A power-up system, comprising: a voltage comparator configured to compare a voltage of a first supply rail (INT VDD) with a voltage of a second supply rail (EXT VDD) and to output an output signal based on the comparison, wherein the voltage comparator comprises: a first voltage-controlled oscillator, wherein a frequency of an output signal of the first voltage-controlled oscillator is controlled by the voltage of the first supply rail; a second voltage-controlled oscillator, wherein a frequency of an output signal of the second voltage-controlled oscillator is controlled by the voltage of the second supply rail; a first counter configured to count a number of periods of the output signal of the first voltage-controlled oscillator to obtain a first count value; a second counter configured to count a number of periods of the output signal of the second voltage-controlled oscillator to obtain a second count value; and a compare circuit configured to generate the output signal of the voltage comparator based on a difference between the first and second count values; and a controller configured to determine whether the voltage of the first supply rail is at least a predetermined percentage of the voltage of the second supply rail for at least a predetermined period of time based on the output signal of the voltage comparator, and, upon a determination that the voltage of the first supply rail is at least the predetermined percentage of the voltage of the second supply rail for at least the predetermined period of time, to initiate switching of a plurality of switches coupled between the first and second supply rails. a voltage comparator configured to compare a voltage of a first supply rail (INT VDD) with a voltage of a second supply rail (EXT VDD) and to output an output signal based on the comparison, wherein the voltage comparator comprises: a first voltage-controlled oscillator, wherein a frequency of an output signal of the first voltage-controlled oscillator is controlled by the voltage of the first supply rail; a second voltage-controlled oscillator, wherein a frequency of an output signal of the second voltage-controlled oscillator is controlled by the voltage of the second supply rail; a first counter configured to count a number of periods of the output signal of the first voltage-controlled oscillator to obtain a first count value; a second counter configured to count a number of periods of the output signal of the second voltage-controlled oscillator to obtain a second count value; and a compare circuit configured to generate the output signal of the voltage comparator based on a difference between the first and second count values; and a first voltage-controlled oscillator, wherein a frequency of an output signal of the first voltage-controlled oscillator is controlled by the voltage of the first supply rail; a second voltage-controlled oscillator, wherein a frequency of an output signal of the second voltage-controlled oscillator is controlled by the voltage of the second supply rail; a first counter configured to count a number of periods of the output signal of the first voltage-controlled oscillator to obtain a first count value; a second counter configured to count a number of periods of the output signal of the second voltage-controlled oscillator to obtain a second count value; and a compare circuit configured to generate the output signal of the voltage comparator based on a difference between the first and second count values; and a controller configured to determine whether the voltage of the first supply rail is at least a predetermined percentage of the voltage of the second supply rail for at least a predetermined period of time based on the output signal of the voltage comparator, and, upon a determination that the voltage of the first supply rail is at least the predetermined percentage of the voltage of the second supply rail for at least the predetermined period of time, to initiate switching of a plurality of switches coupled between the first and second supply rails.