LM3x7 Datasheet Error?

richie00boy · 7th July 2004, 07:19 PM

I'm designing a PSU using LM317T/LM337T regs. The National Semiconductor datasheet states that the output voltage is calculated by:

Vout = 1.25 * (1 + (R2 / R1)) + (Iadj * R2)

where Iadj = 1.25 / R1

However, using this formula on the Typical Applications circuits, I cannot get anything like what voltage they say I should be getting.

This is further complicated by the LM337L datasheet stating that the output voltage is calculated by:

Vout = 1.25 * (1 + (R2 / R1))

Which gives apparantely correct results.

Only this datasheet has the formula in this way, the LM317L, LM317T and LM337T all have the formula as in the first case.

The L suffix is TO92 package and the T suffix is TO220 package.

Has anybody actually got any proven formula or values I can try? I built one using L suffix parts and according to the second formula and it worked fine, but I want to use T suffix parts now.

jackinnj · 7th July 2004, 07:22 PM

I(adj) is the current on the "adjust" pin, not the total current pulled through the device -- it's measured in microamps -- Nat Semi has min/max/avg values on the datasheet. -- something like 50uA

Pjotr · 7th July 2004, 07:41 PM

Hi Richi,

I_adj is indeed the current flowing out of the adjust pin. If you keep the recommended value for R2, 220 - 240 ohms, you can safely ignore I_adj and use the formula for the LM317L as well for the LM317T.

Cheers

peranders · 7th July 2004, 07:55 PM

Here is a spreadsheet which I made for the purpose.

Use it

richie00boy · 7th July 2004, 08:24 PM

Thanks guys.

I was slightly misreading the datasheet and confusing Iadj with the whole current flowing in the programming resistors.

peranders · 7th July 2004, 08:32 PM

Good that you found out. In real life you can have R1 up to 1 kohms without problems. Iadj which cleary is stated

is far less than worst case.

7th July 2004, 07:19 PM	#1
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	LM3x7 Datasheet Error? I'm designing a PSU using LM317T/LM337T regs. The National Semiconductor datasheet states that the output voltage is calculated by: Vout = 1.25 * (1 + (R2 / R1)) + (Iadj * R2) where Iadj = 1.25 / R1 However, using this formula on the Typical Applications circuits, I cannot get anything like what voltage they say I should be getting. This is further complicated by the LM337L datasheet stating that the output voltage is calculated by: Vout = 1.25 * (1 + (R2 / R1)) Which gives apparantely correct results. Only this datasheet has the formula in this way, the LM317L, LM317T and LM337T all have the formula as in the first case. The L suffix is TO92 package and the T suffix is TO220 package. Has anybody actually got any proven formula or values I can try? I built one using L suffix parts and according to the second formula and it worked fine, but I want to use T suffix parts now.

7th July 2004, 08:32 PM	#6
peranders Electrons are yellow and more is better! diyAudio Member Join Date: Apr 2002 Location: Göteborg, Sweden Blog Entries: 4	Good that you found out. In real life you can have R1 up to 1 kohms without problems. Iadj which cleary is stated is far less than worst case. __________________ /Per-Anders (my first name) or P-A as my friends call me

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
Improving the LM3x7 regulator circuit	Geoff	Power Supplies	158	8th January 2012 07:08 PM
KA3x7 vs LM3x7; is there a difference & bargain price	smiler	Parts	8	1st July 2009 06:22 PM
Regulated PS with LM3X7 ?	kacernator	Solid State	0	13th March 2009 04:04 PM
LM3X7 Supply	zei	Power Supplies	25	23rd December 2007 06:52 AM
LM3x7 HV regulating.	w00t	Parts	3	29th February 2004 09:42 PM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read


	Please consider donating to help us continue to serve you. Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving

7th July 2004, 07:22 PM	#2
jackinnj diyAudio Member Join Date: Apr 2002 Location: Llanddewi Brefi, NJ	I(adj) is the current on the "adjust" pin, not the total current pulled through the device -- it's measured in microamps -- Nat Semi has min/max/avg values on the datasheet. -- something like 50uA

7th July 2004, 07:41 PM	#3
Pjotr diyAudio Member Join Date: Nov 2002 Location: Netherlands	Hi Richi, I_adj is indeed the current flowing out of the adjust pin. If you keep the recommended value for R2, 220 - 240 ohms, you can safely ignore I_adj and use the formula for the LM317L as well for the LM317T. Cheers

7th July 2004, 08:24 PM	#5
richie00boy Did it Himself diyAudio Member Join Date: Nov 2003 Location: Gloucestershire, England, UK	Thanks guys. I was slightly misreading the datasheet and confusing Iadj with the whole current flowing in the programming resistors.

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)