Apparatus defining a circuit module for receiving an operating voltage on an input and for generating a voltage of a lower magnitude that is applied to an intermediate bus. A holdover energy source, such as an ultra capacitor or the like, of the circuit module has a voltage lower than said input voltage and the voltage of said intermediate bus. The holdover energy source is connected under control of a voltage supervisor to a boost converter in response to the receipt of a transient on the input. The boost converter is responsive to the connection to generate a boost voltage that maintains a constant voltage on the intermediate bus.