Operating circuit applied to backlight and associated method

An operating circuit applied to a backlight is provided, where the backlight includes a plurality of lighting elements, and the operating circuit includes a plurality of current control circuits, a plurality of switches, a minimum voltage selector, a supply voltage generating circuit and a control unit. The current control circuits are coupled to the lighting elements via a plurality of nodes, respectively. The switches are coupled to the nodes, respectively. The minimum voltage selector is utilized for receiving at least a portion of voltages of the plurality of nodes, and selecting a minimum voltage among the received voltages. The supply voltage generating circuit is utilized for generating a supply voltage of the lighting elements according to the minimum voltage. For each of the switches, the control unit determines an on/off state of the switch by determining whether the corresponding lighting element is an open circuit or not.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an operating circuit applied to a backlight, and more particularly, to an operating circuit applied to a light-emitting diode (LED) backlight.

2. Description of the Prior Art

Please refer toFIG. 1, which illustrates a prior art backlight module control system100. As shown inFIG. 1, the backlight module control system100includes a plurality of LED strings110_1-110_4, a plurality of current control circuits120_1-120_4, a minimum voltage selector130and a voltage booster140, where each of the LED strings110_1-110_4includes a plurality of LEDs, and each of the current control circuits120_1-120_4includes an amplifier (122_1-122_4), a transistor (M1-M4) and a resistor R.

Because each of the LEDs has a semiconductor process variation, the cross voltages of the LED strings110_1-110_4are different while the LED strings110_1-110_4are working. To confirm each of the LED strings110_1-110_4has enough cross voltage while working, the minimum voltage selector130selects a minimum voltage among the voltages VDET1-VDET4of the nodes N1-N4, and the voltage booster140determines a supply voltage Vo of the LED strings110_1-110_4according to the minimum voltage. In other words, the backlight module control system100determines the supply voltage Vo of the LED strings110_1-110_4by referring to the LED string having a largest cross voltage so as to make each of the LED strings110_1-110_4have enough cross voltage.

However, when one the LED strings110_1-110_4is burned out (i.e., being an open circuit), the corresponding voltage VDETwill always be zero. Then, because the minimum voltage VMINoutputted from the minimum voltage selector130is always zero, the voltage booster140will continuously boost the supply voltage Vo until an over voltage protection mechanism is triggered to close the operations of the backlight module control system100, causing the failure of the control mechanism.

SUMMARY OF THE INVENTION

It is therefore an objective of the present invention to provide an operating circuit applied to a backlight to prevent the control mechanism from being failed due to the burned out LED.

According to one embodiment of the present invention, an operating circuit applied to a backlight is provided, where the backlight includes a plurality of lighting elements, and the operating circuit includes a plurality of current control circuits, a plurality of switches, a minimum voltage selector, a supply voltage generating circuit and a control unit. The current control circuits are coupled to the lighting elements via a plurality of nodes, respectively. The switches are coupled to the nodes, respectively. The minimum voltage selector is utilized for receiving at least a portion of voltages of the plurality of nodes, and selecting a minimum voltage among the received voltages. The supply voltage generating circuit is utilized for generating a supply voltage of the lighting elements according to the minimum voltage. For each of the switches, the control unit determines an on/off state of the switch by determining whether the corresponding lighting element is an open circuit or not.

According to another embodiment of the present invention, an operating method applied to a backlight is provided, where the backlight comprises a plurality of lighting elements, each of the lighting elements comprises at least one lighting unit, and the operating method comprises: providing a plurality of current control circuits, coupled to the plurality of lighting elements via a plurality of nodes, respectively; providing a plurality of switches, coupled to the plurality of nodes, respectively; boosting a supply voltage to make the supply voltage greater than a required cross voltage of the plurality of lighting elements; for each of the switches, determining an on/off state of the switch by determining whether the corresponding lighting element is an open circuit; receiving at least a portion of voltages of the nodes, and select a minimum voltage among the received voltages; and adjusting the supply voltage according to the minimum voltage.

DETAILED DESCRIPTION

Please refer toFIG. 2, which illustrates an operating circuit200applied to a backlight according to one embodiment of the present invention, where the backlight includes a plurality of lighting elements, and each of the lighting elements includes at least one lighting unit. In this embodiment, each lighting unit is an LED, and the lighting elements are LED strings210_1-210—k. As shown inFIG. 2, the operating circuit200includes a plurality of current control circuits220_1-220—k, two resistors R1and R2, a control unit230, a plurality of switches SW1-SWk, a minimum voltage selector250and a supply voltage generating circuit, wherein the supply voltage generating circuit is implemented by a voltage booster260in this embodiment. The current control circuit220_1-220—kare coupled to the LED strings210_1-210—kvia nodes N1-Nk, respectively, and each of the current control circuit220_1-220—kincludes an amplifier (222_1-222—k), a transistor (M1-Mk) and a resistor R. The control unit230includes a first comparator232and a plurality of second comparators240_1-240—k.

The resistors R1and R2divide the supply voltage Vo of the LED strings210_1-210—kto generate a divided voltage Vo_div. Then the first comparator232compares the divided voltage Vo_div with a first reference voltage Vref1to generate a comparison signal ENX. The first reference voltage Vref1can be set according to a standard cross voltage of the LED strings210_1-210—kprovided by the client. In one embodiment, the first reference voltage Vref1is set a little lower than a divided standard cross voltage divided by the resistors R1and R2, and when the divided voltage Vo_div is greater than the first reference voltage Vref1, it is represented that the supply voltage Vo is greater than the required cross voltage of the LED strings210_1-210—kand all the LED strings210_1-210—kcan be operated normally, at this time, the comparison signal ENX outputted from the first comparator232is “1”, and the second comparators240_1-240—kare enabled. On the other hand, when the divided voltage Vo_div is not greater than the first reference voltage Vref1, it is represented that the cross voltages of LED strings210_1-210—kmay not be large enough to make all the LED strings210_1-210—koperated normally, at this time, the comparison signal ENX outputted from the first comparator232is “0”, and the second comparators240_1-240—kare disabled.

In other embodiments, the first reference voltage Vref1is set a little greater than the divided standard cross voltage divided by the resistors R1and R2. That is, the second comparators240_1-240—kare enabled after the supply voltage Vo is boosted. When the second comparators240_1-240—kare disabled, the switches SW1-SWk are kept switched on, and only after the second comparators240_1-240—kare enabled, the switches SW1-SWk are controlled by the switch control signals EN1-ENk, respectively.

In light of above, by referring to the statuses of the switches SW1-SWk respectively controlled by the switch control signals EN1-ENk, the minimum voltage selector250receives at least a portion of the voltages VDET1-VDETkof the nodes N1-Nk. If the LED string210_1is open, the switch control signal EN1switches off the switch SW1to make the minimum voltage selector250not receive the voltage VDET1, and the switch control signals EN2-ENk switch on the switches SW2-SWk, respectively, to make the minimum voltage selector250receive the voltages VDET2-VDETk. Therefore, the minimum voltage selector250can output the correct minimum voltage VMINto the voltage booster260to adjust the supply voltage correctly.

During the period that the divided voltage Vo_div is not greater than the first reference voltage Vref1(e.g., the operating circuit200is enabled a short time ago), it is meant that the supply voltage Vo is lower than the required cross voltage of the LED strings210_1-210—k. As the cross voltages of the LED strings210_1-210—kare not enough, the switches SW1-SWk are switched off, the minimum voltage selector250does not receive any one of the voltages VDET1-VDETkof the nodes N1-Nk, the minimum voltage selector250does not output the minimum voltage VMIN, and the voltage booster260does not receive the minimum voltage VMIN. In this embodiment, the voltage booster260has a fast boosting mechanism (not shown) to fast boost the supply voltage Vo when the voltage booster260does not receive the minimum voltage VMIN. After the divided voltage Vo_div is greater than the first reference voltage Vref1, the LED open circuit determining mechanism is enabled, the minimum voltage selector250starts to provide the minimum voltage VMIN, and the voltage booster260starts to adjust the supply voltage Vo according to the minimum voltage VMIN.

In addition, in the embodiment shown inFIG. 2, the switch control signals EN1-ENk are generating by comparing the feedback voltages VFB1-VFBkwith the second reference voltages Vref2, respectively. However, it is not meant to be a limitation of the present invention. In another embodiment of the present invention, the second comparators240_1-240—kcan compare the voltages VDET1-VDETkwith the second reference voltage Vref2to generate the switch control signals EN1-ENk, respectively. This alternative design should fall within the scope of the present invention.

Please refer toFIG. 3, which is a flowchart of an operating method applied to a backlight according to one embodiment of the present invention, where the backlight includes a plurality of lighting elements, and each of the lighting elements includes at least one lighting unit. Referring toFIG. 2andFIG. 3, the flow is described as follows.

Step300: provide a plurality of current control circuits coupled to the plurality of lighting elements via a plurality of nodes, respectively.

Step302: provide a plurality of switches respectively coupled to the plurality of nodes.

Step304: boost a supply voltage to make the supply voltage greater than a required cross voltage of the plurality of lighting elements.

Step306: for each of the switches, determine an on/off state of the switch by determining whether the corresponding lighting element is an open circuit.

Step308: receive at least a portion of voltages of the nodes, and select a minimum voltage among the received voltages.

Step310: Adjust the supply voltage according to the minimum voltage.

Briefly summarized, in the operating circuit and associated method of the present invention, a LED string is detected to determine whether the LED string is an open circuit. Then, if the LED string is determined as the open circuit, its corresponding voltage will not be used to determine the minimum voltage. Therefore, the operating circuit can determine the supply voltage of the LED strings correctly.