Hello,
I am currently running a power-chain simulation using LTspice, and I am experiencing an unexpected oscillation issue that I would like to understand.
In my design, a 28 V input supply is converted to 12 V using U1 (LTM8025).
This 12 V rail then feeds U2 (LTM4644), which is intended to generate four downstream supply rails.
To ensure proper startup sequencing, I connected the PGOOD pin of U1 to the RUN pin of U2 with a pull-up resistor, so that U2 only turns on after U1 is fully regulated and stable.
When U1 is simulated alone, the 12 V output is stable.
However, when U1’s VOUT is connected to the VIN of U2, the 12 V rail can no longer maintain a steady value and instead oscillates continuously, as shown in the second waveform image.
At first glance, there does not appear to be a major schematic error, and all component values follow the datasheet recommendations.
I am trying to understand:
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Why the upstream regulator (U1) becomes unstable only when U2 is connected
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Whether this could be caused by input impedance interaction, startup behavior, or control-loop interaction between the two power modules
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And what design or compensation methods are typically recommended to avoid this type of oscillation
Any insight into the root cause or guidance on how to stabilize this configuration would be greatly appreciated.
Thank you very much for your help.
