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 Tube Buffered Gainclone in work \|Thread

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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.