LED driver having a pre-chargeable feedback for maintaining current and the method using the same

A driving circuit of a light emitting diode (LED), including a driving unit, a current pre-charging unit and a feedback unit, is provided. The driving unit outputs a driving power to drive the LEDs and outputs at least one first feedback signal according to the current conducted in the LEDs. The current pre-charging unit is coupled to an output of the driving unit to provide a current path to the driving unit and generates a second feedback signal. One of the at least one first feedback signal is selected to adjust the driving power when the enable signal is at a first logic level; the second feedback signal is selected to adjust the driving power when the enable signal is at a second logic level so as to maintain a current to drive the LEDs.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of Taiwan application serial no. 98146199, filed on Dec. 31, 2009. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of this specification.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to a driving circuit of a light emitting diode (LED), and more specifically, to a driving circuit for maintaining a current driving ability.

2. Description of Related Art

As concerns for environmental and energy preservation have drastically increased, the light emitting diode (LED) has gradually overtaken traditional incandescent light sources as the light source of the new millennium, owing to its small size, energy efficiency and durability. The LED emits light by converting electrical energy into light energy. Since an LED chip is made of semiconductor materials, the emitted wavelength is determined by the energy gap levels of the materials used, thereby allowing the LED to emit different colors of light. Therefore, not only can the LED be used as a white light source, but it may also be adopted as automobile headlights, traffic signals, text displays, billboards and other large displays, as well as regular and architectural lighting and backlights for a liquid crystal display (LCD), etc.

Since the emitted brightness of the LED is related with the size of the current conducted, the driving capability of the current is emphasized while driving the LED. Especially for large LED billboards, since the purpose is to display images or texts, the response time of the LED is of great importance. In large LED billboards, due to the large number of LEDs, a significant magnitude of current is needed for driving the billboards. If a driving circuit cannot provide enough current in time, then the LED might not be able to display accurate brightness and image quality. Therefore, for an LED display or billboard, a current driving ability of an LED driving circuit becomes a matter of great importance. However, conventional LED driving circuits mainly focus on current stability or driving voltage control, but few have placed emphasis on improving the driving ability of the driving current.

SUMMARY OF THE INVENTION

The present invention provides a driving circuit of a light emitting diode (LED) and a driving method thereof. A current pre-charging unit is disposed in the driving circuit, so as to maintain a current driving capability of the driving circuit to provide a sufficient current to drive the LED when the LED is being switched on. Accordingly, a switching speed of the LED is accelerated, thereby preventing the issue of insufficient current affecting a display speed of the LED.

In light of the foregoing, an aspect of the invention provides a driving circuit adapted to drive at least an LED unit, in which the LED unit includes at least an LED. The driving circuit includes a driving unit, a current pre-charging unit, and a feedback selecting unit. The driving unit outputs a driving power to the LED unit, and outputs at least a first feedback signal according to a current conducted by the LED unit. The current pre-charging unit is coupled to an output of the driving unit to provide a current path to the driving unit, and the current pre-charging unit generates a second feedback signal according to the driving power. The feedback selecting unit is coupled to the driving unit and the current pre-charging unit, and according to an enable signal, the feedback selecting unit selects one of the first feedback signals or the second feedback signal FS as an output. The driving unit adjusts the driving power according to the output of the feedback selecting unit.

In one embodiment of the invention, when the enable signal is disabled and the LED unit is turned off, the feedback selecting unit selects the second feedback signal as the output, so that an output current of the driving unit is maintained at a predetermined value. When the enable signal is enabled and the LED unit is turned on, the feedback selecting unit selects one of the first feedback signals having a smallest voltage as the output.

In one embodiment of the invention, the pre-charging unit includes a first resistor, a second resistor, an inverter, and a switch. The first resistor and the second resistor are serially coupled between the output of the driving unit and the switch. The inverter has an input end coupled to the enable signal and an output end coupled to the switch. When the enable signal is disabled, the switch is turned on, and when the enable signal is enabled, the switch is turned off.

In one embodiment of the invention, the above-described current pre-charging unit includes a first resistor, an adjustable current source, a control unit, and a delay unit. An end of the first resistor is coupled to the output of the driving unit, and the adjustable current source is coupled between another end of the first resistor and the ground. The control unit is coupled to the adjustable current source, and according to an original enable signal, the control unit adjusts a conducted current of the adjustable current source. The delay unit is coupled between the feedback selecting unit, the current controlling circuit, and the original enable signal. The delay unit is configured to delay the original enable signal a predetermined time so as to output the enable signal to the feedback selecting unit and the current controlling circuit. A node between the adjustable current source and the first resistor outputs the second feedback signal. During the predetermined time delay, the control unit changes the conducted current of the adjustable current source in accordance with the original enable signal, so as to increase an output current of the driving unit. Moreover, when the LEDs are turned on (i.e., after the predetermined time delay), the current of the adjustable current source is automatically turned off.

Another aspect of the invention provides a driving method of an LED, including the following steps. A driving power is outputted to drive an LED unit and generate at least a first feedback signal. Moreover, a current path is provided to the driving power, and a second feedback signal is generated according to a current conducted by the current path. According to an enable signal, whether to turn on the LEDs is determined. For example, when the enable signal is at logic level one, the driving power is conducted to the LED unit to turn on the LED s; and when the enable signal is at logic level zero, the driving power is not conducted to the LED unit so as to turn off the LEDs.

When the LED unit is turned off, the driving power is adjusted according to the second feedback signal. When the LED unit is turned on, the driving power is adjusted according to the first feedback signal. Please refer to the foregoing description of the driving circuit for the remaining operating details of the driving method. Hence, a detailed description thereof is omitted.

In summary, according to the invention as embodied and broadly described herein, a current driving capability of the driving circuit is maintained by an advance adjustment to a current, so that a sufficient current may be provided to an LED when the LED is being switched, and thereby accelerating a turn on time of the LED. Moreover, an embodiment of the invention uses the current pre-charging unit to provide feedback signals, so that even when the LED is turned off, the driving circuit may adjust the output of the power converting unit according to the feedback signals.

DESCRIPTION OF EMBODIMENTS

First Embodiment

Referring toFIG. 1A,FIG. 1Ais a schematic block diagram of a driving circuit in accordance with a first embodiment of the invention. The driving circuit is adapted to drive at least a light emitting diode (LED), and the driving circuit includes a driving unit105, a feedback selecting unit130, and a current pre-charging unit150. The driving unit105is coupled to an LED unit160, and the LED unit160includes at least a LED. The LEDs may form an LED string by mutually connecting in series. The driving unit105outputs a driving power VOUT to the LED unit160, and outputs at least a first feedback signal F1-Fnaccording to a current conducted by the LED string (not shown). The current conducted by the LED string can be derived by the first feedback signals F1-Fncorresponding to the divided voltage of each LED string. The current pre-charging unit150is coupled to an output of the driving unit105to provide a current path to the driving unit105. Moreover, the current pre-charging unit150generates a second feedback signal FS according to a current conducted by the current pre-charging unit150, or a voltage level of the driving power VOUT. The feedback selecting unit130is coupled to the driving unit105and the current pre-charging unit150. The feedback selecting unit130selects the first feedback signals F1-Fnor the second feedback signal FS according to an enable signal EN. The driving unit105adjusts the driving power VOUT according to an output of the feedback selecting unit130.

When the LED string in the LED unit160is turned on, the feedback selecting unit130selects one of the first feedback signals F1-Fnas the output (e.g., a feedback signal having a lowest voltage), and the driving unit105adjusts the driving power VOUT according to the selected feedback signal. At this time, the operation of the entire driving circuit is similar to a conventional LED driving circuit, in that a feedback signal of an LED device is used to adjust the driving voltage VOUT. Generally speaking, the first feedback signals F1-Fnare related to a current conducted by a corresponding LED. When the LEDs in the LED unit160are turned off, the feedback selecting unit130selects the second feedback signal FS as the output. The driving unit105adjusts the driving power VOUT according to the second feedback signal FS. In other words, when the LED unit160is turned off, the driving unit105still maintains a magnitude of output current for conducting the LED unit160.

The current pre-charging unit150may be viewed as a second output current path of the driving unit105. When the LEDs in the LED unit160are temporarily turned off (i.e. when switched off by the enable signal EN), an output current of the driving unit105may be maintained by the current pre-charging unit150, so as to prevent the output current from being lowered to zero. Accordingly, when the LEDs are again conducted, the driving unit105is capable of rapidly providing a large current to the LEDs, thereby preventing a delay in conducting the LEDs that may affect normal display. At this time, the current pre-charging unit150may coordinate with the conduction of the LEDs by temporarily shutting down, so as to lower current consumption. Then, according to the enable signal EN, a turn on time of the current pre-charging unit150may be determined.

Next, a circuit of the driving unit105is further illustrated in the following. Referring toFIG. 1B,FIG. 1Bis a circuit diagram of the driving unit105in accordance with the first embodiment of the invention. The driving unit105includes a power converting unit110, a pulse width modulating (PWM) unit120, and a current adjusting unit140. The power converting unit110is coupled to the LED unit160formed by a plurality of LED strings, and the power converting unit110is configured to convert an input voltage VIN to the driving voltage VOUT for the LED strings in the LED unit160. The LED strings in the LED unit160are formed by a plurality of LEDs connected in series respectively. One end of each LED string is coupled to the driving voltage VOUT; and the other end of the LED string is coupled to the current adjusting unit140. The current adjusting unit140has a capability to control a conducting current of each of the LED strings and may selectively conduct specific LED strings when the enable signal EN is enabled. The current pre-charging unit150is coupled to an output of the power converting unit110, and the feedback selecting unit130is coupled to the current pre-charging unit150and the PWM unit120. The feedback selecting unit130may receive the first feedback signals F1-Fncoming from the current adjusting unit140, and may receive the second feedback signal FS coming from the current pre-charging unit150. The first feedback signals F1-Fn feedback voltages of nodes connecting the LED strings and the current adjusting unit140. The second feedback signal FS coming from the current pre-charging unit150changes according to a size of the current conducted.

The PWM unit120may adjust a PWM signal PWMS according to a feedback signal selected by the feedback selecting unit130, so that the power converting unit110outputs a corresponding driving power VOUT. In another embodiment of the invention, the driving unit105may also employ a pulse frequency modulating unit to adjust the driving power VOUT. In other words, the pulse frequency modulating unit may be used to replace the PWM unit120.

When the enable signal EN is enabled, the current adjusting unit140conducts and lights the LED strings. At this time the feedback selecting unit130selects one of the first feedback signals F1-Fncorresponding to the conducted LED strings (e.g., a feedback signal having the lowest voltage at a specified connecting node), and outputs the selected feedback signals to the PWM unit120. The PWM unit120adjusts the output of the power converting unit110according to the selected feedback signal, so that the LED strings have a sufficient driving current. When the enable signal EN is disabled, the current adjusting unit140turns off the LED strings to block the current from flowing into the LED strings. At this time, the feedback selecting unit130selects and outputs the second feedback signal FS to the PWM unit120. The PWM unit120adjusts the output of the power converting unit110according to the second feedback signal FS, so that an output current or an output voltage of the power converting unit110is maintained at a predetermined value.

In other words, the present embodiment adds a current path in the driving circuit of the LEDs, so that the power converting unit110may maintain an output current value, and therefore current output is not discontinued due to a conductive state of the LEDs. In the present embodiment of the invention, when the LEDs are turned off, the magnitude of the conducted current by the power converting unit110may be maintained by the current pre-charging unit150so that the power converting unit110continues to output current. Accordingly, when the enable signal EN is enabled and the power converting unit110may directly provide a required current for conducting the LEDs as long as the current pre-charging unit150discontinues conducting current, and therefore an issue of insufficient driving current is prevented. The magnitude of current conducted by the current pre-charging unit150may be determined according to a design requirement, wherein the magnitude of the conducted current may be controlled by the feedback selecting unit130or by a direct setting. The invention is not limited to the aforementioned methods of setting the magnitude of the conducted current. It is sufficient if the current pre-charging unit150can provide a current path for the power converting unit110. Moreover, the current pre-charging unit150needs not to be continuously conducting. To save power, the current pre-charging unit150may be configured to conduct only before the LED is turned on so that the current driving capability is maintained.

As shown inFIG. 2A, the current pre-charging unit150may be formed by serially coupled resistors.FIG. 2Ais a detailed circuit diagram of a driving circuit in accordance with the first embodiment of the invention. As an illustrative example, the LED unit160is represented as a single LED string, and by setting a corresponding current controlling circuit142according to each specific LED string, the current adjusting unit140controls a magnitude of current conducted by the LED string. TakingFIG. 2Afor example, the current controlling circuit142is coupled to an end of the LED string, and the current controlling circuit142generates a current I1according to the enable signal EN. A first feedback signal F1is generated by a node between the current controlling circuit142and the LED string. It should be noted that, according to a circuit design requirement, a metal-oxide-semiconductor (MOS) device may be disposed between the current controlling circuit142and the LED string to separate the current controlling circuit142and the LED string, and this does not affect the generation of the first feedback signal F1.

The current controlling circuit142is formed by a structure including a current source and a current mirror, for example, although the invention is not limited thereto. The current pre-charging unit150includes a resistor R21and a resistor R22serially coupled between the driving power VOUT and a ground GND. The PWM unit120includes a PWM generator122and an operational amplifier124. An input end of the operational amplifier124is coupled to an output of the feedback selecting unit130and a reference voltage Vref, respectively. An output end of the operational amplifier124is coupled to the PWM generator122.

When the enable signal EN is enabled, the LED string is in a normal display mode (i.e., a conducting mode), and the feedback selecting unit130selects the first feedback signal F1as the output. According to a voltage of the first feedback signal F1, the PWM unit120determines whether a voltage and a current of the driving power VOUT are sufficient to drive the LED string, and outputs the PWM signal PWMS to the power converting unit110. The power converting unit110is a boost circuit or a buck circuit, for example, that uses a duty cycle of the PWM signal PWMS to adjust a voltage value of the driving power VOUT. In conventional techniques, when the LED string is turned off, the power converting unit110lowers the voltage of the driving power VOUT to zero. Therefore, when the LED string is lit again, the power converting unit110cannot timely provide a large current to the LEDs. At this time, the LED string may have an issue of a turn-on delay or an inaccurate brightness. To mitigate this issue, the present embodiment adds a current pre-charging unit150in the driving circuit, thereby providing the power converting unit110another current path so as to maintain the current output of the power converting unit110. When the LED string is conducting again, the required driving current may be rapidly supplied, and thereby a driving speed is accelerated.

When the LED string is turned off, the feedback selecting unit130switches a feedback path to the current pre-charging unit150, so as to output the second feedback signal FS. According to the second feedback signal FS, the PWM unit120adjusts the duty cycle of the PWM signal PWMS, so as to adjust the output voltage of the power converting unit110. According to a current I1when the LED string is turned on, the current pre-charging unit150may set resistance values of the resistors R21and R22, so that the power converting unit110may output a corresponding current of a predetermined value when the LED string is turned off. Thereby, the power converting unit110may maintain a current driving ability thereof for conducting the LED string.

In the present embodiment of the invention, the number of LED strings driven by the power converting unit110is not limited. A plurality of LED strings may be driven by the power converting unit110, and two LED strings are used in an illustrative example hereafter. Referring toFIG. 2B,FIG. 2Bis a circuit diagram of a driving circuit in accordance with the first embodiment of the invention. The power converting unit110is configured to drive two LED strings, in which the LED strings are coupled between the output of the power converting unit110and the current controlling circuits142and144. Nodes between the current controlling circuits142and144and the LED strings respectively output the first feedback signals F1and F2to the feedback selecting unit130. Moreover, the enable signal EN is used to determine whether to conduct the LED strings. In the present embodiment of the invention, the enable signal EN may include a first enable signal EN1and a second enable signal EN2, respectively configured to control the current controlling circuits142and144.

Similarly, when the enable signal is enabled, the feedback selecting unit130returns one of the first feedback signals F1-F2to the PWM unit120. When the enable signal is disabled, the feedback selecting unit130returns the second feedback signal FS to the PWM unit120. The PWM unit120adjusts the output of the power converting unit110according to the feedback signal received. As other parts of the circuit structure betweenFIGS. 2B and 2Aare similar, and the further descriptions thereof are omitted hereinafter.

Second Embodiment

It should be noted that, as shown inFIG. 3A, a switch may be added in the above-described current pre-charging unit150(e.g., implemented by a NMOS transistor M21) to conserve energy.FIG. 3Ais a circuit diagram of a driving circuit in accordance with a second embodiment of the invention. A difference betweenFIGS. 3A and 2Alies in the NMOS transistor M21. A current pre-charging unit350depicted inFIG. 3Aincludes resistors R21and R22along with the NMOS transistor M21. The NMOS transistor M21is coupled between the transistor R22and the ground GND, and controlled by an inverted enable signal EN. The NMOS transistor M21may selectively conduct the current pre-charging unit350. Moreover, when the LED strings need to be turned off for a period of time, or when a display power is turned off, the current pre-charging unit350may turn off a current path to conserve power. During normal operation, when the enable signal EN is enabled, the NMOS transistor M21is turned off. When the enable signal EN is disabled, the NMOS transistor M21is turned on. Moreover, the NMOS transistor M21may be controlled by the feedback selecting unit130in accordance with the enable signal EN, or the NMOS transistor M21may be controlled by an independent control circuit such as an inverter310, and the invention is not limited thereto. As other parts of the circuit structure betweenFIGS. 3A and 2Aare similar, and the further descriptions thereof are omitted hereinafter.

Similarly,FIG. 3Amay be adapted to drive a plurality of LED strings, as shown inFIG. 3B.FIG. 3Bis a circuit diagram of a driving circuit in accordance with the second embodiment of the invention.FIG. 3Bincludes two LED strings, andFIGS. 2B and 3Bmay be referred to for a manner for driving the LED strings, and thus the further descriptions are omitted hereinafter.

Third Embodiment

Next, referring toFIG. 3C,FIG. 3Cis a circuit diagram of a driving circuit in accordance with a third embodiment of the invention. A difference betweenFIGS. 3C and 3Alies in a current pre-charging unit355. The current pre-charging unit355includes the resistor R21, an adjustable current source320, a control unit360, and a delay unit370. The resistor R21and the adjustable current source320are coupled between the driving power VOUT and the ground GND. A node between the resistor R21and the adjustable current source320outputs the second feedback signal FS. The control unit360is coupled between an original enable signal OEN and the adjustable current source320. According to the original enable signal OEN, the control unit360adjusts a current conducted by the adjustable current source320. The delay unit is coupled between the feedback selecting unit130and the current controlling circuit142. The delay unit370is configured to delay the original enable signal OEN, so as to generate the enable signal EN for the feedback selecting unit130and the current controlling circuit142. During a delayed predetermined time of the original enable signal OEN, the control signal360adjusts beforehand the adjustable current source320in accordance with a magnitude of current to be conducted by the LED strings, so that the output current of the power converting unit110is increased in advance. After the predetermined time, the control unit360disables the adjustable current source320, so that current originally conducted by the current pre-charging unit355is directed to the LED strings which are designated to turn on. Likewise, the current pre-charging unit355may be configured inFIG. 3Bto replace the current pre-charging unit350.

The above-described current controlling circuits142and144are configured to control a conducted current of an LED string. According to a design requirement, there may be a plurality of circuit structures for implementing the current controlling circuits142and144, for example as shown inFIG. 4.FIG. 4illustrates a plurality of circuit structures of a current controlling circuit in accordance with an embodiment of the invention. Taking the current controlling circuit142as an example and referring toFIGS. 4(a)-4(c), inFIG. 4(a) the current controlling circuit142may be formed by a current source410and a current mirror420, configured to determine a current conducted by a LED string. It should be noted that, whether the current mirror420is enabled is controlled by the enable signal EN, and an exemplary implementation thereof may be a switch disposed in a current conducting path of the current mirror420.

InFIG. 4(b), to implement the current controlling circuit142, a resistor R41is connected in series with a NMOS transistor M41between the first feedback signal F1and the ground GND. The NMOS transistor M41is controlled by the enable signal EN, and a size of a current conducted by the NMOS transistor M41may be determined by a voltage of the enable signal EN. InFIG. 4(c), the current controlling circuit142may be formed by an operational amplifier430, a NMOS transistor M42, and a resistor R42. The input ends of the operational amplifier430are respectively coupled to the enable signal EN and a source of the NMOS transistor M42. The resistor R42is coupled between the source of the NMOS transistor M42and the ground GND. It should be noted that, the circuit structures in the above-describedFIG. 4are merely exemplary embodiments of the circuit controlling circuit142, and the invention is not limited thereto.

The current pre-charging unit150is configured to provide the power converting unit110another current path, and the current pre-charging unit150is adapted to maintain the output current of the power converting unit110when the LEDs are turned off. Moreover, in another embodiment of the invention, when the LEDs are about to be lit, the current pre-charging unit150may dynamically adjust the output current of the power converting unit110according to a corresponding magnitude of current of the enable signal EN, so that the LEDs may receive the needed current in time. For example, when the enable signal EN is about to conduct a current I1needed by a LED string, the current pre-charging unit150increases the output current of the power converting unit110by the current I1. When the enable signal EN is about to conduct a current two times I1needed by a LED string, the current pre-charging unit150increases the output current of the power converting unit110by two times the current I1. In other words, the current pre-charging unit150dynamically adjusts the magnitude of current conducted according to the enable signal EN, so that the output current of the power converting unit150may perform adjustments in advance according to the enable signal EN.

The adjustable current source320in the current pre-charging unit355may be implemented by various circuit structures according to a design requirement, for example as shown inFIG. 5.FIG. 5illustrates a plurality of circuit structures of a current pre-charging unit in accordance with an embodiment of the invention. Referring toFIG. 5(a), in the current pre-charging unit150, a resistor R21and an adjustable resistor R51are connected in series between the driving power VOUT and the ground GND. By adjusting a resistance value of the adjustable resistor R51, the current pre-charging unit150may adjust the output current of the power converting unit110. In another embodiment of the invention, the adjustable resistor R51may be directly adjusted by using the feedback selecting unit130according to the enable signal EN to adjust the output current of the power converting unit110.

Next, referring toFIG. 5(b), a difference betweenFIGS. 5(a) and5(b) lies in a NMOS transistor M51coupled between a resistor R52and the ground. A gate of the NMOS transistor M51is coupled to the control unit360. When the driving circuit enters an energy saving mode, a current path of the current pre-charging unit355may be turned off to conserve power consumption.FIG. 5(c) illustrates a circuit structure for a constant current, including a resistor R21, a NMOS transistor M52, an operational amplifier510, and a resistor R53. The resistor R21, the NMOS transistor M52, and the resistor R53are coupled between the driving power VOUT and the ground GND. A negative input end of the operational amplifier510is coupled to a source of the NMOS transistor M52. A current conducted by the current pre-charging unit355may be controlled through a positive input end of the operational amplifier510.

Referring toFIG. 5(d),FIG. 5(d) is formed by a resistor R21and a plurality of current mirrors520and530. The resistor R21is connected to the current mirrors520and530through a switch SW51and a switch SW52, respectively. The current mirrors520and530respectively mirror a current I1and a current I2from a current source551and a current source552. By controlling whether the switches SW51and SW52are turned on, the current flowing through the resistor R21may be adjusted. The switches SW51and SW52may also be controlled by the control unit360. The current mirrors520and530depicted inFIG. 5(d) may be directly integrated with a current source in the current adjusting unit140, so as to mirror the conducted currents of each of the LED strings. The current mirrors520and530may adopt a typical current mirror design, and the width to length ratios of transistors used in the current mirrors520and530may be based upon a current ratio, thus no further description is provided hereinafter. Moreover, as shown inFIG. 5(e), the above-described current mirrors520and530depicted inFIG. 5(d) may be directly represented as current sources. InFIG. 5(e), a resistor R21is coupled to a current source553, a current source554, and a current source555through a switch SW53, a switch SW54, and a switch SW55, respectively. By controlling whether the switches SW53, SW54, and SW55are turned on, the current flowing through the resistor R21may be adjusted. Likewise, the switches SW53, SW54, and SW55may also be controlled by the feedback selecting unit130.

As described above inFIGS. 5(a)-5(e), the circuit structure of the current pre-charging unit150may be formed by a resistor and an adjustable current source, as shown inFIG. 5(f). InFIG. 5(f), a resistor R21and an adjustable current source560are connected in series between the driving power VOUT and the ground GND. For the circuit structure of the adjustable current source560, please refer to the implementations illustrated in the aforementionedFIGS. 5(a)-5(e), although the invention is not limited thereto. The current magnitude of the adjustable current source560may be adjusted according to the enable signal EN or design requirement, so that the current magnitude of the adjustable current source560may be set at a predetermined value, so as to maintain the magnitude of output current from the power converting unit110.

In light of the foregoing, an embodiment of the invention adds a current pre-charging unit (e.g.,150,350, or355) at the output end of the power converting unit110. This current pre-charging unit may be enabled in advance of the LEDs being conducted, so that the output current of the power converting unit110is raised beforehand to increase a driving capability thereof. When the LEDs are turned on, this current path is automatically turned off, thereby allowing the LEDs to rapidly receive a required magnitude of current. In a dynamic display process, the current pre-charging unit may coordinate with the adjustable current source to delay the enable signal of the LEDs a period of time. In this delay time period, the output current of the power converting unit110is raised in advance. When the LEDs are turned on, a current path is automatically turned off, so as to allow the LEDs to obtain the required current magnitude. By setting the current pre-charging unit, the output current of the power converting unit110may be adjusted beforehand, so that the LEDs may be rapidly turned on.

Moreover, the above-described current adjusting unit140and the current pre-charging units150,350, and355are configured to control a current magnitude, and to generate feedback signals for adjusting the output of the power converting unit110. The circuit structures depicted in the above-describedFIGS. 4 and 5serve only as exemplary embodiments of the current adjusting unit140and the current pre-charging unit150, and thus the invention is not limited thereto. Persons of ordinary skill in the art may easily derive other feasible circuit structures by referring to the disclosure of the invention, and details are not further described hereinafter. The NMOS transistor depicted inFIGS. 4 and 5may be implemented as a PMOS transistor, with corresponding adjustments to the circuit structure.

Fourth Embodiment

From another perspective, a driving method of an LED may be generalized from the aforementioned embodiment, as illustrated inFIG. 6.FIG. 6is a flow chart of a driving method in accordance with a fourth embodiment of the invention. The driving power is outputted to drive an LED unit and generate a first feedback signal (Step S610). In this step, the first feedback signal is used to adjust a voltage and a current of the driving power, so that the LED unit receives a sufficient driving current. Another current path is provided to the driving power, and according to a current conducted by this current path, a second feedback signal is generated (Step S620). In a driving process of the LED unit, the enable signal determines whether to turn on the LEDs in the LED unit (Step S630). When the LEDs in the LED unit are turned on, the driving power is adjusted according to the first feedback signal (Step S640), and when the LED unit is turned off, the driving power is adjusted according to the second feedback signal (Step S650). Since the driving power is adjusted in accordance with the second feedback signal when the LED unit is turned off, the driving power may be adjusted by modifying the current conducted by the above-described current path, so that a sufficient current driving capability is provided in advance.

The aforementioned driving method provides an extra current path in the driving circuit of the LEDs. By adjusting a current magnitude of the driving power through this current path, the current driving capability of the driving power may match a current requirement for conducting the LEDs, so that a switching speed of the LEDs is accelerated. Please refer to the foregoing description ofFIGS. 1-5for the remaining operating details of the driving method. Thus, a detailed description thereof is omitted.

In light of the foregoing, an embodiment of the invention provides a method to enhance a driving capability of an LED driving circuit, in which an extra current path is disposed in the driving circuit for maintaining a current output capability thereof. Alternatively, according to a current magnitude required for conducting an LED string, an output current of the driving circuit may be adjusted in advance, so that the LEDs may be switched rapidly to receive the required driving current.