I just bought five 12v stepup converters ( U3V12F12) according to the website it should be able to deliver about 275mA at 3.3Vin. But the output voltage starts to drop at 75mA and at 100mA the output voltage drops to 5.5V (the input current is 300mA). The converter is supplied with a 3A 3.3V lab supply (no current limit) and an electronic load (in constant current mode). But at 5V it is able to deliver 0.4A.
I want to use the stepup converter to drive a load of 50ohm (resistive heater between 2 and 3 watt) from a lipo battery (3V to 4.2V).
And at some transitions it makes a high pitch noise.
What I’m I doing wrong?
I am sorry you are having trouble getting your regulators to work in your system. Even though the efficiency plots for that regulator shows that it can handle about 275mA, it cannot do that for very long. Those graphs are meant to show the efficiency of the regulator all the way out to the maximum current it can handle in short bursts; they do not represent the maximum continuous current the regulator can output.
Ultimately, we do not expect this regulator to continuously power a 240mA load, like it seems you need. However, the measurements you mentioned seem worse than I would expect. If you post pictures that clearly show your connections and setup, including where you are taking your measurements, I might be able to determine why you are measuring those values.
As for the noise, that is a result of how quickly the regulator is switching (which varies with load), so it makes sense that it could at some times be more audible than others.