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I have been puzzling over this for days now, and cannot for the life of me figure it out...

I have a constant current source set up to use as an analog dimmer for my LM3429 LED driver. I have made very slight changes in order to use a digital potentiometer and avoid issues that arise from bad tolerancing of that part.

Here is a stripped back version of the cicruit I am testing on.

What I have observed so far:

  • When unconnected the LM4120 reference give a stable 5V.
  • The potentiometer is definitely changing
  • When top of potentiometer is connected to reference, Reference drops to 3.9V

I really have no idea what is going on here. Can anyone help me debug this?

enter image description here

LM4120 DS: http://www.ti.com/lit/ds/symlink/lm4120.pdf

LM3429 DS (page 14): http://www.ti.com/lit/ds/symlink/lm3429.pdf

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  • \$\begingroup\$ Are you sure this schematic is drawn correctly? Shouldn't IN/EN be tied to 6.6V and the reference pin floating? \$\endgroup\$ – user49628 Dec 4 '15 at 18:42
  • \$\begingroup\$ My mistake, well spotted. It is correct on the testing circuit, I just cobbled that schematic together quickly. Obviously trying to diagnose the problem has taken it's toll on me! Fixed it now. \$\endgroup\$ – Sensors Dec 4 '15 at 18:48
  • \$\begingroup\$ What power supply does your digital pot use? \$\endgroup\$ – Andy aka Dec 4 '15 at 19:05
  • \$\begingroup\$ The pot uses the same 3v supply that powers the control side of things. I didn't think that should affect the operation since the digital pot seems to just be a series of switches on resistor banks. Is that a correct assumption? \$\endgroup\$ – Sensors Dec 4 '15 at 19:52
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    \$\begingroup\$ Following on Andy's inquiry- your digital pot needs a supply at least as high as the highest voltage any of the 'pot' terminals will see. So it needs a 5V supply. \$\endgroup\$ – Spehro Pefhany Dec 4 '15 at 21:29

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