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I'm trying to power a FPGA evaluation board using a 5V linear voltage regulator. I supply the 5V through the USB connector as shown: (this goes to the microUSB on the board)

USB connector with soldered wires

For a basic test, I power the board up with no attached loads. I have a 12V AC/DC adapter that powers the 5V regulator. I have a 22uF capacitor at the output of the regulator, as the datasheet recommended this for stability.

When I plug this all in, I probe the voltage output of the regulator - it appears to work for a minute or so (holds a constant 5.0 V), then it rapidly drops to below 5V, and starts erratically pulsing. The FPGA stops performing and the LEDs on the board pulse.

My guess is that it's drawing too much current and overheating? The regulator is rated for 5A, which I figured was plenty for this application. The evaluation board doesn't seem to have a datasheet, but the FPGA does have a datasheet (download here).

Any thoughts as to why the regulator stops working? Any suggestions of a better method to provide the 5V to the board? I'd like to use the 12V AC/DC adapter as a power source if I can.

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    \$\begingroup\$ If you are going to use a USB port as a 5V source, at least cut the cable and solder the wires directly, don't mangle the connector and short things out on one side or the other. Also, depending on what you are connecting to, you may only be getting 500mA of current, depending on what your board is running this may be too much load for the (wall wart?) power side causing it to oscillate. \$\endgroup\$
    – Ron Beyer
    Commented Apr 12, 2020 at 1:33
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    \$\begingroup\$ And what kind of heat sink do you employ on your LT1084CT linear regulator? Did you try to hold the LT1084 with your fingers when you run the experiment (I hope you didn't)... \$\endgroup\$
    – Ale..chenski
    Commented Apr 12, 2020 at 2:10
  • \$\begingroup\$ POwer dissipated in regulator ~= Vdropx I = (12v-5V) x I = 7Watts per Amp Or 7 mW per mA. How long it will last deep-ends on heatsinking. Photo(s) a very good idea. \$\endgroup\$
    – Russell McMahon ♦
    Commented Apr 12, 2020 at 2:11
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    \$\begingroup\$ This SE answer will probably tell you more than you need or want to know on the subject of regulator heating - My linear voltage regulator is overheating very fast \$\endgroup\$
    – Russell McMahon ♦
    Commented Apr 12, 2020 at 2:13
  • \$\begingroup\$ the first link you provided states that the eval board can be powered from a PC USB connector \$\endgroup\$
    – jsotola
    Commented Apr 12, 2020 at 2:28

1 Answer 1

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What typical current drain when failing?
What heatsinking?

LT1084 datasheet here

Probably but not certainly a heatsinking issue.
You may be able to dissipate 1 to 2 watt in a TO220 package without a heatsink deep-ending on environment.
That's say 150-300 mA max and probably not 300 mA.

Here are some thermal resistance for various package Rthjc (Thermal resistance junction to case) and Rthja (junction to air 0 usually on a 4 layer PCB.
A TO220 package has Rthja of about 30 C/W ona 4 layer board any maybe 60 C/W in air with no heatsink.

Temperature rise = Watts_dissipated x Rthja in the environment in question.

Table adapted from here -
MCP1826 - 000 mA, Low Voltage, Low Quiescent Current LDO Regulator

enter image description here

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Power dissipated in regulator ~= Vdropx I = (12v-5V) x I = 7Watts per Amp Or 7 mW per mA. How long it will last deep-ends on heatsinking.
Photo(s) a very good idea.

This SE answer will probably tell you more than you need or want to know on the subject of regulator heating - My linear voltage regulator is overheating very fast

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  • \$\begingroup\$ I measured the current being drawn, it looks like 1.4 A! So if my quick math is right, that'd be (12V-5V)*1.4A = 9.8 W. With the TO-220 package in open air @ 60 °C/W, that'd be a toasty 588°C (1090 °F). \$\endgroup\$
    – epiolba
    Commented Apr 12, 2020 at 19:49
  • \$\begingroup\$ I added two heat syncs like this (i.sstatic.net/yEKrR.jpg) back-to-back on the regulator and it was stable for much longer (maybe 6 minutes) before becoming unstable. So I think heat is definitely the cause... \$\endgroup\$
    – epiolba
    Commented Apr 12, 2020 at 19:50
  • \$\begingroup\$ Thanks for that previous stack link, it is super helpful \$\endgroup\$
    – epiolba
    Commented Apr 12, 2020 at 19:51

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