Why is my 12v regulator outputting 5v when it shouldn't be outputting anything?

So I made this little boost regulator for powering 12V LED bulbs:

And I followed the datasheet exactly: http://www.ti.com/lit/ds/symlink/lm27313.pdf

And it seemed to work perfectly fine. I only just recently noticed however that it's not working as expected at all. It will output 12v when the SHDN pin is pulled high, but when it's pulled low, the module outputs 5V, when I expected it to output 0V. I mean, typically that's what a shutdown pin is for on a regulator right? To make it stop outputting?

The reason I hadn't noticed this before now is thankfully most 12V LED bulbs won't light if powered by 5V. I'm not sure if they're still drawing power or not, but at least the board works for its intended application. I did notice that some bulbs light with 9V though, and I recently ordered some even smaller bulbs which I hope do not light with 5V or that is going to be a costly mistake.

Anyway, I suspect that the 5V is just flowing straight through that inductor, since it goes right from the 5V input pin to the output side of the regulator. But that's how it was wired up in the datasheet, so I'm confused why they even have the shutdown pin wired up there if all that is ever going to do is take the regulator from 12V down to Vin.

It seems like if I want this regulator to function the way I want it would require a complete redesign, with a new regulator, or by adding a mosfet in there to actually work as a proper shutdown pin.

So, what are your thoughts on this? Is the board functioning properly as designed in their schematic? Is there an easy fix without adding a mosfet? And what is that shutdown pin really for?

[edit]

I guess what the pin is really for is this: "This part has a logic-level shutdown pin that can be used to reduce quiescent current and extend battery life."

But it still does not make sense why one would want to reduce the output from 12v to 5v instead of reducing it to 0v, if the goal is to save power.

Did you make the board? If not, can you give a link to the board you bought? the link you gave was for an IC (must be the one used on the board, right?). Looks like the label on the board does not indicate "shut down", just signal. So maybe if the URL for the board has specs, we can find out what the signal will do.

Thanks, Jack

Anyway, I suspect that the 5V is just flowing straight through that inductor, since it goes right from the 5V input pin to the output side of the regulator.

Right... That's a "nice" current path through the inductor & diode.

I don't have an explanation.

Would make a good Flash programmer.

jackwp: Did you make the board? If not, can you give a link to the board you bought? the link you gave was for an IC (must be the one used on the board, right?). Looks like the label on the board does not indicate "shut down", just signal. So maybe if the URL for the board has specs, we can find out what the signal will do.

Yes, I designed the board, following the datasheet pretty much exactly. The "signal" pin is just a direct link to the shutdown pin on the regulator, with the 51K pulldown resistor there. This board is designed to be connected via a servo cable to an Arduino-like board of my own design, and I just pull the signal pin high or low to control it.

That circuit can do nothing else. In fact, it should be a few 10ths of a volt below 5V. Being a buck regulator, it has a straight path through the inductor and diode.

Edit: Doh! I meant to say "boost regulator".

Congrats on the board. I have wanted to but never have designed, and produced boards in mass quantity. So, what you thought the Ic would do, did not happen. I guess you have tried more than one of them?

The datasheet only says that you can shutdown the regulator to save battery power... it doesn't actually claim that your power supply output voltage will go to zero... it's only really shutting off the regulator... so yeah, you basically get the Unregulated Voltage you fed in (minus the shottky Vforward) with it off... whatever that is.

jackwp: Congrats on the board. I have wanted to but never have designed, and produced boards in mass quantity. So, what you thought the Ic would do, did not happen. I guess you have tried more than one of them?

Yeah, I mean looking at it now it's kinda obvious why it's working the way it is, but I just assumed when I didn't know anything about boost converters that something was going on that I didn't understand that would make the shutdown pin work the way I expected it to. I still can't understand why if one was concerned about saving energy, one would simply reduce the voltage to 5V rather than shutting the thing down completely, but I guess someone's gotta be using it that way. I now wonder though if there's any boost converter chips out there that would work the way I want, or if I would have to add a mosfet to any of them. I also wonder what the side effects of doing PWM with a regulator like this would be. It seems like PWM-ing the mosfet and thus turning the regulator on and off rapidly could have an undesirable effect. I suppose you should be able to do it on the output side, and leave the SHDN pin alone. But then you'd be drawing power through that switch all the time. So PWM the input then? Or PWM the output and SHDN at the same time? Not sure what the best approach woud be.

And yes, I tried more than one of them.

It'll continue to work fine for what I needed it for in the first place, but it's not going to be suitable for a wider range of uses that I had in mind unfortunately. I'm glad I didn't redesign the thing to include a diode for a motor only to discover the issue when the motors never turned off. That would have been an even more costly mistake than possibly buying some leds that won't turn off.