# How to correctly set output voltage with LT3080

I have designed the schematic below, wanting to build a simple power supply for a project. Problem is, the LT3080 is outputting around 15V no matter what R1 I use.

Trying to find the root cause, I have removed all components following the LT3080 (except for R1), but to no avail.

I am measuring 11Vac across the transformer (it is rated 7.5V @ 1.5VA) and 15Vdc across the bridge. Measuring the tab of the LT3080 gives 14.7V (same voltage if I measure across C4. All of this with a 470kO R1... which if I'm not mistaken should output 4.7V

Can anyone shed some light on this?

• According to a discussion on the EEVblog forum, the LT3080 has been silently upgraded to other parts by LT due to a design problem that can lead to latchup. – replete Jan 14 '17 at 14:21
• I know it has been a long time, but which discussion, link? – Lazyboy Apr 22 at 15:07
• I'm 99% sure this was the thread: eevblog.com/forum/projects/… – CatalinM Apr 22 at 17:17

It's normal for regulators to require a minimum load before they can take effect. From the datasheet, the minimum load current for your part is 0.5mA. If you remove everything in your circuit after the LT3080, it won't regulate.

• That might be a very valid point. I will check again (while using a load). – CatalinM Jan 14 '17 at 15:14
• Also, quick question: does C3 have the correct polarity in my schematic? – CatalinM Jan 14 '17 at 15:15
• C3 has the correct polarity but I would test without this capacitor for now. The problems with the LT3080 I mentioned in my comment are related to the difference between the SET and OUT pins. I do suggest you read that thread. – replete Jan 14 '17 at 15:19
• Thanks. I'll be connecting the status led now, without any other bypass caps and see if it behaves correctly. – CatalinM Jan 14 '17 at 15:21
• The main thing to avoid in your testing for now is shorts on the output. SET and OUT must remain within 0.3V per the datasheet, but with the cap the datasheet shows on the SET pin this isn't happening. You can still get the part to work for you if you understand the issues. – replete Jan 14 '17 at 15:24
• V OUT = R SET • 10µA
• Rmin to get 1.1a depends on heatsink Rth and $Delta(V_{in-out})*I*R_{th}[\deg C/W]$ for 85'Cmax