How to understand an unusual internal power supply?
The ATmega169P runs on 330µA (that's micro amperes.)  So, 0.33mA.

The Trinket uses a ATtiny85 that runs on 300µA.  It also, however, has an LED that turns on when it is running.  The Trinket schematic shows the LED connected through a 470Ohm resistor.  Given a 2Volt forward voltage for the green LED, and powered from 3.3Volts, you get about 3mA for just the LED.  That's a whole order of magnitude more than the original processor (ATmega169P.)

It is entirely possible that the high voltage side supplies very little current to the low voltage side since the original processor didn't need much current.  Your Trinket pulls 10 times as much current, and so may be just too much for the power supply.

You might try removing the series resistor from the LED on the Trinket (open the circuit to the LED so it doesn't light up.)  That should greatly reduce the current, and may get things running.



I've answered what I think is the meat of your question.  Let me know if I've guessed wrong, and please do consider simplifying the question.