Light-emitting Diode (LED) Driving Circuit

Abstract

A light-emitting diode (LED) driving circuit for driving a plurality of first lightbars and a plurality of second lightbars is provided. The LED driving circuit includes a first current mirror, a second current mirror and a control circuit. The first current mirror, if enabled, balances currents of the first lightbars. The second current mirror, if enabled, balances currents of the second lightbars. During a first period, the control circuit disables the second current mirror and adjusts the duration of enabling the first current mirror according to a dimming signal. During a second period, the control circuit disables the first current mirror and adjusts the duration of enabling the second current mirror according to the dimming signal. Therefore, only the first lightbars or the second lightbars are driven in each period.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting diode (LED) driving circuit. More particularly, the present invention relates to an LED driving circuit for driving a plurality of lightbars each including a plurality of LEDs coupled in series.

2. Description of the Prior Art

FIG. 1 is a block diagram illustrating a conventional LED driving circuit. Referring to FIG. 1, an LED driving circuit 1 is adapted to driving a plurality of lightbars LB1-LBm, and each lightbar LBi includes a plurality of LEDs D1-Dn coupled in series, where m and n are positive integers, and i is an integer from 1 to m. The LED driving circuit 1 includes a direct-current to direct-current (DC/DC) converter 11 and an LED controller 12. The DC/DC converter 11 such as a buck or boost converter converts a DC input voltage Vin to a DC voltage V1b sufficient to drive the lightbars LB1-LBm. Each lightbar LBi has a first terminal coupled to the DC/DC converter 11 to receive the DC voltage V1b and a second terminal coupled to a corresponding channel terminal CHi of the LED controller 12. The LED controller 12 detects current of each lightbar LBi and controls current of each lightbar LBi to become equal to a predetermined value by built-in constant current sources or variable resistors; that is, the LED controller 12 balances currents of the lightbars LB1-LBm. The LED controller 12 further outputs a feedback signal from a feedback terminal FB to control the DC/DC converter 11 to adjust the DC voltage V1b.

If too many lightbars LB1-LBm are employed, or if LEDs D1-Dn with high brightness are employed, the total current of the lightbars LB1-LBm may cause the LED controller 12 to be destroyed. Accordingly, there is a need for an LED driving circuit to employ external control manner shown in FIG. 2. Referring to FIG. 2, an LED driving circuit 2 includes a DC/DC converter 21, an LED controller 22, a plurality of transistors M1-Mm and a plurality of resistors R1-Rm. The transistor Mi and the resistor Ri are coupled in series between the second terminal of a corresponding lightbar LBi and a ground. The LED controller 22 detects current of each lightbar LBi from a corresponding current sensing terminal ISi and outputs a signal from a corresponding channel terminal CHi to control current of each lightbar LBi to become equal to a predetermined value. The LED controller 22 further detects a voltage at the second terminal of each lightbar LBi from a voltage detecting terminal VDi to provide a short protection for the lightbars LB1-LBm.

The LED driving circuits 1 and 2 employ the LED controllers 12 and 22 which are specific-purpose integrated circuits (ICs). However, a commercially available LED controller IC supports a fixed number of lightbars. It may be necessary to employ a plurality of LED controller ICs to drive the lightbars as the number of the lightbars increases. The number of the transistors M1-Mm and the resistors R1-Rm employed in the LED driving circuit 2 will increase as the number of the lightbars increases. Therefore, as the number of the lightbars increases, the conventional LED driving circuits become more complex and expensive to design and manufacture.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide an LED driving circuit employing a simple driving structure with reduced components and without using a specific-purpose LED controller IC.

The present invention provides an LED driving circuit for driving a plurality of first lightbars and a plurality of second lightbars. Each of the first lightbars and the second lightbars includes a plurality of LEDs coupled in series. Each of the first lightbars and the second lightbars has a first terminal coupled to receive a direct-current (DC) voltage and a second terminal. The LED driving circuit includes a first current mirror, a second current mirror and a control circuit. The first current mirror is coupled to the second terminals of the first lightbars. The first current mirror balances currents of the first lightbars when the first current mirror is enabled, and causes the currents of the first lightbars to become zero when the first current mirror is disabled. The second current mirror is coupled to the second terminals of the second lightbars. The second current mirror balances currents of the second lightbars when the second current mirror is enabled, and causes the currents of the second lightbars to become zero when the second current mirror is disabled. The control circuit is coupled to the first current mirror and the second current mirror. During a first period, the control circuit disables the second current mirror and adjusts the duration of enabling the first current mirror according to a dimming signal. During a second period, the control circuit disables the first current mirror and adjusts the duration of enabling the second current minor according to the dimming signal. The first period and the second period are repeated alternatively.

The control circuit includes a controller, a first switch, a first current detector, a second switch and a second current detector. The controller outputs a first pulse-width modulation (PWM) signal and a second PWM signal. A duty cycle of the first PWM signal is determined by the dimming signal and the total current of the first lightbars, and a duty cycle of the second PWM signal is determined by the dimming signal and the total current of the second lightbars. The first switch has a first terminal coupled to the first current mirror to receive the total current of the first lightbars, a second terminal and a control terminal coupled to the controller to receive the first PWM signal. The first current detector has a first terminal coupled to the second terminal of the first switch and the controller and a second terminal coupled to a ground. The first current detector detects the total current of the first lightbars. The second switch has a first terminal coupled to the second current mirror to receive the total current of the second lightbars, a second terminal and a control terminal coupled to the controller to receive the second PWM signal. The second current detector has a first terminal coupled to the second terminal of the second switch and the controller and a second terminal coupled to the ground. The second current detector detects the total current of the second lightbars.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a block diagram illustrating a conventional LED driving circuit;

FIG. 2 is a block diagram illustrating another conventional LED driving circuit;

FIG. 3 is a block diagram illustrating an LED driving circuit according to a preferred embodiment of the present invention;

FIG. 4 is a schematic diagram illustrating the LED driving circuit shown in FIG. 3; and

FIG. 5 is a timing diagram illustrating time division control of the LED driving circuit shown in FIG. 4 under maximum brightness.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.

FIG. 3 is a block diagram illustrating an LED driving circuit according to a preferred embodiment of the present invention. Referring to FIG. 3, an LED driving circuit 3 is adapted to driving a plurality of first lightbars LB11-LB1m and a plurality of second lightbars LB21-LB2m, where m is a positive integer. Each of the first lightbars LB11-LB1m and the second lightbars LB21-LB2m includes a plurality of LEDs D1-Dn coupled in series, where n is a positive integer. Each of the first lightbars LB11-LB1m and the second lightbars LB21-LB2m has a first terminal coupled to receive a DC voltage V1b and a second terminal. The DC voltage V1b, for example, is provided by the DC/DC converter 11 shown in FIG. 1.

The LED driving circuit 3 includes a first current mirror 31, a second current mirror 32 and a control circuit 33. The first current mirror 31 is coupled to the second terminals of the first lightbars LB11-LB1m to receive currents I11-I1m of the first lightbars LB11-LB1m. The first current mirror 31 balances the currents I11-I1m of the first lightbars LB11-LB1m when the first current mirror 31 is enabled, and causes the currents I11-I1m of the first lightbars LB11-LB1m to become zero when the first current mirror 31 is disabled. The second current mirror 32 is coupled to the second terminals of the second lightbars LB21-LB2m to receive currents I21-I2m of the second lightbars LB21-LB2m. The second current mirror 32 balances the currents I21-I2m of the second lightbars LB21-LB2m when the second current mirror 32 is enabled, and causes the currents I21-I2m of the second lightbars LB21-LB2m to become zero when the second current mirror 32 is disabled. The control circuit 33 is coupled to the first current mirror 31 and the second current mirror 32. During a first period, the control circuit 33 disables the second current mirror 32, and the control circuit 33 enables the first current mirror 31 and adjusts the duration of enabling the first current mirror 31 according to a dimming signal Vdim. During a second period, the control circuit 33 disables the first current mirror 31, and the control circuit 33 enables the second current mirror 32 and adjusts the duration of enabling the second current mirror 32 according to the dimming signal Vdim. The first period and the second period are repeated alternatively.

FIG. 4 is a schematic diagram illustrating the LED driving circuit shown in FIG. 3. Referring to FIG. 4, the first current mirror 31 includes a plurality of first transistors Q11-Q1m. Each first transistor Q1i has a first terminal, a second terminal and a control terminal, where i is an integer from 1 to m. The first terminal of each first transistor Q1i is coupled to the second terminal of a corresponding first lightbar LB1i. The second terminals of the first transistors Q11-Q1m are coupled to one another and to the control circuit 33. The control terminals of the first transistors Q11-Q1m are coupled to one another and to the first terminal of one of the first transistors Q11-Q1m (e.g. Q11). The second current mirror 32 includes a plurality of second transistors Q21-Q2m. Each second transistor Q2i has a first terminal, a second terminal and a control terminal. The first terminal of each second transistor Q2i is coupled to the second terminal of a corresponding second lightbar LB2i. The second terminals of the second transistors Q21-Q2m are coupled to one another and to the control circuit 33. The control terminals of the second transistors Q21-Q2m are coupled to one another and to the first terminal of one of the second transistors Q21-Q2m (e.g. Q21). The first transistors Q11-Q1m and the second transistors Q21-Q2m are matched to one another.

The control circuit 33 includes a controller U1, a first switch M1, a first current detector Rd1, a second switch M2 and a second current detector Rd2. The controller U1 outputs a first pulse-width modulation (PWM) signal Vpwm1 and a second PWM signal Vpwm2. A duty cycle of the first PWM signal Vpwm1 is determined by the dimming signal Vdim and the total current I31 of the first lightbars LB11-LB1m, and a duty cycle of the second PWM signal Vpwm2 is determined by the dimming signal Vdim and the total current I32 of the second lightbars LB21-LB2m, in which the total current I31 is the sum of the currents I11-I1m, and the total current I32 is the sum of the currents I21-I2m. The first switch M1 has a first terminal coupled to the second terminals of the first transistors Q11-Q1m to receive the total current I31 of the first lightbars LB11-LB1m, a second terminal and a control terminal coupled to receive the first PWM signal Vpwm1. The first current detector Rd1 has a first terminal coupled to the second terminal of the first switch M1 and the controller U1 and a second terminal coupled to a ground. The first current detector Rd1 detects the total current I31 of the first lightbars LB11-LB1m. The second switch M2 has a first terminal coupled to the second terminals of the second transistors Q21-Q2m to receive the total current I32 of the second lightbars LB21-LB2m, a second terminal and a control terminal coupled to receive the second PWM signal Vpwm2. The second current detector Rd2 has a first terminal coupled to the second terminal of the second switch M2 and the controller U1 and a second terminal coupled to the ground. The second current detector Rd2 detects the total current I32 of the second lightbars LB21-LB2m.

In this embodiment, the controller U1 is a general-purpose PWM controller IC such as TL494 or OZ9938. The controller U1 has a dimming terminal DIM, output terminals G1 and G2 and a current sensing terminal IS. The controller U1 receives the dimming signal Vdim from the dimming terminal DIM, obtains the detection result of the total current I31 of the first lightbars LB11-LB1m and the total current I32 of the second lightbars LB21-LB2m from the current sensing terminal IS, and outputs the first PWM signal Vpwm1 and the second PWM signal Vpwm2 from the output terminals G1 and G2. The first transistors Q11-Q1m and the second transistors Q21-Q2m are N-channel field-effect transistors (FETs). The first switch M1 and the second switch M2 are implemented by N-channel FETs. The first current detector Rd1 and the second current detector Rd2 are implemented by resistors. In an alternative embodiment, the first transistors Q11-Q1m and the second transistors Q21-Q2m are NPN bipolar junction transistors (BJTs), and the first switch M1 and the second switch M2 are implemented by NPN BJTs.

FIG. 5 is a timing diagram illustrating time division control of the LED driving circuit shown in FIG. 4 under maximum brightness. Referring to FIG. 5, a pulse signal Vpulse generated by an internal oscillator (not shown) of the controller U1 is served as an operating frequency of the controller U1. Time can be divided into a plurality of periods each having a duration of T. Each period includes a pulse having a duration (or pulse width) of Td, where Td is much less than T. These pulses are adapted to avoiding overlap between the first PWM signal Vpwm1 and the second PWM signal Vpwm2 so that the first lightbars LB11-LB1m are turned on when the second lightbars LB21-LB2m are turned off, and the first lightbars LB11-LB1m are turned off when the second lightbars LB21-LB2m are turned on. It is defined that odd periods are first periods T1 and even periods are second periods T2; hence, the first period T1 and the second period T2 are repeated alternatively.

During the first period T1, the second PWM signal Vpwm2 remains at low level and controls the second switch M2 to be open to disable the second current mirror 32, then the disabled second current mirror 32 controls the second lightbars LB21-LB2m to be turned off and causes the currents I21-I2m of the second lightbars LB21-LB2m to become zero (here, taking the total current I32, the sum of the currents I21-I2m, as an example shown in FIG. 5). The first PWM signal Vpwm1 is at high level except in the interval from j×2T to (j×2T+Td), where j is a non-negative integer. The first PWM signal Vpwm1 at high level of a duration Ton1 controls the first switch M1 to be closed to enable the first current mirror 31, then the enabled first current mirror 31 controls the first lightbars LB11-LB1m to be turned on and balances the currents I11-I1m of the first lightbars LB 11-LB1m (here, taking the total current I31, the sum of the currents I11-I1m, as an example shown in FIG. 5). The first PWM signal Vpwm1 at low level of a duration Td controls the first switch M1 to be open to disable the first current mirror 31, then the disabled first current mirror 31 controls the first lightbars LB11-LB1m to be turned off and causes the currents of the first lightbars LB11-LB1m to become zero. The duty cycle of the first PWM signal Vpwm1, Ton1/2T, will be adjusted by the controller U1 according to the dimming signal Vdim and the total current I31 of the first lightbars LB11-LB1m; that is, the duration of enabling the first current mirror 31 or the duration of the first PWM signal Vpwm1 at high level, Ton1, will be adjusted.

During the second period T2, the first PWM signal Vpwm1 remains at low level and controls the first switch M1 to be open to disable the first current mirror 31, then the disabled first current mirror 31 controls the first lightbars LB11-LB1m to be turned off and causes the currents I11-I1m of the first lightbars LB11-LB1m to become zero. The second PWM signal Vpwm2 is at high level except in the interval from (2j+1)×T to ((2j+1)×T+Td). The second PWM signal Vpwm2 at high level of a duration Ton2 controls the second switch M2 to be closed to enable the second current mirror 32, then the enabled second current mirror 32 controls the second lightbars LB21-LB2m to be turned on and balances the currents I21-I2m of the second lightbars LB21-LB2m. The second PWM signal Vpwm2 at low level of a duration Td controls the second switch M2 to be open to disable the second current mirror 32, then the disabled second current mirror 32 controls the second lightbars LB21-LB2m to be turned off and causes the currents I21-I2m of the second lightbars LB21-LB2m to become zero. The duty cycle of the second PWM signal Vpwm2, Ton2/2T, will be adjusted by the controller U1 according to the dimming signal Vdim and the total current I32 of the second lightbars LB21-LB2m; that is, the duration of enabling the second current mirror 32 or the duration of the second PWM signal Vpwm2 at high level, Ton2, will be adjusted.

In this embodiment, the dimming signal Vdim is a DC signal whose magnitude influences the duty cycles of the first PWM signal Vpwm1 and the second PWM signal Vpwm2. In alternative embodiment, the dimming signal Vdim is a PWM signal whose duty cycle influences the duty cycles of the first PWM signal Vpwm1 and the second PWM signal Vpwm2.

In summary, the LED driving circuit of the present invention divides all lightbars into the first lightbars and the second lightbars for time division control, and only the first lightbars or the second lightbars are driven in each period. For example, only the first lightbars are driven in the first period, and only the second lightbars are driven in the second period. It reduces the amount of current provided by the DC/DC converter for driving the lightbars in each period; hence, the DC/DC converter can reduce voltage ripple therein and employ a filter capacitor of smaller capacitance at its output to reduce its cost. In addition, the LED driving circuit of the present invention employs a simple driving structure with reduced components and without using a specific-purpose LED controller IC to reduce its cost.

It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.

Claims

1. A light-emitting diode (LED) driving circuit for driving a plurality of first lightbars and a plurality of second lightbars, each of the first lightbars and the second lightbars comprising a plurality of LEDs coupled in series, each of the first lightbars and the 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 first lightbars for balancing currents of the first lightbars when the first current mirror is enabled, and causing the currents of the first lightbars to become zero when the first current mirror is disabled;a second current mirror coupled to the second terminals of the second lightbars for balancing currents of the second lightbars when the second current mirror is enabled, and causing the currents of the second lightbars to become zero when the second current mirror is disabled; anda control circuit coupled to the first current mirror and the second current mirror 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 period and the second period being repeated alternatively;wherein the control circuit comprises:a controller for outputting a first pulse-width modulation (PWM) signal and a second PWM signal, a duty cycle of the first PWM signal being determined by the dimming signal and the total current of the first lightbars, a duty cycle of the second PWM signal being determined by the dimming signal and the total current of the second lightbars;a first switch having a first terminal coupled to the first current mirror to receive the total current of the first lightbars, a second terminal and a control terminal coupled to the controller to receive the first PWM signal;a first current detector having a first terminal coupled to the second terminal of the first switch and the controller and a second terminal coupled to a ground, the first current detector detecting the total current of the first lightbars;a second switch having a first terminal coupled to the second current mirror to receive the total current of the second lightbars, a second terminal and a control terminal coupled to the controller to receive the second PWM signal; anda second current detector having a first terminal coupled to the second terminal of the second switch and the controller and a second terminal coupled to the ground, the second current detector detecting the total current of the second lightbars.

2. The LED driving circuit according to claim 1, wherein the first current mirror comprises a plurality of first transistors, each first transistor having a first terminal, a second terminal and a control terminal, the first terminal of each first transistor being coupled to the second terminal of a corresponding first lightbar, the second terminals of the first transistors being coupled to one another and to the control circuit, the control terminals of the first transistors being coupled to one another and to the first terminal of one of the first transistors; and wherein the second current mirror comprises a plurality of second transistors, each second transistor having a first terminal, a second terminal and a control terminal, the first terminal of each second transistor being coupled to the second terminal of a corresponding second lightbar, the second terminals of the second transistors being coupled to one another and to the control circuit, the control terminals of the second transistors being coupled to one another and to the first terminal of one of the second transistors, the first transistors and the second transistors being matched to one another.

3. The LED driving circuit according to claim 2, wherein the first transistors and the second transistors comprise NPN bipolar junction transistors (BJTs).

4. The LED driving circuit according to claim 2, wherein the first transistors and the second transistors comprise N-channel field-effect transistors (FETs).

5. The LED driving circuit according to claim 1, wherein the controller is a general-purpose PWM controller integrated circuit.

6. The LED driving circuit according to claim 1, wherein the first switch and the second switch are implemented by N-channel PETs.

7. The LED driving circuit according to claim 1, wherein the first current detector and the second current detector are implemented by resistors.