what is undervoltage
Actually the undervoltage detector senses the voltage on the bootstrap capacitor, or the Vcc voltage, depending on which driver you refer to. But this voltage is used by the corresponding output stage to drive the MOSFET gate, so sensing it makes perfect sense, because the driver will only lose a few hundred millivolts. So in good approximation this voltage can be considered the MOSFET drive voltage.
Note that this is a DC voltage, and therefore, easier to sense. The MOSFET gate is driven at a certain frequency with a certain duty-cylce, so sensing it would be much more complicated.
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