LM317 Vout-Vin > 3V limitation

[bt36]

Hello All,

I've purchased quite a few LM317's in order to create +5v stable analog voltage from a the +5v of a USB jack.

Problem is, and this unfortunately is not very clearly stated in the datasheet (Fairchild),
Vout - Vin must be at least 3v. Which is not the case.

Question: any idea of a work-around? saving my already purchased 25 or so IC's?

Any help would be greatly appreciated

BT.

 

[hock]

there is no way you can use lm317 in this case. if you refer to the data-sheet you will see that the minimum input to outputs difference is 2.5V or more. so on 5 v supply you can get only 2.5V. even you use a LDO like LT1117-5, it needs 200mV drop, so the minimum voltage is 5.2V. if you can work with a bit lower voltage you can use this LDO. if you need 5V only then use some buck- boost regulator and then a LDO..


hock

 

[IanP]

None of linear regulators, including LDOs, will produce nice Vout=5V out of Vin=5V+/-X% ..
Only buck-boost configuration can do that ..
You will have to find different application for your investment ..

IanP

 

[FvM]

You should also consider, that the minimum specified voltage supplied by a Host/Hub is 4.75V (high power) respectively 4.4V (low power). The cable voltage drop has to be added to the voltage budget. See USB spec clause 7.2 for details.

5V analog voltage isn't feasible without a boost converter "by design".

 

[bt36]

Hi Guys,

thanks very much for the replies

so,

- do I need buck-boost + LDO (2 chips) or can a single buck-boost do the job?
- any recommendations for specific IC's?

Thanks again!

BT.

 

[IanP]

You can boost the voltage up to, say 6-6.5V and then use any popular +5V LDO, or you can try something like the LTC3534 ..
Here is it's data sheet:
**broken link removed**

IanP

 

[bt36]

Thanks IanP

The LT part does not seem to be that expensive at digikey. I wish I'd know about this prior to my board design and my purchase.


Last question (I promise) : any recommendation for the 5nH inductor?

do inductors come in e.g. 1206 package as all the rest of my resistors that I've purchased or is that a (yet another) custom / different PCB foot-print?

Thanks again, BT.

 

[IanP]

Don’t make a mistake: the schematic diagram shows 5µH, not 5nH ..
Inductors come in all sort of packages but keep in mind that it has to have low dc resistance and also has to be rated for at least the output current ..
5uH inductor will be not easy to get, the closest is 4.7µH, but you can consider connecting two inductors in series, for example 2.2µH and 3.3µH ..

Try: LQH32CN4R7 (murata) ..
https://www.futureelectronics.com/en/Technologies/Product.aspx?ProductID=LQH32CN4R7M23LMURATA7649957

IanP

 

[karesz]

Hi,
Their are recommendation for Inductor Vendors on page12:
Coilcraft (847) 639-6400 (847) 639-1469, www.coilcraft.com
FDK (408) 432-8331 america@fdk.com www.fdk.com
Murata (814) 237-1431, (800) 831-9172, (814) 238-0490 www.murata.com
Sumida USA: (847) 956-0666, Japan: 81(3) 3607-5111
USA: (847) 956-0702,Japan: 81(3) 3607-5144,www.sumida.com
TDK (847) 803-6100 (847) 803-6296,ww.component.tdk.com
TOKO (847) 297-0070 (847) 699-7864,ww.tokoam.com

In text you can read "select some SMD inductances for 1..2A, then peak current are ca. 4x higher as Iout"...
So an inductor has special footprint (but shurly bigger as 1210) with ca. 0.5-1cm in diameter.
K.