Switching controller having switching frequency hopping for power converter

1. A switching controller having switching frequency hopping for a power converter, comprising:

a first oscillator, generating a pulse signal and a maximum duty-cycle signal for determining a switching frequency of a switching signal;

a pattern generator with a second oscillator, generating a digital pattern code in response to a clock signal, wherein the clock signal is generated by the second oscillator;

a programmable capacitor, coupled to the pattern generator and the first oscillator for modulating the switching frequency of the switching signal in response to the digital pattern code; and

a PWM circuit, coupled to the first oscillator for generating the switching signal in accordance with the maximum duty-cycle signal, a maximum on-time of the switching signal being limited by the maximum duty-cycle signal, wherein the switching signal is utilized to switch a transformer of the power converter.

2. The switching controller as claimed in claim 1 , wherein a switching period of the pulse signal is correlated to a switching period of the switching signal.

3. The switching controller as claimed in claim 1 , wherein a switching period generated by the first oscillator is independent of a switching period generated by the second oscillator.

4. The switching controller as claimed in claim 1 , wherein a switching period of the pulse signal is independent of a switching period of the clock signal.

5. The switching controller as claimed in claim 1 , wherein the digital pattern code controls switching-capacitor sets to connect a saw-tooth capacitor in parallel for modulating an oscillation signal, and different capacitances of the saw-tooth capacitor cycle-by-cycle generates frequency variation of the switching signal.

6. The switching controller as claimed in claim 1 , wherein the programmable capacitor comprises a plurality of switching-capacitor sets connected to one another in parallel, the switching-capacitor sets are formed by several switches and capacitors connected in series respectively, and the switches are controlled by the digital pattern code.

7. The switching controller as claimed in claim 1 , wherein the second oscillator comprises:

a second switch, coupled to a second charging current, the second switch being controlled by the clock signal;

a second capacitor, coupled to the second charging current and connected to the second switch in parallel, wherein the second capacitor is charged by the second charging current once the second switch is turned off, and the second capacitor is discharged when the second switch is turned on; and

a second trigger and an inverter, coupled to the second switch, the second charging current, and the second capacitor for generating the clock signal.