Brown-out detector and method thereof. For example, the brown-out detector includes a variable resistor and a P-type current mirror including a P-type transistor associated with a threshold on-voltage. The P-type current mirror is configured to generate a characteristic voltage of a supply voltage and generate a characteristic current flowing through the variable resistor to generate a reference voltage based at least in part on the characteristic current. Additionally, the brown-out detector includes a voltage comparator configured to compare the characteristic voltage and the reference voltage and output a signal indicating a comparison result, and a controller. The threshold on-voltage is associated with a first rate of thermal change, and the characteristic current is associated with a second rate of thermal change.