LP2951 Voltage Regulator - "Shutdown" pin not giving 0 volts! - Page 1
Forum section: electronics / projects

author: radian
author_rank: Contributor
timestamp: July 06, 2013, 03:15:47 am
content: EDIT: PROBLEM SOLVED. SEE POST BELOW.--------Hi all,Bought three LP2951 programmable voltage regulators. The huge draw for me was the TTL "shutdown" feature. What I assumed this meant was that with a TTL signal I would be able to turn off the output completely (to zero volts). I received the parts today, eagerly wired them up, only to discover feeding in a logic high signal (shutdown enable) the output is 1.27V! Now I'm thinking this voltage has to do with the error amplifier voltage reference (see the second figure on page 2 here:http://www.onsemi.com/pub_link/Collateral/LP2950-D.PDF), but not positive. Now interestingly, if I feed in 9V (input voltage) into the shutdown pin, I can get the output to 0.2V. I think this is more of a hack though.Any thoughts on this? I'm a missing something?

author: David_AVD
author_rank: Super Contributor
timestamp: July 06, 2013, 03:24:32 am
content: Are these genuine parts from a reputable distributor?

author: radian
author_rank: Contributor
timestamp: July 06, 2013, 03:25:59 am
content: Absolutely, purchased from Digi-Key. Authentic ON parts. This user reported a similar issue:http://www.microchip.com/forums/m93594.aspx

author: c4757p
author_rank: Super Contributor
timestamp: July 06, 2013, 03:30:08 am
content: What happens if you load the output with 1k or so? It says it will give around 10uA during shutdown, so this may translate to a voltage when unloaded.

author: radian
author_rank: Contributor
timestamp: July 06, 2013, 03:47:01 am
content: Just tried with a 1.2K. No change. Also tried powering an AVR (8mA draw), no change again.Tried the other two parts - same output.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 06, 2013, 03:58:40 am
content: Quote from: radian on July 06, 2013, 03:15:47 amHaha, let me guess you get 1.25V out. It is by design and absolutely normal.

author: radian
author_rank: Contributor
timestamp: July 06, 2013, 04:10:02 am
content: Quote from: Alexei.Polkhanov on July 06, 2013, 03:58:40 amOh super. I suppose for $0.93 a piece, it isn't a big investment. On the plus side, it regulates the voltage quite well, so that's good at least. I could always throw in a logic level p-fet for the shutdown feature. Thanks for the sanity check.

author: Bored@Work
author_rank: Super Contributor
timestamp: July 06, 2013, 05:25:07 am
content: Quote from: radian on July 06, 2013, 04:10:02 amNo, they are not designed to have 1.25V in shutdown mode and it is not normal. Check figure 11 in the datasheet you refer to. Output is 0V while Vshutdown = 2V, until t = 0, when Vshutdown is switched to 0V. Then you see Vout starting to rise to 5V.If you take the datasheets from other LP2951 manufacturers you see most of them have similar diagrams. E.g. TIhttp://www.ti.com/lit/ds/symlink/lp2950-n.pdffigure 45 on page 14.So it is more likely one or more of the following happens, in no particular order:- You got fakes- You got broken ones- You connected them wrong- Something is back-feeding power into your circuit- The thing oscillates( because you omitted caps)

author: radian
author_rank: Contributor
timestamp: July 06, 2013, 03:09:57 pm
content: Problem solved! Thanks Bored@Work, your comments forced me to try a couple more tests. So here's the deal, apparently you can't use the output from the regulator to shutdown the regulator (self kill). Here's some images to depict the setup:Input voltage: 9VOutput cap: 1uFError resistor: 100KOutput load: 1.2K resistorDMM: Connected on bottom rail next to output resistorWithout shutdown enabled:With shutdown enabled (powered from output):With shutdown enabled (powered from 5V from LM7805):I suppose I'm ok with this functionality. I don't think I'll have any need for the regulator to self kill.Thanks everyone

author: c4757p
author_rank: Super Contributor
timestamp: July 06, 2013, 04:16:50 pm
content: Quote from: radian on July 06, 2013, 03:09:57 pmWell, of course you can't! Think about it - if the output is used to feed the shutdown pin, as soon as the output drops below the shutdown level, shutdown is lost and it will turn back on. It will either oscillate, or settle at precisely the threshold. You'd need a latching shutdown to do that.

author: radian
author_rank: Contributor
timestamp: July 06, 2013, 05:06:39 pm
content: Quote from: c4757p on July 06, 2013, 04:16:50 pmHaha, well that makes sense now. When prototyping it's easy to grab any voltage nearby, without thinking of the consequences. Interesting little chips though, and now that the shutdown works, I think it's a bargain at under $1. Figure 26 shows a way to boost output current to 2A with low dropout.

author: David_AVD
author_rank: Super Contributor
timestamp: July 07, 2013, 12:22:16 am
content: Quote from: radian on July 06, 2013, 03:09:57 pmLMFAO! Sorry, but that was pretty funny.At least you learned something new.

author: Alexei.Polkhanov
author_rank: Frequent Contributor
timestamp: July 10, 2013, 11:49:46 pm
content: Quote from: Bored@Work on July 06, 2013, 05:25:07 amYes,1.2 is what you get when you put LM317 ADJ pin to the ground (that sometimes used as pseudo-shutdown). This regulator has separate shutdown pin, you are right it is totally different story. Sorry for confusion.