A number of high-side switches (e.g. the NCP380) go into constant-current mode when the programmed maximum current is reached. This is typically set with an external resistor according to the required max current. (e.g. page 17 from the NCP380 datasheet). In this case they seam like a perfect replacement for a constant-current source with all the added protections in one package.
My question is can i use a voltage controlled variable resistor in its place to adjust the constant-current limit during operation e.g. from the output of a DAC. Then i could choose the current limit according to the load that is connected.
Best Answer
You can use a Voltage controlled resister to give variable current limit .You can also switch in different resister values for different currents .I have used a P channel mosfet as a voltage controlled resister to implement variable peak current limiting on Buck converters using standard hard switching peak current mode chips and on descrete component designs that soft switch .The barefoot P channel mosfet is not very accurate due to on resistance spreads but as temp increases so does on resistance .For most of my jobs this is a good thing for reliability at elevated temperatures like in cars .If you need very accurate current for say a science application then your voltage controlled resister would not be a simple bare foot mosfet .If the negative temp co of output current is not desirable like on say a fan driver then set up at more current than needed and regulate the fan voltage .Summing up if the reason for current limit is really for protection then the simple P channel works very well.If you need accuracy then Spreads in mosfet on resistance will make this concept a production disaster if you just use one barefoot mosfet .Remember that a cheap resister can easily be 1% or better but a mosfet is about 30% .