Patent ID: 7550933

Claim:
An offline control circuit of LED driver to control a plurality of LEDs, comprising: a switching circuit generating a plurality of LED currents through a transformer to supply the LEDs; a voltage divider coupled to the LEDs and sensing a voltage across the LEDs for generating a voltage-feedback signal; a voltage-feedback circuit coupled to the voltage divider and sensing the voltage-feedback signal for generating a voltage loop signal; a current-feedback circuit coupled to the LEDs and sensing the LED currents for generating a current loop signal in accordance with a plurality of current-feedback signals of the LEDs, and in response to a maximum current of the LEDs, the current-feedback circuit comprises: a plurality of operational amplifiers, receiving the current-feedback signals for generating the current loop signal; and a second capacitor coupled from outputs of the operational amplifiers to a ground for frequency compensation; wherein the operational amplifiers are trans-conductance operational amplifier and are connected each other in parallel; and a buffer circuit coupled to the voltage-feedback circuit and the current-feedback circuit, generating a feedback signal in accordance with the voltage loop signal and the current loop signal; wherein the feedback signal is coupled to the switching circuit to control a maximum voltage of the LEDs and the maximum current of the LEDs.