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) is desired. Several choices are available, each with its own disadvantages or limitations.

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

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

TODO disclaimer.

Available Component Classes

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.

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

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: