I've used the eDesigneSuite tool to draw a buck converter. Specs are simple: Input Voltage: 185~265 Volts, Output Voltage: 4.5 Volts; Output current: 0.4 Ampere using the Viper26LN.
The attached picture show also a limit to the current at ~500mA (just to be avoid saturation in the inductance).
I've built the circuit using the BOM provided.
With very light load (0.5 ma) the output is of 4.5 Voltage. As soon I start to increase load (i.e. 35ma), the output voltage drop around 3.6 Volts, but the voltage after the diode D2 is at 4.5 voltage.
Increasing the load up to 300mA, gives the same results: current is stable, and also the output voltage, but at a lower value, leaving the voltage stable at 4.5 (and so a perfectly regulated 3.3 voltage) for the FB pin.
I think the viper stuck in burst mode, also if the load increase.
Looking with the scope, I can see a the 'single pulse' on the inductor increase the frequency, but far below the 60Khz I expected to see.
The Scope screenshot show the output with a load of 30mA. You can see the output voltage, around 3.9 Volts and the bursts from the viper, with a period of 228uSec.
Any idea/suggestions on what is happening?