Control a current limiting pin

I would start with a current mirror as the voltage is fixed and current range limited. with voltage-controlled current with a matched pair package or similar.

Hi,

it´s not clear what you want to achieve.

What does "control a pin" mean?
The voltage at this pin should be constant.
Maybe you want to control the current at the REF pin. --> then simply change the resistance of R_REF.

Nucrino said:

The datasheets explains that the output current can be set with the Rref resistance

Click to expand...

R_Ref is connected to REF_pin. Change of R_Ref will change the current at the REF_pin.
...and further this will set the range of the output current of OUTAx pins.

Nucrino said:

Would it be possible to put a voltage controlled current source there and by doing so create a Iref proportional to the voltage input ?

Click to expand...

For sure you may use a current source ... as long as you are within the datasheet given limits.

***
I´m still unsure if this is what you want.
Then you get something that is called a "multiplying DAC".

***
Requirements should come with values. We don´t know your analog_voltage range, the expected REF_pin current range, the frequency bandwidth....

Klaus

A digital potentiometer will work.
https://www.analog.com/media/en/new...ection-guide/Choosing_the_Correct_Digipot.pdf

I you are only interested in changing between a few numbers of predefined amplitudes, you might simply connect different resistors (in parallel) between thr REF pin and ground via transistors. N transistor, N resistors and N GPIO pins, allow 2^N different settings.

Nucrino said:

but i need it to be analog.

Click to expand...

sorry, I haven't catched that, so a digital potentiometer or a digital controlled resistor bank will not be suitable for you :/

KlausST said:

Hi,

it´s not clear what you want to achieve.

What does "control a pin" mean?
The voltage at this pin should be constant.
Maybe you want to control the current at the REF pin. --> then simply change the resistance of R_REF.


R_Ref is connected to REF_pin. Change of R_Ref will change the current at the REF_pin.
...and further this will set the range of the output current of OUTAx pins.


For sure you may use a current source ... as long as you are within the datasheet given limits.

***
I´m still unsure if this is what you want.
Then you get something that is called a "multiplying DAC".

***
Requirements should come with values. We don´t know your analog_voltage range, the expected REF_pin current range, the frequency bandwidth....

Klaus

Click to expand...

Hi, first of all thank you for your answer.
Yes, when I said "control the pin" I meant control the current to the REF pin.
What i am trying to do is to set the current output with an analog signal, so changing the resistance is a no go.
As for my recquirements :
- the analog signal vary from 0V to 28V.
- the LEDs I intend to drive have a If of 35 mA maximum, so I would need to vary Iout from 0 to 35 mA. The datasheet shows the graph below, it seems doable to at least make it range from 8mA to 35mA.

From what I understood, the output current is equal to Iref * K, K being programmable (default value is 512) . So using a Voltage controlled current source, I would just need to send a current equal to Iout_wanted/K no ?
Hope i gave you enough informations, if not do not hesitate.
Thanks

crutschow said:

Below is the LTspice sim of a current-mirror circuit as Tony suggested, which would seem to be the simplest solution:
The values shown give a 0-200µA max current (yellow trace) from the Ref pin for a 0-5V Cntl input (green trace).
The max current is based upon the lowest value of resistor (1.235V / 6.34KΩ) given in Table 7.1.

The current range is controlled by the input voltage and the value of R1 below.
For example R1=11.5kΩ would give the same 0-200µA range for a 0-3V input.

View attachment 188912

Click to expand...

Thanks for your answer
As I said in my answer to Klaus, my analog signal has a 0-28V range, and I would need my output current to range from at least 8mA to 35mA
Now, if my pre-programmed K is equal to 512, then i would need a maximum of 70µA to achieve 35mA in the output (assuming my calculations are correct )
So your solution would work, as for the range of the analog signal, maybe I could use a voltage divider to have my analog signal range from 0 to 2.8V ? (divide by 10)
Thank you !

stenzer said:

A digital potentiometer will work.
https://www.analog.com/media/en/new...ection-guide/Choosing_the_Correct_Digipot.pdf

I you are only interested in changing between a few numbers of predefined amplitudes, you might simply connect different resistors (in parallel) between thr REF pin and ground via transistors. N transistor, N resistors and N GPIO pins, allow 2^N different settings.

Click to expand...

a digital potentiometer or a digital controlled resistor bank will not be suitable for you :/

Click to expand...

No worries I explained my problem extremely bad.
As for the Transistors, i have been looking in that direction, but I would need to create a lot of parallel branches to make it work the way I intend to.

Hi,

Yes, I think a voltage controlled current source (or better say "current sink") is the way to go.

I´d divide the 0..28V down to let´s say 0..0.6V or so (half of the 1.23V to leave enough headroom) using two resistors.
then use a MOSFET with Shunt (0.6V / 70uA = 8.5kOhms)
Then use an OPAMP as regualtor: IN+ = input from voltage divider, IN- is feedback from shunt.
(eventually add some RC for stability improvement)

Schematics are all over the internet.

Klaus

Nucrino said:

my analog signal has a 0-28V range

Click to expand...

Nucrino said:

Now, if my pre-programmed K is equal to 512, then i would need a maximum of 70µA to achieve 35mA in the output (assuming my calculations are correct

Click to expand...

That looks correct.
Below is the current-mirror circuit to give that current from 28V in:

The Ref output mirror-current is basically (Vcntl - 0.6V) / R1.