I've got an MCU with a 3.3V DAC onboard and I'd like to convert the 0-3.3V signal to a 4-20mA output. Any ideas on how to do this?
You can use one of the ICs called "voltage to current" converters and follow one of the appnotes or datasheet examples to achieve your goal. You get a lot with them, but of course you pay in terms of cost and single-source.
A way is to use a +V power source to provide whatever voltage compliance you want (with an output impedance of some kind, usually sized larger for perhaps a watt or more) and a BJT to sink current to ground. In the emitter leg of the BJT, there is a current measuring resistor that develops a ground-referenced voltage that is used to feed back to an opamp that is driving the BJT. But you need to worry about oscillation and perhaps calibration with offset and gain adjustments. Often, that is handled by using a dual op-amp package like the cheap and decent TL062A, with one of the opamps accepting the voltage input and providing offset and gain adjustments and the other opamp doing the current monitoring and driving the BJT sink.
You haven't said a word about the necessary voltage compliance for the 4-20mA output. Nor the specific MCU and the characteristics of the DAC (like its precision, for example, let alone output characteristics.) Nor what kind of accuracy you need and whether or not you care about offset errors and/or gain errors, differential or integral linearity, etc.
Also, what have you googled and rejected?