Light-emitting diode (LED) driving circuit

A light-emitting diode (LED) driving circuit for driving a plurality of first lightbars and a plurality of second lightbars, each of the plurality of first and second lightbars comprising a plurality of LEDs coupled in series, each of the first and second lightbars having a first terminal coupled to receive a direct-current (DC) voltage and a second terminal, the LED driving circuit comprising:

• a first current mirror coupled to the second terminals of the plurality of first lightbars for balancing current among the plurality of first lightbars when the first current mirror is enabled, and causing current of the plurality of first lightbars to become zero when the first current mirror is disabled, wherein the first current mirror comprises a first constant current source, a plurality of first transistors and a second transistor, the first constant current source providing a first constant current, each of the plurality of first transistors and the second transistor having a first terminal, a second terminal and a control terminal, the first terminal of each first transistor coupled to the second terminal of a corresponding first lightbar;
• a second current mirror coupled to the second terminals of the plurality of second lightbars for balancing current among the plurality of second lightbars when the second current mirror is enabled, and causing current of the plurality of second lightbars to become zero when the second current mirror is disabled, wherein the second current mirror comprises a second constant current source, a plurality of third transistors and a fourth transistor, the second constant current source providing a second constant current having the same magnitude as the first constant current, each of the plurality of third transistors and the fourth transistor having a first terminal, a second terminal and a control terminal, the first terminal of each third transistor coupled to the second terminal of a corresponding second lightbar;
• and
• a control circuit coupled to the first and second current mirrors for, during a first period, disabling the second current mirror and adjusting the duration of enabling the first current mirror according to a dimming signal;
• and, during a second period, disabling the first current mirror and adjusting the duration of enabling the second current mirror according to the dimming signal, the first and second periods being repeated alternatively, wherein the second terminals of the plurality of first transistors and the second transistor coupled to one another and to the control circuit, the control terminals of the plurality of first transistors and the second transistor coupled to one another and to the first terminal of the second transistor, the first constant current source and the control circuit, the plurality of first transistors and the second transistor being matched to one another, wherein the second terminals of the plurality of third transistors and the fourth transistor coupled to one another and to the control circuit, the control terminals of the plurality of third transistors and the fourth transistor coupled to one another and to the first terminal of the fourth transistor, the second constant current source and the control circuit, the plurality of third transistors and the fourth transistor being matched to one another, wherein the control circuit comprises:
• a controller for outputting first and second pulse-width modulation (PWM) signals, with duty cycles of the first and second PWM signals being determined by the dimming signal;
• a first switch having a first terminal coupled to the second terminals of the plurality of first transistors and the second transistor, a second terminal coupled to ground and a control terminal coupled to receive the first PWM signal;
• a second switch having a first terminal coupled to the second terminals of the plurality of third transistors and the fourth transistor, a second terminal coupled to ground and a control terminal coupled to receive the second PWM signal;
• a first switch unit having a first terminal coupled to the control terminal of the second transistor, a second terminal coupled to ground and a control terminal coupled to receive a third PWM signal;
• a second switch unit having a first terminal coupled to the control terminal of the fourth transistor, a second terminal coupled to ground and a control terminal coupled to receive the third PWM signal;
• a short protection circuit coupled to the second terminals of the plurality of first and second lightbars for outputting a short signal, the short signal being valid when a voltage at the second terminal of one of the plurality of first and second lightbars is larger than a threshold voltage, the short signal being invalid when a voltage at the second terminal of one of the plurality of first and second lightbars is smaller than the threshold voltage;
• a third switch unit having a first terminal coupled to receive the third PWM signal, a second terminal coupled to the control terminal of the first switch unit and a control terminal coupled to receive the short signal, the third switch unit being open when the short signal is valid and closed when the short signal is invalid, the first switch unit being open or closed according to the third PWM signal when the third switch unit is closed, the first switch unit being closed when the third switch unit is open;
• and
• a fourth switch unit having a first terminal coupled to receive the third PWM signal, a second terminal coupled to the control terminal of the second switch unit and a control terminal coupled to receive the short signal, the fourth switch unit being open when the short signal is valid and closed when the short signal is invalid, the second switch unit being open or closed according to the third PWM signal when the fourth switch unit is closed, and the second switch unit being closed when the fourth switch unit is open.