Electronic – DC current limiter micro-amps

currentdc/dc converter

Let me start by saying that I'm a student, so I apologize if this is a stupid question.

I have a "power supply" that initially supplies 2 mA of current for a few seconds and then drops down to 100 uA over 30 seconds. I'm trying to find a way to limit the output current from this supply to 100 uA or less, i.e. prevent it from exceeding 100 uA, but every IC that I've checked doesn't allow for this either at the lower limit of quiescent current or the upper limit of quiescent current.

An LDO like the LM317A seems to fit the bill, but I'm worried that this is overkill since the input current is so low and it kills the efficiency when compared to a switching regulator.

I briefly considered using a digital potentiometer controlled via microcontroller, but that's possibly the most complicated way imaginable to tackle this.

Is there a simpler way to limit the current to 100 uA that I'm unaware of? All of the ICs that I've looked at seem like overkill.

EDITED: This power supply is from a small photovoltaic cell. I would prefer to keep the voltage as low as possible, which is around 0.5 volts from the PVC since it's so much easier to add resistance than take it away, but I'm open to any ideas. Think Ti-30 calculators that use a small cell.

Best Answer

Siliconix Application Note AN103 may be of interest.

The combination of low associated operating voltage and high output impedance makes the FET attractive as a constant-current source. An adjustable-current source (Figure 1) may be built with a FET, a variable resistor, and a small battery. For optimum thermal stability, the FET should be biased near the zero temperature coefficient point.

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Figure 1. Simple FET current source.

The application note goes into considerable detail so you may find something suitable.

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