What is that MOSFET-looking thing between SW and the output pin in these converter block diagrams?

You are reading it mostly right, acutally. That P-channel device is the main high-side switch of the converter, and the SW pin you see is basicly its drain terminal. The chip just draws it sitting right at the switching node beacuse that is where it physically lives, so it can look odd if you were expecting it drawn off the input rail instead.

The small switch attached to it is the interesting part. It represents the body connection of the transistor. A power MOSFET has an internal body region that forms a diode between drain and source wheather you want one or not, and where that body region gets tied determines how that diode behaves. Normally the chip ties the body to the source side, so the diode points the useful direction for a buck converter and stays out of the way otherwise.

The reason it is drawn as a switch here is that this part has a pass-through mode. When the input voltage comes down close to the output voltage, there is almost nothing for the converter to do, so instead of switching away and burning overhead in the process, the chip stops toggling and just holds that P-channel device fully on. Input passes straight through to the output, minus a small drop across the FET and the inductor winding. The little switch in the diagram is how they show the body connection being steered to whichever side sits at the higher potential, wich keeps the diode from conducting backward in that mode.

One thing that confused me on the same drawing at first: the gate looks tied to the source, which whould mean the FET never turns on. That is just diagram shorthand. The gate drive circuitry pulls the gate below the source when it wants the device conducting, the simplified block diagram simply does not show it.

That explanation regarding the body connection being tied to the higher potential rail makes perfect sense regarding the necessery diode isolation, altough I’ve had some expirience with converters where that body connection is explicitly tied to the source input before the gate signal arrives. If the device is operating in a standard buck configuration, the implications of how that body tie behaves during the off state is that it determines the forward recovery characteristics of that internal diode which essentially bridges the drain and the source. It is a subtle but vital distinction for minimizing switching losses in the high frequency operation.

I'm not sure that the body connection switch is strictly there for the pass-through mode behavior. While managing the body drive is key to diode isolation, pinning its inclusion solely on that feature feels like an oversimplification of the chip's design constraints. Typically, designers manage the necessary body tie potential across the entire operational range, even during switching, to maintain consistent efficiency and prevent issues with leakage, not just when the input and output voltages are nearly matched. It seems more like a general layout choice than a specific feature for linear operation.

The body drive management is much broader than just what happens during that pass-thru phase. While the body tie simplifies the low-load scenario, managing that connection is critical for handling the reverse recovery losses during switching. If you don't keep the body potential stable, the parasitic diode can start conducting when it shouldn't, and that adds heat. On the larger industrial boards I work on, I always design for the voltage drop across the FET to stay under 10 millivolts when running at 2 amps, because that's where efficiency starts to tank rapidly.

I totally agree with the idea that you have to manage that body connection right across the whole operational range, not just in low load. When I was designing a small bench supply last year, I noticed the datasheet only talked about the body tie in a specific operating region. However, if you push the current limits, you see noise across that pin that suggests the body potential is fluctuating. It seems like keeping that tie solid helps maintain predictable switching behavior under heavy stress, even when the converter is fully toggling.