I have redesigned the regulator circuit on my board from one based on a TI device to one using the MP2459. The spec is 12V in, 5V out with load current from 0.1 - 0.5A. The design is copied from that produced by the online DC-DC designer.
On power up when the processor comes out of reset I can see a voltage dip on the 5V output of around 1.5V. At this point, the current draw from the regulator is only around 150mA. You can see the voltage dip on the 5V output here, the blue trace where it drops to around 3.57V
And here is the current profile of the board. The voltage dip happens when the board comes out of reset and the current goes from around 80mA to 140mA.
Thank you for visiting the MPS Technical Forum. Based on the information you provided, it looks like your MP2459 circuit could be outputting a bad transient response.
What is the slew rate at which the output current (i.e. when the processor turns on) rises to 140mA from 80mA? It is strange to me that such a small current step-up would cause that big of an undershoot.
Here is what I recommend. Please try one method at a time. Either one should result in smaller undershoot:
Increase your output capacitance. Add 1x or 2x low ESR 22uF.
Add a feedforward network to your FB resistor divider. Use your circuit parameters and desired specs to properly size Cff and Rff.
