with R2 in order to limit the high end of the voltage range.

Edited to add: Boy, is that module cheap, why would you want to make your own?

Edited again: Actually, you want to replace just R2 with a pot, and probably adjust R1 to get the correct range.

Bob

 

[khankll]

In the datasheet:

http://www.ti.com/lit/ds/symlink/lm2596.pdf

Look at the section called "Test Circuit and Layout Guidelines" . Go to the second figure. "Adjustable Output Versions." Replace R1 and R2 with a POT and you are done. You probably want a fixed resistance in series with R2 in order to limit the high end of the voltage range.

Edited to add: Boy, is that module cheap, why would you want to make your own?

Edited again: Actually, you want to replace just R2 with a pot, and probably adjust R1 to get the correct range.

Bob

it needs paypal and its not available in my country..and neither do we have credit cards.. like visa,,master.. these things are aliento ordinary,normal,average man.,,, only very rich ppl have it.here everything is done on hard cache
yes i agree its very very very cheap .. and has free shipping...

 

[khankll]

and what about the inductors and caps ....? as the are chosen for one specific case of desired output conditions .. if i change its output voltage then i wil have to recalculkte the inductor etc values as per DS ..

 

[BobK]

You would want to choose the max inductor needed over the range. From the formulae given in the datasheet, it looks like that is going to be closer to the middle of the range rather than the low or high end.

You can also see the values on the board you linked to. But the inductor is already out of range of the ones listed in their design guidelines, so go figure.

Bob

 

[khankll]

thanks i have done a little work on it.. made few tables and tried to find where can i compromise ..

found that the following conditions will be ok..

Vin 31 Volts ...(will always be at this )
vout is between 5 and 25
current range from .3 or .5 amp minimum to close to 3 amp max..

i made a table for E.T values where vin was 31 volts.and vout varied from 3 volts to 25 volts with increment of 1 volt.. found that the min ET value was 20.7 and max 52.2 ... [however i may at some times use vin=18volts which scales the ET range from 8 to 28 for vout of 2volts to 15.5volts... after a bit more carefull observation i noticed that there is a line from et value fo 9 to 30 on vertical axis and that can provide me with current of .6-3 amps .. and it crosses/covers 3 inductor ranges namely L23,L32,L40 and i looked in the table of inductor to find what these numbers correspond to and these all correspond to 33 uH inductor.. am i correct ? these tables are table 7 on page 17 and page 18 of National Lm2596 DS]

[ also for the ET range of 20 to 52 i found that there is a line starting just below 20 and extends to 60 which covers 3 inductors ranges/areas namely L21,L30,L38,L44 if i look at the inductor areas below the line.. and these represent 68uH inductor..and if look above this line then the inductor reginons are L29,L37,L43 and this range indicates inductor of 100 uH... what range is correct ? above the line or below the line ? [i can further squezze the range and compromise on available output ]

so what should be the sutable inductor ..
i have trouble in identifing which area to look for.. the manual says where the I max intersects the ET that area covered is the suitable inductor...
the design that refered is using 33uH inductor..

thanks

 

[BobK]

Hey, we are in agreement! I did the calculation myself and came up with a max of 51.8 with 31V input and 14.7V output. You take that, and your 1.5A max current and look it up on the chart in figure 7, and I get L36 which is 150uH 2.5A.

Bob

 

[khankll]

Hey, we are in agreement! I did the calculation myself and came up with a max of 51.8 with 31V input and 14.7V output. You take that, and your 1.5A max current and look it up on the chart in figure 7, and I get L36 which is 150uH 2.5A.

Bob

thanks i have done a little work on it.. made few tables and tried to find where can i compromise ..

found that the following conditions will be ok..

Vin 31 Volts ...(will always be at this )
vout is between 5 and 25
current range from .3 or .5 amp minimum to close to 3 amp max..

i made a table for E.T values where vin was 31 volts.and vout varied from 3 volts to 25 volts with increment of 1 volt.. found that the min ET value was 20.7 and max 52.2 ... [however i may at some times use vin=18volts which scales the ET range from 8 to 28 for vout of 2volts to 15.5volts... after a bit more carefull observation i noticed that there is a line from et value fo 9 to 30 on vertical axis and that can provide me with current of .6-3 amps .. and it crosses/covers 3 inductor ranges namely L23,L32,L40 and i looked in the table of inductor to find what these numbers correspond to and these all correspond to 33 uH inductor.. am i correct ? these tables are table 7 on page 17 and page 18 of National Lm2596 DS]

[ also for the ET range of 20 to 52 i found that there is a line starting just below 20 and extends to 60 which covers 3 inductors ranges/areas namely L21,L30,L38,L44 if i look at the inductor areas below the line.. and these represent 68uH inductor..and if look above this line then the inductor reginons are L29,L37,L43 and this range indicates inductor of 100 uH... what range is correct ? above the line or below the line ? [i can further squezze the range and compromise on available output ]

so what should be the sutable inductor ..
i have trouble in identifing which area to look for.. the manual says where the I max intersects the ET that area covered is the suitable inductor...
the design that refered is using 33uH inductor..

thanks

i still cant make it clear.. .. which one is to be chosen ? range above the line or range below the line ,,their own examples use below the line range..
i checked lm2576 another buc converter and that uses same method to build ckt.. and uses the values under the line ..

so if i have to implement the ET values from 20 to 52 i have to choose 68uH inductor.. what is your opinion on it ?

Thanks

 

[khankll]

also how they chosed the Cff of 560pF in DS ? going by equattion it should be 2094pF..

 

[CocaCola]

it needs paypal and its not available in my country..and neither do we have credit cards.. like visa,,master.. these things are aliento ordinary,normal,average man.,,, only very rich ppl have it.here everything is done on hard cache
yes i agree its very very very cheap .. and has free shipping...

Have you consider talking to you local supplier and having them ship one in for you? It's a win win as far as I can see, he makes a buck or two for ordering it for you and you get it without any credit card hassles for small upcharge...

 

[khankll]

no such services exist oin this part o world.. its daydreaming..

so its out of question..
i wont mind the cost if i make it myself that way i will learn something and be sure about its specs... i tried hard to find any alternative but only came accross one which gave only fixed 5 volts and that cost 20 USD equivbalent here...

so i m much better with one of my own..

 

[CocaCola]

no such services exist oin this part o world.. its daydreaming..

I'm betting if you asked you would find someone willing to do it, people like making money with little work... Where do you get your parts now, have you asked the shop owner about custom orders, you said you have to pay cash so I'm guessing it's a local brick and mortar shop... Why would they turn down a guaranteed paid in advance sale, that is just poor business...

I have done it for friends myself and had friends do it for me when we needed stuff that was easier to obtain in one place or another, like specific region only merchandise... Heck I paid an online broker to purchase stuff for me off Japan's Yahoo auctions, the guy was plenty happy taking my money to act as a middle man, that only required he forward the package to me...

 

[BobK]

i still cant make it clear.. .. which one is to be chosen ? range above the line or range below the line ,,their own examples use below the line range..
i checked lm2576 another buc converter and that uses same method to build ckt.. and uses the values under the line ..

so if i have to implement the ET values from 20 to 52 i have to choose 68uH inductor.. what is your opinion on it ?

The higher the EV value, the more demanding it is of the inductor, so you go with the max of 52. That, plus your max current of 1.5A gives you 2 coordinates that give you single point in the graph. That is how I came up with L36. It is always okay to use a larger inductor, except that it will limit the speed of response to changing current demands. But you can't have everything. I notice the device you linked to used a 330uH, which is not even on the chart.

Edit: added quote to make clear what I am answering.

Bob

 

[khankll]

^^ that is 33uH inductor.. the third digit is 10 raised to the power multiplier . i.e 33 X 10^0=33 uH

whatr sort of inductor is that.. like i have seen inductors like resistors shape and color coding .. wont they work ?

what exact type of that inductor is ?

also i found that hey r using a feedback capacitor whose value depends on R2..which in muy case will be a pot .. so any change in r2 value will need a new cap how do i overcome it ?

 

[BobK]

It is a surface mount power inductor. If it is indeed 33uH it is way too small, I have some like it at home that are 470uH, when I get home I will check to see if they are marked 470 or 471.

Bob