EEHPRT Simple Low-Current Constant-Current Sources

Introduction

Whether driving LEDs or for transducer excitation, a need arises for a simple (ideally, two-leaded part) constant-current source which is simple to use, compact, versatile, adjustable, and has a wide span of current adjustment and excitation voltage supply range.

Unfortunately, such a part does not really exist if "small" constant current (≤20mA, even more so for ≤10mA) is desired. Several choices are available, each with its own disadvantages or limitations.

Even less choices are available if adjustment of current output is required, and/or a low tempco (temperature coefficient of changes in output without any adjustment of setpoint) is also needed.

This article primarily relies on components as currently stocked on DigiKey.

https://www.digikey.com/en/products/filter/pmic-current-regulation-management/734

TODO DK disclaimer.

Available Component Classes

Constant-Current Diode

Constant-current diodes were popular back in the day, but they are relatively rare these days (although LED-drive applications are causing a comeback, even if minimum current regulation for those who bothered to make devices is already too high at 10-20mA for state of the art modern LEDs).

We will discuss Central Semiconductor Corp CMJ series here, as a representative example for options below 10mA (more options exist for ≥10mA from onsemi et al).

Total supply voltage range: 50/100V

Minimum voltage headroom for regulation: On the order of 10V???

Current adjustment range: --

Minimum regulated current:

Maximum regulated current:

Maximum voltage delta:

Maximum power: 500mW

Does not require ground reference ("two-terminal" operation)?: Yes

Accuracy:

Reverse-voltage protection?:

Advantages:

Disadvantages: Not every current choice is available, especially exact selections like 4.0mA. Non-trivial tempco. Shitty datasheet. Large required voltage headroom???

Since these devices have a shitty datasheet, there is not much more we can determine from the datasheet. Our best understanding of what is inside these devices is that they are simply binned JFETs with controlled gate width/length, and with two terminals interconnected (see section below on making such a device yourself).

LM134 / LM234

LM134/LM234/LM334 is a strange old device, in that its current is highly dependent on IC junction temperature. With an added diode and resistor, at total component count of four devices, tempco can be minimized but NOT eliminated to the point of acceptable standards in modern times. We suppose this device still has archaic use cases. We wish there was its equivalent with zero tempco by now!!!

Total supply voltage range: 1V to 40V

Minimum voltage headroom for regulation: 1.0V

Current adjustment range: 1μA to 10mA

Minimum regulated current: 1μA (BS cover sheet claim, too optimistic per rest of datasheet tables and plots)

Maximum regulated current: 10mA

Maximum voltage delta: 40V

Maximum power: 400mW

Does not require ground reference ("two-terminal" operation)?: Yes

Accuracy: ±3%.

Reverse-voltage protection?: Yes

Advantages: Old stuff, small headroom

Disadvantages: dependence of current output on IC junction temperature, four components required, adjustable zero-tempco operation is not practical because both resistors' value would have to be adjusted in a tracking manner.

TODO schematic

TODO modern diode

LM317L

LM317 is well-known to be capable of constant-current operation (instead of more common constant-voltage operation) with slight rearrangement of external wiring.

TODO schematic

Reading LM317 datasheet, however, the reader will realize that minimum output current is rather high at TODO.

The solution is LM317L variant. Capable of only 100mA maximum output, the minimum output current also decreases to 1.5-2.5mA.

Total supply voltage range: 35V max

Minimum voltage headroom for regulation: 2.5V

Current adjustment range: 2.5mA(max) to 100mA

Minimum regulated current: 1.5mA typical, 2.5mA maximum

Maximum regulated current: 100mA

Maximum voltage delta: 35V

Maximum power:

Does not require ground reference ("two-terminal" operation)?: Yes

Accuracy:

Reverse-voltage protection?: TODO

Advantages: Very well-known, easy to use, commonly available.

Disadvantages: Headroom voltage higher than possible with other devices. Load (or load+pull-down resistor provided in schematic) must draw a minimum current of 1.5mA for constant current source to remain in regulation. All load current passes thru setpoint resistor, thus resistor power dissipation rating must be appropriate for application.

Other Circuits

TODO

Total supply voltage range:

Minimum voltage headroom for regulation:

Current adjustment range:

Minimum regulated current:

Maximum regulated current:

Maximum voltage delta:

Maximum power:

Does not require ground reference ("two-terminal" operation)?: Yes

Accuracy:

Reverse-voltage protection?:

Advantages:

Disadvantages:

Discrete Circuits

Single JFET

JFET constant-current source

A single JFET can be interconnected as shown to make a rather simple constant-current source, which can either be fixed or adjustable.

The first significant downside is that part selection and binning is required to obtain a particular current value. This is not a problem in a hobby situation, but would be unacceptable in a manufacturing environment.

Another probable downside is that the voltage regulation overhead would be higher than other solutions shown in this page.

TODO plot one of these myself!!!

TODO

Total supply voltage range:

Minimum voltage headroom for regulation:

Current adjustment range:

Minimum regulated current:

Maximum regulated current:

Maximum voltage delta:

Maximum power:

Does not require ground reference ("two-terminal" operation)?: Yes

Accuracy:

Reverse-voltage protection?:

Advantages:

Disadvantages:

Conclusion

As bizarre as it sounds, few choices exist for simple constant-current regulation below about 10mA, especially if adjustment is required, and tempco must be low. If several considerations are not a deal-killer, the trusty LM317L is probably best. Some solutions exist for niche applications like LED drive, but often have an unacceptably (for many applications) minimum required voltage headroom.