According circuit schematic on page-17 of TPS55332-Q1 datasheet, it's required input diode(=D1). The customer is asking why it's needed. They tried to short anode and cathode of D1, dcdc converter was unstable.
The diode located at D1 is required. The VREG input is connected to the output voltage and the device runs off of this voltage for input supply voltages less than 5V. VREG is routed internally back to VIN and D1 blocks current from flowing from VIN back to the supply.
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