A voltage monitoring circuit is configured to monitor the input voltage in a power converter and to assert a reset signal to disable operation of the power converter in response to the input voltage falling below a threshold level. The voltage monitoring circuit may include a power-on-reset (POR) block that asserts the reset signal in response to the input voltage falling below a first threshold at a first rate, and a brown-out block that asserts the reset signal in response to the input voltage falling below a second threshold at a faster second rate (e.g., the input voltage falls quickly to zero or near zero such as during a brown-out event). The brown-out block includes a backup supply voltage that maintains some positive voltage level even in the absence of the input voltage for a certain period of time and a discharge circuit designed to quickly assert the reset signal.