# Adjustable Voltage Regulator that operates in the uA region as a Precision Current Regulator?

I've been looking at all sorts of voltage regulators to suit my needs, I'm arranging an adjustable voltage regulator in such a way that I'm actually using it as a precision current source. Turns out I can't paste an image, but you can see the "Precision Current Limiter Circuit" on pg12 of the LM317 datasheet here: http://www.ti.com/lit/ds/symlink/lm317.pdf

The idea is that there is a fixed voltage (enforced by the regulator) between the adjustable pin and the output pin. A resistor between those two nodes determines the output current.

My question is this: Is there an adjustable voltage regulator I can use at this setting for an output current of ~150uA? I haven't been able to find one that operates in this way below about 2mA. This is the only way I know of to make a precision current regulator.

EDIT: here is a good image of a voltage regulator being used as a precision current source: http://diyaudioprojects.com/Technical/Current-Regulator/1-Amp-Current-Regulator-Scheamtic.png.

This is what I am trying to achieve but the voltage regulators don't operate as intended at such low current and so I haven't been able to achieve a regulated current regardless of the load.

• voltage regulator or current regulator ... pick one, you can't have both at the same time Jan 25, 2018 at 5:59
• You may need a current mirror or servo-loop current source to get down to uA levels. I am assuming you mean current regulator. As jsotola mentioned you cannot have both from one circuit.
– user105652
Jan 25, 2018 at 6:20
• Thanks for the comments. The chip is a "linear voltage regulator" (low drop out voltage), but the way I am using it in my circuit is as a precision current regulator (see: diyaudioprojects.com/Technical/Current-Regulator/…). The picture is an adjustable voltage regulator wired to operate as a precision current regulator/source. What I want is the precision current regulator, 150uA, but I don't know of any way to achieve it at the moment because the voltage regulators I have used don't operate as intended at such low current Jan 25, 2018 at 6:46
• I've added the image for you. Feel free to edit to place it in line with your text. Jan 25, 2018 at 6:51
• If you must use a voltage regulator (X) for your Y problem, then go to Analog, Texas or Maxim and do a search for micropower adjustable voltage regulators. If you want to solve it with Y, then use a current mirror or servoed current source. Jan 25, 2018 at 7:40

The LT3092 can be pretty useful: -

Set the ratio of Rset to Rout at 15 and you get 150 uA. Read the data shett for preferred values and performance expectations of course.

The REF200 can also be pretty good and you can set it to produce 150 uA (see figure 25 in data sheet for factoring a value of "N"): -

• Neat, but pricey. I like suggestions of simple good ICs. Jan 25, 2018 at 15:24
• Thank you. I've ordered a component similar to the LT3092 :) Jan 28, 2018 at 23:02

To use a voltage regulator in this mode requires an ADJ pin current << output current. They will be difficult to find for 150uA output current.

Better to use a current mirror (low precision) or a current servo (higher precision)

simulate this circuit – Schematic created using CircuitLab

The current mirror is a very common idiom to get tracking currents. D1 offsets Vbe, in a reasonably temperatutr compensated way. The ratio R1/R2 sets the ratio of the output current to the control current. Alternatively you can simply set the voltage of Q1 base. This is fairly low precision as it depends on high gain in Q1.

The servoed current source is higher precision, as it measures the voltage in the sense resistor itself, rather than with a Vbe offset. As shown, the base current is an error to the output current, so Q2 should have a high and stable hfe for good output accuracy. Q2 can be replaced with a darlington (higher gain) or a PFET (zero gate current) to reduce or eliminate this source of error.