Hi ,

I am planning to design a constant current source using LT3092. Below is the circuit given in the datasheet.


The equation for setting the current is given below.


My DAC maximum output is 3.3V and current at 3.3V should be 20mA.

For DAC controlled current source in the datasheet schematic there is no RSET.

May I know how do I approach this.

 

I got the answer

 

I got the answer

Now post the answer. Learning is a two-way street.

 

Sorry for the delay.You can connect as shown below.
For generating 20mA the RSET(R1 in schematic) value needs to be 165 Ohm . Please see my circuit below.




The issue what I am facing is my load will come in series with RSET and this is changing the desired current. May I know is there any way to solve this issue.


Below is the circuit with load.You can see that the current is changed.



I want my output current to be independent of load.

 

I think you are using the part wrong.
What do you want to do? I think you want current at the "output" pin to be 20mA.
Do you want to change the current by a voltage?
What supply voltages do you have. 5V.
That is the load? 5 ohms to 100 ohms?

 

I want to make a DAC controlled current source which will send current to my load.
The load current depends on DAC voltage .When the DAC voltage is maximum (3.3V in my case) the load current needs to be 20mA.

The load will vary like 10 Ohm,30 Ohm,50 Ohm etc..

The current should be independent of load.I mean for example when the DAC voltage is 3.3V what ever load I connect at the
current source output,the current must be 20mA

 

V2 is a voltage that goes from 0 to 3.3 and back down to 0. DAC
Rload is the load. Current is 0 to 20mA.
Transistors should be something like the 2N2222a. 2N2907.

Now that I think about it, I can make it with less parts. lol

 

Below is the LTspice sim of a modified Howland single RR op amp, constant-current circuit with a BJT buffer output driving a grounded load.

The V1 input is stepped from 0V to 3.3V (top plot) as the output load (2nd plot) varies from 1mΩ to 250Ω
As you can see, for an input of 3.3V (top blue trace) and a 5V supply, the output is a constant 20mA (bottom blue trace) until the output saturates at about 4V (third blue trace) when the load reaches 200Ω.

An interesting feature of the circuit is that the inputs (V1 and V2) are differential.

For the shown differential circuit gain of 0.1, the constant-current is 0.1(V1-V2) / R5.