We have had cases where a customer has called in and told us that the output voltage of their low drop-out (LDO) voltage regulator was out of spec when they powered it up. The output voltage was significantly higher than where it should have been.
The first question when such a problem arises should be, “What is your load current?”
Often the answer is “I haven’t applied any load current yet. I am just testing the regulator now to make sure it operates correctly before I apply a load to it.”
And therein lies the problem. As it turns out, many LDOs require a minimum load current for them to operate and regulate the voltage properly. Not all LDOs have this requirement so one needs to read the datasheet carefully. Often the specification is not clearly advertised in the front page hype of the datasheet, so one needs to look beyond page one.
In some cases they will not state a minimum load current directly, but merely imply it within another spec. Take, for example, a snippet of page 2 of a typical LDO shown here:
Note how the “Output Voltage Load Regulation” spec states a condition of “Iout = 10mA to 3A”. This implies that if the output current goes higher than 3A or lower than 10mA, that the output voltage may not remain within spec. To be fair, this particular datasheet mentions the minimum load current requirement later on in the “Functional Description” section.
The point is, some LDOs require a minimum load current for the device to regulate the voltage properly. One should always carefully read the datasheet of the chosen device and not just make assumptions about how it will function.