What is the reason for the sudden drop in output voltage of a DC/DC buck regulator when the input voltage exceeds a certain value?

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Joined Sep 22, 2023
I am a beginner and designed a circuit, as shown in the partial diagram below. The main goal is to step down the input voltage Vpp to 15V, 5V, 3.3V, and 1.8V. The 15V and 5V outputs are directly obtained from Vpp using LT8631. The 3.3V and 1.8V outputs are further stepped down from the 5V output. When testing the board, I found that when the input voltage Vpp is below 15V, all the outputs work fine. However, when Vpp exceeds 15V, the 15V output remains normal, but the 5V output suddenly drops to around 1.5V, causing the 3.3V and 1.8V outputs to be limited to 1.5V as well. As I continue to increase the voltage, the outputs do not change. If I lower the voltage back below 15V, all the outputs become normal again. Additionally, since the 3.3V output is connected to an LED, I noticed that as the voltage approaches 15V from a lower value, the LED flickers randomly as if the voltage is unstable.
The datasheet of LT8631 states that it can accept input voltages from 3V to 100V, and the circuits for the 15V and 5V outputs are identical except for the output voltage divider resistors. It feels like some kind of circuit protection is being triggered, but I'm unsure of the specific cause. Could an expert help me analyze this issue? Thank you!