LT3092 Programmable current source - Page 1
Forum section: electronics / projects

author: grimmjaw
author_rank: Regular Contributor
timestamp: December 05, 2013, 06:13:58 pm
content: Hallo eevbloger,I currently playing LT3092 Programmable current source.I'm using it as a current source by driving it with PSoC5LP.Please refer to pic for the circuit .My problem is when I set the Vdac to 0V, the R1 read ~50mV which mean that5mA is flowing throug the load.When I set the Vdac to 40mV , R1 read 90mV, and it seem like the is an offset of 50mV at Vdac.Does anyone knows what happening here?The set pin has a 10uA flowing out of it.Maybe this is the culprit?

author: c4757p
author_rank: Super Contributor
timestamp: December 05, 2013, 06:47:39 pm
content: The only way they ever show it being used in the datasheet is with a programming resistor. It looks like it should work when driven by a DAC, but have you tried doing it the way the datasheet recommends?Though I think it would be massive overkill for a non-floating, DAC-driven current source, that can just be an op amp...

author: grimmjaw
author_rank: Regular Contributor
timestamp: December 06, 2013, 07:15:23 am
content: I followed the schematic from the datasheet ,page 16.I know it is an overkill, but it should be working, and LT seem to be very proud of this chip

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 06, 2013, 10:51:02 am
content: Quote from: grimmjaw on December 05, 2013, 06:13:58 pm* Ground the set pin, what do you measure across R1 now?* Use a resistor to set the current, is it correct now?* Using the DAC, what voltage do you measure on the set pin?

author: tszaboo
author_rank: Super Contributor
timestamp: December 06, 2013, 02:14:57 pm
content: Do you have at least 1.2V on the "in" pin?

author: c4757p
author_rank: Super Contributor
timestamp: December 06, 2013, 02:18:21 pm
content: Quote from: grimmjaw on December 06, 2013, 07:15:23 amDamn, how did I miss that one?QuoteThe "Typical Applications" section of a somewhat expensive chip almost always lists a bunch of implausible uses in the hope that you will use it for everything. Sure, use it as a DAC-controlled current source! Hell, try to use it as a decoupling capacitor, we don't care, as long as you buy a lot!It looks like a nice, handy chip for a resistor-controlled current source, because the reference and all is built right in, and the specs are decent. But when controlling it with a DAC, most of those nice specs become completely irrelevant!

author: grimmjaw
author_rank: Regular Contributor
timestamp: December 08, 2013, 11:20:48 am
content: okay, to answer few questionI got 0mA when set pin is grounded, and 50mV when Vdac is attached (Vdac set to 0V)Vcc is 5v should be okay.@c4757p: It sound like LT is some evil corporation lol.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 08, 2013, 11:30:02 am
content: Quote from: grimmjaw on December 08, 2013, 11:20:48 amWhat voltage do you measure on the set pin when the DAC is set to 0?My guess is that it is not 0V.

author: grimmjaw
author_rank: Regular Contributor
timestamp: December 08, 2013, 11:37:43 am
content: Vdac is set to 0V, but you are right, after connected to the set pin I got 50mV, which put the LT3092 to 5mA output

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 08, 2013, 11:43:25 am
content: Quote from: grimmjaw on December 08, 2013, 11:37:43 amThought so, I think the dac is not capable of sinking the set pin. An opamp should be able to solve that.

author: grimmjaw
author_rank: Regular Contributor
timestamp: December 08, 2013, 12:34:28 pm
content: @PA0PBZ: sorry for the noob question, could you elaborate

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 08, 2013, 12:36:32 pm
content: A voltage follower, like this:Use the opamp with positive and negative supply, otherwise it will not go to 0V.What dac are you using?

author: grimmjaw
author_rank: Regular Contributor
timestamp: December 08, 2013, 12:39:15 pm
content: duh...I though some op amp+transistor combo..anyway will give it try. thanks for the tipi'm using build in dac in PsoC5lp

author: minime72706
author_rank: Frequent Contributor
timestamp: December 08, 2013, 04:44:00 pm
content: Just thought I'd chime in with a small piece: The LT3092 has a near-identical topology to the LT3080, which is a voltage regulator that is often discussed here.I think they made the set current so low so that it can be safely overridden by applying a voltage source (DAC output, preferably buffered?) to the set pin.I make this assumption because such a low set current often requires larger resistor values than I might use in a precision circuit. Johnson-Nyquist noise and all that.

author: PA0PBZ
author_rank: Super Contributor
timestamp: December 08, 2013, 05:52:03 pm
content: Quote from: grimmjaw on December 08, 2013, 12:39:15 pmThere you go:8.10.2 Voltage DACFor the voltage DAC (VDAC), the current DAC output is routedthrough resistors. The two ranges available for the VDAC are 0to 1.02 V and 0 to 4.08 V. In voltage mode any load connectedto the output of a DAC should be purely capacitive (the output ofthe VDAC is not buffered).

author: grimmjaw
author_rank: Regular Contributor
timestamp: December 09, 2013, 10:28:13 am
content: yup, that was the culprit.I'm using an internal volatge follower and set was reduce to 5mV,it seem that I need an external opamp with negative supply to get 0V.Thanks for the help