Having issues getting an LM4040 reference working - Page 1
Forum section: electronics / beginners

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: August 30, 2018, 12:25:48 am
content: I'm doing some ADC, and I have an LM4040 to set the 5V reference on the arduino to something reliable.It's an LM4040BIZ-5.0, and I ordered it from DigiKey.http://www.ti.com/lit/ds/symlink/lm4040-n.pdfIt's a pretty simple reference schematic that I'm trying to use, with a 460ohm resistor. This should give 2.5mA, which is more then the 1mA requirement. But I'm getting 12v at the output. One small (heh) detail, I might have put too much current through it - I'm noticing now the absolute maximum is 10/20mA. The magic smoke was never released... but I guess it's something we need to consider. Maybe I blew it...

author: floobydust
author_rank: Super Contributor
timestamp: August 30, 2018, 01:28:56 am
content: Check your PCB pinout is correct and pin 3 is N/C or connected to pin 2 anode.12V-5V/460R is high at over 15mA- which is the max. for the LM4040. Try the math again, it should be k ohms.Otherwise, it might be dead from an overcurrent spike.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: August 30, 2018, 01:42:36 am
content: Ahh, I see.Okay, I think it's probably safe to assume I nuked it

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2018, 07:58:02 am
content: You've made an error in your calculations.I = (VIN- VREF)/RIt's rated to give 5V at just 65μA, worst case, so why do you need to run it at over 15mA. How much current does the ADC need for the reference? Set the current for around 1mA and add a 100nF capacitor in parallel with the reference.R = (VIN- VREF)/I = (12 - 5)/0.001 = 7000.Use the nearest standard value. It doesn't have to be accurate, E6 20% tolerance will do, which gives 6k8.https://en.wikipedia.org/wiki/E_series_of_preferred_numbers#ListsStill, I doubt 15mA would fry it. There must be an error somewhere else. Check all the connections and the output voltage again with a 6k8 resistor.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: August 30, 2018, 12:46:48 pm
content: Gave it a shot with 6k8 - no dice, still 12V. Time to find another batch of items to get from Digikey!

author: exe
author_rank: Unknown
timestamp: August 30, 2018, 04:04:32 pm
content: I'd double check polarity (I don't believe it's a diode/zener, I think datasheet shamelessly lies, should be a band-gap).

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: August 30, 2018, 11:19:20 pm
content: I flipped it as well with no success

author: exe
author_rank: Unknown
timestamp: August 31, 2018, 07:09:54 am
content: Just in case, one of leads is not connected.BTW, I buy all (cheap) ICs in more quantity than I need just for this reason.

author: stj
author_rank: Super Contributor
timestamp: September 01, 2018, 11:54:48 am
content: one revision of the LM4040 datasheet has the wrong pinouts!!this cost me a couple of devices - i'll try to find which revision is good or bad.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: September 03, 2018, 04:32:00 pm
content: Ah, !@#. Bottom view... nice.

author: joeyjoejoe
author_rank: Frequent Contributor
timestamp: September 03, 2018, 04:56:54 pm
content: Yup, totally working nowI'm seeing 5.001V on the Fluke 117.