Patent ID: 8570014

Claim:
A method for switching the clock controlling a switch mode power supply between an internal clock and an external clock, the method comprising: (a) determining when a predetermined plurality of external clock pulses of substantially equal switching period have been sequentially detected and generating an external clock select signal in response; (b) causing the operation of the power supply to be switched over to said external clock when the external clock select signal is generated; (c) monitoring said external clock when it is controlling the operation of the power supply to determine if its switching period exceeds a predetermined dropout threshold; (d) generating a replacement clock pulse for operating said power supply whenever the external clock switching period exceeds said dropout threshold; (e) counting the number of replacement clock pulses generated sequentially and generating a reset signal when the count reaches a predetermined number; (f) causing the operation of the power supply to be switched back to said internal clock in response to said reset signal; and (g) repeating of (a) to (f).