A start-up circuit for a ring current-controlled oscillator (CCO) includes a replica CCO current generator, a replica ring CCO, and a buffer. The ring CCO is connected to a CCO driver and the buffer. The CCO driver generates a CCO current based on a reference current. The ring CCO generates a CCO output voltage at a first oscillating frequency based on the CCO current. The replica CCO current generator generates a replica CCO current based on a reference voltage. The replica ring CCO generates a replica CCO output voltage at a second oscillating frequency based on the replica CCO current. The buffer provides a first current to the ring CCO when the first oscillating frequency is lower than a desired oscillating frequency, and drains a second current from the ring CCO when the first oscillating frequency is greater than the desired oscillating frequency.