LLC controller with programmable fractional burst frequency

A controller for use in an LLC resonant converter, comprising:

• first and second fractional current mirrors coupled to be driven in response to a maximum frequency signal;
• a third fractional current mirror coupled to be driven in response to a feedback signal representative of an output of the LLC resonant converter;
• a state machine coupled to the first, second and third fractional current mirrors to control a first, second and third current through the first, second and third fractional current mirrors, respectively;
• a first comparator coupled to generate a first clamp signal in response to a comparison of a feedback current and the first current;
• a second comparator coupled to generate a second clamp signal in response to a comparison of a third current and the second current;
• and
• an oscillator coupled to control a switching frequency of the LLC resonant converter in response to the feedback signal, wherein the oscillator is coupled to decrease the switching frequency in response to an increase in load and increase the switching frequency in response to a decrease in load, wherein a programmable burst stop frequency of a burst mode of operation is responsive to the first or the second clamp signal being activated, wherein a programmable burst start frequency of the burst mode is responsive to the first and second clamp signals being deactivated.

A method of controlling a resonant converter, comprising:

• switching the resonant converter at a switching frequency in response to a feedback signal to control a transfer of energy to an output of the resonant converter;
• decreasing the switching frequency as low as a minimum switching frequency in response to an increase in load at the output of the resonant converter;
• increasing the switching frequency as high as a maximum switching frequency in response to a decrease in load at the output of the resonant converter;
• switching the resonant converter at a burst mode frequency of a burst mode of operation of the resonant converter in response to a low load or no load condition at the output of the resonant converter, wherein the burst mode frequency is a fraction of the maximum switching frequency;
• wherein the burst mode frequency includes a burst start frequency and a burst stop frequency, wherein the burst start frequency is less than the burst stop frequency;
• wherein the burst start frequency and the burst stop frequency are fractions of the maximum switching frequency;
• and programming the burst stop frequency and the burst start frequency in response to the maximum switching frequency.