I have a need for a power supply circuit to deliver both +12V @ 1.5A and +5V @ 1.5A from an input voltage supply of +9V to +30V. Using the Webench design Centre, I have a design that uses a LM3481 is SEPIC confguration for the +12V rail and a LM22676 for the +5V rail.
I have been able to prove the SEPIC regulator circuit, from the full input volatge supply range. But I can only do this by isolating (removing) the LM22676 regulator. If I put the LM22676 back into the circuit (i.e re-solder to board) and apply an input volatge (to both LM3481 and LM22676), the input supply drops dramatically as if there is a huge load applied. This results in neither a +12V or +5V output.
What could be causing the input supply large voltage drop? See the attached circuit diagram as suggested from Webench.
