Looks like it might be an MP65151DJ from Monolithic Power Systems. For reference from the datasheet (pdf):
Note the TSOT23-6 chip marking of AAN for an active-high chip (pretty sure the vertical line in your picture is the pin 1/polarity indicator). The circuit on your board seems to match the pinout as well (pin 2 = GND, pin 5 is a resistor to GND, pin 6 looks like an output with some output capacitance, etc.)