# Patent

## Title
LED driving circuit having a large operational range in voltage

## Abstract

An LED driving circuit includes a current selecting circuit. The current selecting circuit controls the current transmission path in the plurality of LEDs according to respective threshold voltages of corresponding LEDs and a plurality of current limits.

## Background
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to an LED driving circuit, and more particularly, to an LED driving circuit having a large operational voltage range.
2. Description of the Prior Art
Compared to incandescent lamps, light emitting diodes (LEDs) are characterized in low power consumption, long lifetime, small size and fast optical response. LEDs can easily be manufactured as miniaturized or array devices, which are widely used in various electronic products. Common LED applications include outdoor stationary displays (such as billboards, signboards or traffic signs) and portable devices (such as mobile phones, notebook computers or PDAs).
Reference is made to FIG. 1 for a voltage-current chart of an LED. When the forward-bias voltage of the LED is smaller than its threshold voltage Vb, the LED only conducts a negligible amount of current and the two ends of the LED are substantially open-circuited. When the forward-bias voltage of the LED is larger than its threshold voltage Vb, the current flowing through the LED exponentially increases with the forward-bias voltage and the two ends of the LED are substantially short-circuited. In an LED driving circuit, a current source is normally adopted for driving multiple LEDs so as to provide uniform luminescence.
Reference is made to FIG. 2 for a diagram of a prior art LED driving circuit 300. The LED driving circuit 300, including a voltage source VS and a current source IS, is configured to drive a luminescent device 10. The voltage source VS can provide a driving voltage Vf for turning on the luminescent device 10, while the current source IS can stabilize a driving current If which flows through the luminescent device 10 so as to maintain uniform luminescence. Since the LED is a current-driven device whose luminescence is proportional to its driving current, the luminescent device 10 normally includes a plurality of serially-coupled light-emitting diodes LED1-LEDn in order to provide sufficient and uniform light in large-size applications. Assuming all the light-emitting diodes LED1-LEDn have the ideal threshold voltage Vb, then a driving voltage Vf equal to n*Vb is required for turning on the luminescent device 10. In the prior art LED driving circuit 100, while more light-emitting diodes can provide higher light intensity, the forward-bias voltage of the luminescent device 10 also increases accordingly, thereby reducing the effective operational voltage range.
Reference is made to FIG. 3 for a diagram of another prior art LED driving circuit 400. The LED driving circuit 400, including a power supply circuit 110, a voltage detecting circuit 410 and a current-regulating circuit 420, is configured to drive a luminescent device 10. The power supply circuit 110 includes a voltage source VS and a bridge rectifier 20. The voltage source VS can output an alternating current (AC) voltage which periodically switches between positive and negative phases, while the bridge rectifier 20 is configured to convert the AC voltage outputted in the negative phase. The power supply circuit 110 can thus provide a direct current (DC) voltage Vf for driving the luminescent device 10, wherein the value of the driving voltage Vf periodically varies with time. The current-regulating circuit 420 includes a plurality of current sources IS1-ISn respectively configured to control the light intensity of corresponding light-emitting diodes LED1-LEDn in the luminescent device 10. The voltage detecting circuit 410 can detect the value of the driving voltage Vf, thereby turning on/off the current sources IS1-ISn of the current-regulating circuit 420 accordingly. Assuming all the light-emitting diodes LED1-LEDn have the ideal threshold voltage Vb: when the driving voltage Vf reaches the threshold voltage (Vb) of the light-emitting diode LED1, the voltage detecting circuit 410 turns on the current source IS1 and turns off the current sources IS2-ISn, thereby providing a current path which starts from the voltage source VS and sequentially passes through the light-emitting diode LED1 and the current sources IS1; when the driving voltage Vf reaches the overall threshold voltage of the light-emitting diodes LED1 and LED2 (2Vb), the voltage detecting circuit 410 turns on the current source IS2 and turns off the current sources IS1 and IS3-ISn, thereby providing a current path which starts from the voltage source VS and sequentially passes through the light-emitting diode LED1, the light-emitting diode LED2 and the current sources IS2; . . . ; similarly, when the driving voltage Vf reaches the overall threshold voltage of the light-emitting diodes LED1-LEDn (n*Vb), the voltage detecting circuit 410 turns on the current source ISn and turns off the current sources IS1-ISn−1, thereby providing a current path which starts from the voltage source VS and sequentially passes through the light-emitting diodes LED1-LEDn and the current sources ISn.
However, due to variations in material and manufacturing processes, the light-emitting diodes LED1-LEDn may not have the ideal threshold voltage Vb. The prior art voltage detecting circuit 410 is unable to control each current source according to the actual threshold voltage of a corresponding light-emitting diode. For example, assuming the actual threshold voltage Vb1 of the light-emitting diode LED1 is larger than the ideal threshold voltage Vb. If the voltage detecting circuit 410 turns on the current source IS1 when Vf=Vb, the light-emitting diode LED1 cannot be turned on. Thus for non-ideal light-emitting diodes, the voltage detecting circuit 410 is normally configured to turn on the current source IS1 when the detected driving voltage Vf reaches a switching voltage Vb′ larger than Vb. If the voltage detecting circuit 410 turns on the current source IS1 until Vf=Vb′, the extra voltage (Vb′−Vb1) not only increases the power consumption of the current source IS1, but also reduces the effective operational voltage range of the LED driving circuit 400.
SUMMARY OF THE INVENTION
The present invention provides a driving circuit having a large operational voltage range and configured to drive a plurality of serially-coupled luminescent units. The driving circuit comprises a current-selecting circuit configured to control current paths in the plurality of luminescent units according to a plurality of current limits and respective threshold voltages of corresponding light emitting diodes in the plurality of luminescent units.
The present invention further provides a display device having a large operational voltage range and comprising a plurality of serially-coupled luminescent units; a power supply circuit coupled to plurality of serially-coupled luminescent units; and a driving circuit configured to drive the plurality of serially-coupled luminescent units. The driving circuit comprises a current-selecting circuit configured to control current paths in the plurality of luminescent units according to a plurality of current limits and respective threshold voltages of corresponding light emitting diodes in the plurality of luminescent units.
These and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after reading the following detailed description of the preferred embodiment that is illustrated in the various figures and drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a voltage-current chart of an LED.
FIG. 2 is a diagram of a prior art LED driving circuit.
FIG. 3 is a diagram of another prior art LED driving circuit.
FIGS. 4 and 5 are diagrams illustrating LED driving circuits according to the embodiments of the present invention.
FIG. 6 is a diagram illustrating the operation of an LED driving circuit according to the present invention.



DETAILED DESCRIPTION
FIG. 4 is a diagram illustrating an LED driving circuit 100 according to a first embodiment of the present invention. FIG. 5 is a diagram illustrating an LED driving circuit 200 according to a second embodiment of the present invention. The LED driving circuit 100 having a current-selecting circuit 120 and the LED driving circuit 200 having a current-selecting circuit 220 are configured to drive a luminescent device 10 coupled in series with a power supply circuit 110.
The power supply circuit 110 includes a voltage source VS and a bridge rectifier 20. The voltage source VS can output an AC voltage which periodically switches between positive and negative phases, while the bridge rectifier 20 is configured to convert the AC voltage having negative phase. The power supply circuit 110 can thus provide a DC voltage Vf for driving the luminescent device 10, wherein the value of the driving voltage Vf periodically varies with time. The luminescent device 10 may include a plurality of luminescent units D1-Dn+1 each having a single LED or multiple LEDs. For illustrative purpose, each luminescent unit depicted in FIG. 4 includes a single LED, but this structure does not limit the scope of the present invention. The voltages established between two adjacent luminescent units among the luminescent units D1-Dn+1 are represented by V1-Vn, respectively.
In the LED driving circuit 100 according to the first embodiment of the present invention, the current-selecting circuit 120 includes a plurality of variable current sources IS1-ISn and a plurality of adjusting circuits CKT1-CKTn. The variable current sources IS1-ISn provide adjustable current limits, based on which the currents flowing through the corresponding luminescent units D1-Dn are regulated at respective predetermined values, thereby providing brightness control and device protection. The adjusting circuits CKT1-CKTn can respectively detect the values of the voltages V1-Vn, thereby adjusting the current limits of the variable current sources IS1-ISn accordingly.
As previously illustrated, the driving voltage Vf periodically varies with time. For illustration, assume that the driving voltage Vf gradually rises from 0 after initialization. When the voltage established across the luminescent unit D1 exceeds the threshold voltage of the luminescent unit D1, the luminescent unit D1 is turned on, thereby providing a current path which starts from the voltage source VS and sequentially passes through the luminescent unit D1 and the current sources IS1. At this time, the current flowing through the luminescent unit D1 is maintained at a constant value by the variable current source IS1. Next, as the voltage V1 increases with the driving voltage Vf, the luminescent unit D2 is turned on when the voltage established across the luminescent unit D2 exceeds the threshold voltage of the luminescent unit D2. The adjusting circuit CKT1 then detects the voltage V2 or the current flowing through the luminescent unit D2, thereby gradually lowering the current limit of the variable current source IS1 to zero as the current flowing through the luminescent unit D2 increases. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1, the luminescent unit D2 and the current sources IS2, while the currents passing through the luminescent units D1 and D2 are maintained at respective constant values by the variable current sources IS1 and IS2, respectively. Similarly, as the driving voltage Vf gradually increases, the voltages V1-Vn also increase accordingly, thereby sequentially turning on the luminescent units D1-Dn. On the other hand, the adjusting circuits CKT1-CKTn respectively detect the voltages V2-Vn±1 or respectively detect the currents flowing through the luminescent units D2-Dn+1, thereby sequentially lowering the current limits of the variable current sources IS1-ISn to zero.
Assuming that when the driving voltage Vf provided by the power supply circuit 110 has a maximum value, all of the luminescent units D1-Dn are turned on and the current limits of the variable current sources IS1-ISn−1 are zero. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1-Dn and the current source ISn, while the current passing through the luminescent units D1-Dn is maintained at a constant value by the variable current source ISn. After the driving voltage Vf begins to decrease, the luminescent unit Dn is the first to be turned off due to insufficient applied voltage. The adjusting circuit CKTn−1 then detects the voltage Vn or the current flowing through the luminescent unit Dn, thereby gradually raising the current limit of the variable current source ISn−1 from zero. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent units D1-Dn−1 and the current source ISn−1, while the current flowing through the luminescent units D1-Dn−1 is maintained at a constant value by the variable current source ISn−1. Similarly, as the driving voltage Vf gradually decreases, the voltages Vn-V1 also decrease accordingly, thereby turning off the luminescent units Dn-D1 sequentially. On the other hand, the adjusting circuits CKTn−1-CKT1 respectively detect the voltages Vn-V2 or respectively detect the currents passing through the luminescent units Dn-D1, thereby sequentially increasing the current limits of the variable current sources ISn−-IS1.
In the LED driving circuit 200 according to the second embodiment of the present invention, the current-selecting circuit 220 includes a plurality of constant current sources IS1-ISn, a plurality of switches SW1-SWn and a plurality of judging units CM1-CMn. The current sources IS1-ISn provide constant current limits, based on which the currents flowing through the corresponding luminescent units D1-Dn are regulated at respective predetermined values, thereby providing brightness control and device protection. Each of the switches SW1-SWn includes a first end coupled between two corresponding adjacent luminescent units among the luminescent units D1-Dn (respectively denoted by V1-Vn), and a second end coupled to a corresponding current source among the current sources IS1-ISn. The judging units CM1-CMn can respectively detect the values of the voltages V1-Vn, thereby turning on/off the corresponding switches SW1-SWn accordingly.
As previously illustrated, the driving voltage Vf periodically varies with time. For illustration, assuming that at initialization, the driving voltage Vf is equal to 0 and all switches SW1-SWn are turned on (short-circuit). As the driving voltage Vf gradually increases, the luminescent unit D1 is turned on when the voltage established across the luminescent unit D1 exceeds the threshold voltage of the luminescent unit D1, while the luminescent unit D2 remains off. At the time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1, the switch SW1 and the current source IS1, while the current flowing through the luminescent unit D1 is maintained at a constant value by the current source IS1. Next, as the voltage V1 increases with the driving voltage Vf, the luminescent unit D2 is turned on when the voltage established across the luminescent unit D2 exceeds the threshold voltage of the luminescent unit D2, while the luminescent unit D3 remains off. At the time, the voltage V2 also increases with the driving voltage Vf. After having detected that the voltage V2 has reached a predetermined value, the judging unit CM1 turns off the switch SW1. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1, the luminescent unit D2, the switch SW2 and the current source IS2, while the current flowing through the luminescent unit D1-D2 is maintained at a constant value by the current source IS2. Similarly, as the driving voltage Vf gradually increases, the voltages V1-Vn also increase accordingly, thereby sequentially turning on the luminescent units D1-Dn. On the other hand, the judging units CM1-CMn respectively determine whether the voltages V2-Vn+1 have reached respective predetermined values, thereby sequentially turning off the switches SW1-SWn.
Assuming that when the driving voltage Vf provided by the power supply circuit 110 has a maximum value, the luminescent units D1-Dn are turned on (short-circuit), the switches SW1-SWn−1 are turned off (open-circuit), and the switch SWn is turned on. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1-Dn, the switch SWn and the current source ISn, while the current passing through the luminescent units D1-Dn is maintained at a constant value by the current source ISn. As the voltage Vn decreases with the driving voltage Vf and falls to a predetermined value, the judging unit CMn−1 turns on the switch SWn−1 and the luminescent unit Dn is turned off due to insufficient applied voltage. At this time, the current path starts from the voltage source VS and sequentially passes through the luminescent unit D1-Dn−1, the switch SWn−1 and the current source ISn−1, while the current passing through the luminescent units D1-Dn−1 is maintained at a constant value by the current source ISn−1. Similarly, as the driving voltage Vf gradually decreases, the voltages Vn-V1 also decrease accordingly, thereby turning off the luminescent units Dn-D1 sequentially. On the other hand, the judging units CMn−1-CM1 respectively determine whether the voltages Vn-V2 have reached respective predetermined values, and sequentially turn off the SWn−1-SW1. On the other hand, the luminescent units Dn-D1 are also sequentially turned off as respective applied voltages gradually drop.
Reference is made to FIG. 6 for a diagram illustrating the operation of the LED driving circuit 100 or 200 according to the present invention. Assuming that the LED driving circuit 100 or 200 includes five current sources IS1-IS5 which provide identical current limit, and the luminescent device 10 includes five luminescent units D1-D5 whose threshold voltages are respectively represented by Vb1-Vb5. In FIG. 6, Vf represents the DC voltage provided by the power supply circuit 110, Vb represents the overall voltage established across all the turned-on luminescent units among the luminescent units D1-D5, and ID1 represents the current flowing through the luminescent unit D1. As shown in FIG. 6, the present invention can provide a large operational voltage range (between t1 and t2), as well as can reduce the power consumption of the current sources IS1-IS5 (the differences between Vf and Vb, denoted by dotted regions in FIG. 6).
In conclusion, the present invention can control the current limit of each current source according to the actual threshold voltage of the corresponding luminescent unit, such as the digital adjustment provided by the current-selecting circuit 120 of the first embodiment or the analog adjustment provided by the current-selecting circuit 220 of the second embodiment. The current paths in the LED string can be controlled based on the threshold voltage of each LED without using filter capacitor or detecting the input voltage. Even the LEDS of each luminescent unit may have different threshold voltages, the present invention can still provide accurate current limits accordingly, thereby enlarging the effective operational voltage range and improving optical efficiency and power factor.
Those skilled in the art will readily observe that numerous modifications and alterations of the device and method may be made while retaining the teachings of the invention.

## Claims

1. A driving circuit having a large operational voltage range and configured to drive a plurality of serially-coupled luminescent units, the driving circuit comprising:
a current-selecting circuit configured to control current paths in the plurality of luminescent units according to a plurality of current limits and respective threshold voltages of corresponding light emitting diodes in the plurality of luminescent units;
wherein the current-selecting circuit comprises:
a plurality of current sources respectively configured to provide the plurality of current limits; and
a plurality of adjusting circuits respectively configured to adjust the plurality of current limits according to voltages established between two corresponding adjacent luminescent units among the plurality luminescent units.

2. The driving circuit of claim 1, wherein the plurality of current sources are variable current sources.

3. The driving circuit of claim 1, wherein the current-selecting circuit and the plurality of serially-coupled luminescent units are arranged in a matrix.

4. The driving circuit of claim 1, wherein each luminescent unit includes a light emitting diode (LED).

5. The driving circuit of claim 1, wherein each luminescent unit includes a plurality of serially-coupled LEDs.

6. A driving circuit having a large operational voltage range and configured to drive a plurality of serially-coupled luminescent units, the driving circuit comprising:
a current-selecting circuit configured to control current paths in the plurality of luminescent units according to a plurality of current limits and respective threshold voltages of corresponding light emitting diodes in the plurality of luminescent units;
wherein the current-selecting circuit comprises:
a plurality of current sources respectively configured to provide the plurality of current limits;
a plurality of judging units respectively configured to generate a plurality of switch control signals according to voltages established between two corresponding adjacent luminescent units among the plurality luminescent units; and
a plurality of switches respectively configured to control signal transmission paths between the plurality of current sources and the plurality luminescent units according to the plurality of switch control signals.

7. The driving circuit of claim 6, wherein the plurality of current sources are constant current sources.

8. A display device having a large operational voltage range and comprising:
a plurality of serially-coupled luminescent units;
a power supply circuit coupled to plurality of serially-coupled luminescent units; and
a driving circuit configured to drive the plurality of serially-coupled luminescent units, the driving circuit comprising:
a current-selecting circuit configured to control current paths in the plurality of luminescent units according to a plurality of current limits and respective threshold voltages of corresponding light emitting diodes in the plurality of luminescent units;
wherein the current-selecting circuit comprises:
a plurality of current sources respectively configured to provide the plurality of current limits; and
a plurality of adjusting circuits respectively configured to adjust the plurality of current limits according to voltages established between two corresponding adjacent luminescent units among the plurality luminescent units.

9. The display device of claim 8, wherein the plurality of current sources are variable current sources.

10. The display device of claim 8, wherein the current-selecting circuit and the plurality of serially-coupled luminescent units are arranged in a matrix.

11. The display device of claim 8, wherein the power supply circuit comprises:
a power source configured to provide an alternative current (AC) voltage which periodically switches between positive and negative phases; and
a bridge rectifier configured to convert the AC voltage outputted in the negative phase, thereby providing a direct current (DC) voltage for driving the plurality of serially-coupled luminescent units.

12. The display device of claim 8, wherein each luminescent unit includes an LED.

13. The display device of claim 8, wherein each luminescent unit includes a plurality of serially-coupled LEDs.

14. A display device having a large operational voltage range and comprising:
a plurality of serially-coupled luminescent units;
a power supply circuit coupled to plurality of serially-coupled luminescent units; and
a driving circuit configured to drive the plurality of serially-coupled luminescent units, the driving circuit comprising:
a current-selecting circuit configured to control current paths in the plurality of luminescent units according to a plurality of current limits and respective threshold voltages of corresponding light emitting diodes in the plurality of luminescent units;
wherein the current-selecting circuit comprises:
a plurality of current sources respectively configured to provide the plurality of current limits;
a plurality of judging units respectively configured to generate a plurality of switch control signals according to voltages established between two corresponding adjacent luminescent units among the plurality luminescent units; and
a plurality of switches respectively configured to control signal transmission paths between the plurality of current sources and the plurality luminescent units according to the plurality of switch control signals.

15. The display device of claim 14, wherein the plurality of current sources are constant current sources.