Adaptive feedback control in LED driving circuits

A method for generating a feedback control signal for regulating a bias voltage used for supplying a plurality of LED strings, wherein each LED string has a first terminal and a second terminal, and the plurality of LED strings receive a bias voltage at the first terminals and provide at the second terminals a plurality of headroom detecting voltages having a one-to-one correspondence with the second terminals of the plurality of LED strings, the method comprising:

• comparing each headroom detecting voltage with a high headroom threshold voltage and a low headroom threshold voltage;
• generating a modulating signal based on the comparison between each headroom detecting voltage and the high headroom threshold voltage and the comparison between each headroom detecting voltage and the low headroom threshold voltage, wherein the modulating signal decreases when any of the plurality of headroom detecting voltages is lower than the low headroom threshold voltage, the modulating signal increases when each of the plurality of headroom detecting voltages is higher than the high headroom threshold voltage, and the modulating signal keeps unchanged when each of the plurality of headroom detecting voltages is between the low headroom threshold voltage and the high headroom threshold voltage;
• and
• generating the feedback control signal based on the modulating signal, wherein the feedback control signal is used for regulating the bias voltage so that each of the plurality of headroom detecting voltages is between the high headroom threshold voltage and the low headroom threshold voltage.

An LED driving system driving a plurality of LED strings, wherein each LED string has a first terminal and a second terminal, and the plurality of LED strings receive a bias voltage at the first terminals and provide at the second terminals a plurality of headroom detecting voltages having a one-to-one correspondence with the second terminals of the plurality of LED strings, the LED driving system comprising:

• a feedback control circuit, comprising:
• a status detecting circuit configured to receive the plurality of headroom detecting voltages and to generate an up self-status signal based on comparing each headroom detecting voltage with a low headroom threshold voltage and a down self-status signal based on comparing each headroom detecting voltage with a high headroom threshold voltage;
• a counting circuit configured to generate a counting signal counting in a first direction or in a second direction opposite to the first direction or keeping unchanged based on the up self-status signal and the down self-status signal;
• and
• a modulating circuit configured to generate a modulating signal varying with the counting signal;
• wherein based on the modulating signal, the feedback control circuit is configured to generate a feedback control signal, and
• a voltage converter coupled to the feedback control circuit to receive the feedback control signal and configured to generate and regulate the bias voltage based on the feedback control signal so that each of the plurality of headroom detecting voltages is between the high headroom threshold voltage and the low headroom threshold voltage.