Non-synchronous buck converters require an external catch diode between SW pin and GND. For proper operation this diode must be a low forward voltage Schottky diode.
Basis for this requirement is the maximum allowable negative voltage at the SW pin. Devices like MP(Q)2451, MP2456 and MP(Q)2459 specify a maximum negative voltage of -0.3V ! But typical Schottky diodes (e.g.PMEG4010) have a maximum forward voltage drop of up to 640mV.
Which diodes can be used with above non-sync converters not to violate the datasheet specification ?
In a non-synchronous buck converter the diode provides the free-wheeling path for the inductor current. During the conduction time of the diode, SW-node is at the negative potential of -Vdiode. To not disturb the IC, the clamping with the external (Schottky)-Diode must prevent IC substrate diodes from turning ON. So called Ultra-Fast silicon diodes have a too high forward voltage and result in IC disturbance du to substrate currents.
Your point is entirely correct. If you believe the rating it can’t work. So since we know it works, the abs max rating is bogus. Some other companies rate the Vsw to -1V and then per the datasheet it will work. I bet this part works to -1V as well. Try putting 2 diodes in series see if it still works, try 3 diodes in series etc to convince yourself that in fact it isn’t a problem. Ideally this rating would be adjusted to reflect reality and to provide comfort to careful engineers.