LM317: load current lower than 3.5mA?

By the way.

Customer could push it down to 1.6mA by increasing the resistor value between "adjust" and "output", but then the operating conditions of the chip are not maintained anymore... as the voltage between "adjust" and "output" is not kept constant to 1.25V anymore, but it increases drastically, and he can't rely anymore on the output.

If a 1mA load is not achievable by this device, he is not strictly constrained to find a replacement for this specific component. He just need to achieve a current regulation in this range, so feel free also to suggest any current source device / solution that might work with such range and, possibly, with a high operating voltage input.

Thanks and regards,

Art

Hi Art,

No the device cannot operate with a load current below 3.5mA, as your customer has discovered. One way to achieve a load condition that could always force 3.5mA min, is to add a load resistance. This would assure that regardless of the load that is applied there is always at least 3.5mA draw from the device a forcing it into operation. The sizing of this resistor would need to be such that you knew the specific value of the load current. Is the customer wanting the current source to modulate in operation, or are there multiple places on the design where they require different currents spanning that range?

Regards,

John

Thanks again Jonathan. Customer just wanted to know why TI do not have devices that can operate below 3.5mA? They mentioned REF102 can be used in a current-source regulator configuration that can go in the order of uA or even nA

Hi Art,

Primarily, it is due to the different architectures of LDO's versus References. They are used for different operations, and as such have different outputs and limitations on their operations. LDO's are voltage driven devices, the current configurations in the application sections of our parts are almost all from older parts that contain an adjust pin, and therefore have floating grounds. CMOS architectures are not designed with floating grounds, and as such do not generally have this capability to operate in fixed current outputs.

Before making any recommendations for how to move forward, one question is how the current will be changed. Are they looking to change this current from 1-20mA on a single device during operation using some digital signal? Or is the current range of 1-20mA for different devices on the board, but fixed at each of the locations, like having a 1mA rail, 5mA rail..., 20mA rail?

Regards,

John

Hello John,

Thank you for your reply. Below are the response from the customer:

"How many devices do I want to use?

I need to use only 1 device being able to range from 1 to 20 mA in order to drive an LED for optogenetic application.


How do I want to change the current value?

If, for example, you see how the lm317 or the ref102+Opa operate, they both allow generating a fixed output current by tuning a resistor value.
That's exactly why I initially chose these devices, as they are very simple to control. So the idea is to tune the current by only using an external potentiometer.


Again, I am open to any solution also in using a completely different architecture. I just need to control the output of a current regulator circuit, with high input voltage (this allows to have voltage compliance)."

Let me know for any additional information needed.

Thanks and regards,

Art