Patent Application
20090261743
This application claims the priority benefit of Taiwan application serial no. 97114262, filed on Apr. 18, 2008. The entirety of the above-mentioned patent application is hereby incorporated by reference herein and made a part of specification.
1. Field of the Invention
The present invention generally relates to a light emitting diode module (LED module), and more particularly, to a driving module for driving an LED.
2. Description of Related Art
Due to low power consumption and high luminance, LEDs have been effectively applied in various applications, for example, illumination light, electronic bulletin board and traffic light. In particular, in display field, LEDs have excellent color performance within the gamut set out by National Television Standard Committee (NTSC); therefore, LEDs are gradually substituting a cold cathode fluorescent lamps (CCFL) employed by a backlight module of a display panel and the CCFL is a dominated light source used in a backlight module before.
However, the LEDs served as the light source of a backlight module of a display panel confront two stubborn problems. One of the problems is how to make a plurality of light emitting diode strings (LED strings) in a backlight module produce uniform luminance so as to have better display effect with a display panel. The luminance produced by an LED string is controlled by the current flowing through the LED string. Once only a fixed voltage is used to drive different LED strings, the characteristic difference between individual LED strings would result in nonuniform luminance as a whole.
To solve the above-mentioned problem, many different conventional schemes were provided. One of the conventional schemes is to utilize a plurality of sets of voltage-to-current converters for individually adjusting luminance of each of the LED strings. Although the above-mentioned scheme is able to individually adjust luminance of each of the LED strings to effectively overcome the problem resulted by the characteristic difference between the LED strings, but the conventional scheme requires a numerous voltage-to-current converters, which is not economical solution. Moreover in the prior art, there is time-division-multiplexing (TDM) scheme, by which the luminance corresponding to different LED string is adjustable to achieve balance of luminance. The conventional TDM scheme requires a clock signal with a high frequency and a plurality of switching signals produced based on the clock signal for switching a plurality of switches. The frequent switching of the switches tends to produce inrush currents leading to serious electromagnetic interference (EMI).
Accordingly, the present invention is directed to an LED driving module for dynamically adjusting the voltage and current of driving an LED string so as to thereby increase the light-emitting efficiency and luminance uniformity of the LED strings.
The present invention provides an LED driving module suitable to drive a plurality of LED strings in parallel connection. Each of the LED strings herein has a first terminal and a second terminal. The LED driving module includes a voltage converting apparatus, a conduction voltage detecting apparatus, a reference voltage generating apparatus and a current-adjusting apparatus. The voltage converting apparatus produces a driving voltage at the first terminal of each the LED string according to a conduction voltage. The conduction voltage detecting apparatus is coupled to the second terminal of each the LED string for detecting the conducting states of the LED strings and thereby producing the above-mentioned conduction voltage and a plurality of enabling signals. The reference voltage generating apparatus generates a first reference voltage according to the above-mentioned enabling signals. In addition, the current-adjusting apparatus produces a plurality of driving currents respectively flowing through each the LED string according to the first reference voltage.
Since the present invention adopts a conduction voltage detecting apparatus for detecting the minimal voltage required by the LED strings and thereby providing the most effective driving voltage. The present invention further employs a current-adjusting apparatus for adjusting the driving currents provided to the LED strings so as to stabilize the entire luminance of LED strings. Moreover, the present invention uses a current-balancing device for reducing the differences of the driving currents between the LED strings, which further ensures the luminance uniformity of the LED strings.
Other objectives, features and advantages of the present invention will be further understood from the further technological features disclosed by the embodiments of the present invention wherein there are shown and described preferred embodiments of this invention, simply by way of illustration of modes best suited to carry out the invention.
Reference will now be made in detail to the present preferred embodiments of the 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.
The voltage converting apparatus 111 is for producing driving voltages Vdrv of a set of LED strings 120 consisted of LED strings 121-123. Usually, the voltage converting apparatus 111 can be implemented by a DC-to-DC converter based on voltage-boosting or a charge pump. No matter which of the above-mentioned circuit is used, the voltage converting apparatus 111 needs to uses a feedback voltage Vt as a reference voltage for voltage-boosting. The driving voltage Vdrv is a multiple of the feedback voltage Vt (the multiple herein is not necessarily an integer number). Regarding producing the feedback voltage Vt, the following depiction of the conduction voltage detecting apparatus 112 would explain it further.
In the first embodiment, the conduction voltage detecting apparatus 112 is coupled to the second terminals S1-S3 of the LED strings 121-123 so as to measure the voltages at the second terminals S1-S3. The conduction voltage detecting apparatus 112 uses the received voltages at the second terminals S1-S3 of the LED strings 121-123 for detecting any LED string in open-loop state (open-loop is produced by probably burning out or removing away). Then, the conduction voltage detecting apparatus 112 selects the minimal voltage among the voltages at the second terminals S1-S3 of the LED strings excluding the LED string in open-loop state and outputs the selected one as the feedback voltage Vt.
It can be seen from the described above, the driving voltage Vdrv is a multiple of the feedback voltage Vt; therefore, the driving voltage Vdrv produced by the voltage converting apparatus 111 at the time should be the minimal required voltage. That is to say, the voltage converting apparatus 111 functions to provide a most effective driving voltage Vdrv.
In addition, the conduction voltage detecting apparatus 112 would send an enabling signal EN reflecting the conducting states of the LED strings 121-123 to the reference voltage generating apparatus 113. The function and operation of the reference voltage generating apparatus 113 are explained in follows.
The reference voltage generating apparatus 113 uses the received enabling signal EN to obtain the number of the LED strings in conducting state in the present set of LED strings 120. The reference voltage generating apparatus 113 produces a reference voltage Vref according to the above-mentioned number, wherein the operation is mainly in response to that when more LED strings get conductive, a larger driving circuit is needed, thus the reference voltage Vref must be accordingly increased; in contrast, when more LED strings are in open-loop state, a less driving voltage is needed and the reference voltage Vref must be accordingly decreased.
The current-adjusting apparatus 114 outputs a driving current in response to the reference voltage Vref. In this way, the driving current output from the current-adjusting apparatus 114 is not fixed, so that when an LED string is in open-loop state, the luminance variation due to increasing currents flowing through the rest LED strings can be avoided. In addition, the undesired power consumption can be avoided as well.
The operation of the conduction voltage detecting apparatus 112 can be depicted in more detail in association with an implement of the conduction voltage detecting apparatus 112 according to the first embodiment of the present invention.
The conduction voltage detector 210 includes NOT-gates 211-212, a transmission-gate 213 and a transistor M1, wherein the input terminal of the NOT-gate 211 is coupled to the second terminal S1 of the LED string 121 and an enabling signal EN1 is produced at the output terminal of the NOT-gate 211. The input terminal of the NOT-gate 212 is coupled to the output terminal of the NOT-gate 211, which is coupled to the gate of the transistor M1. The first source/drain of the transistor M1 is coupled to the system voltage VDD and the second source/drain thereof produces a detection voltage Vdet. In addition, two enabling terminals of the transmission-gate 213 are respectively coupled to the input terminal and the output terminal of the NOT-gate 212; two data terminals of the transmission-gate 213 are respectively coupled to the input terminal of the NOT-gate 211 and the second source/drain of the transistor M1.
When an LED string is in open-loop state (for example, the LED string 121 is in open-loop state), the voltage at the second terminal S2 approaches the grounded voltage (i.e., usually, 0 V). Meanwhile, the NOT-gate 211 outputs a logic high-level voltage (enabling signal EN1) and the NOT-gate 212 outputs a logic low-level voltage. The transistor M1 in the embodiment is a P-type metal-oxide-semiconductor field-effect transistor (P-MOSFET); therefore, when the transistor M1 is turned on, the second source/drain thereof produces a detection voltage Vdet almost equal to the system voltage VDD.
In contrast, if the LED string 121 is not in open-loop state, the NOT-gate 211 would output the enabling signal EN and the enabling signal EN is the logic low-level voltage; meanwhile, the NOT-gate 212 would output the logic high-level voltage. At the time, the transistor M1 is turned off and the second source/drain thereof produces the detection voltage Vdet almost equal to the voltage at the second terminal S2 of the LED string 121. In summary, when an LED string is in open-loop state, the corresponding conduction voltage detector outputs a detection voltage Vdet and the detection voltage Vdet must be higher than the detection voltage Vdet output from the conduction voltage detectors corresponding to the LED string in conducting state.
The wirings and the operations of the conduction voltage detectors 210-230 are the same as the conduction voltage detector 210 and they are omitted to describe.
At the time, the voltage comparator 240 is able to compare the detection voltages produced by the conduction voltage detectors 210-230 with each other and select the minimal detection voltage as the conduction voltage Vt provided to the voltage converting apparatus 111 for use.
When an LED string is turned on, the enabling signal produced by the corresponding conduction voltage detector would enable a corresponding switch, so that a current source connected in series to the switch outputs a current flowing through the resistor R1. Thus, the more the LED strings are turned on, the larger current flows through the resistor R1. Note that the reference voltage Vref is equal to the voltage across both ends of the resistor R1; therefore, the more the LED string are turned on, a higher reference voltage Vref is established.
On the other hand, when an LED string is in open-loop state, the real driving current flowing through the set of LED strings 120 is reduced. For example, if the set of LED strings 120 has eight LED strings and assuming the current required by each LED string is the same Id, the maximal driving current required by the set of LED strings 120 would be equal to 8×Id. Once one of the LED strings is burned out and in open-loop state, the driving current required by the set of LED strings would be 7×Id. It can be seen from the described above, the driving current needs to be further adjusted through dynamically adjusting the reference voltage Vref which is the base for producing the driving current.
A plurality of implements for the current-adjusting apparatus in charge of adjusting currents is depicted as follows, wherein the method for adjusting a driving current can be understood more clearly.
The amplifier 540 compares the reduced voltage Vfb formed at an end of the resistor R2 with the reference voltage Vref and produces a control voltage for controlling the driving current sources 510-530. In order to make the LED strings provide different luminance corresponding to a certain gray level on a display panel, the PWM 550 and the pulse-width basic circuit 560 are used to convert the voltage at the output terminal A1 of the amplifier 540 into a periodic signal. The ratio of positive pulse over entire period of the periodic signal is corresponding to a certain gray level on the display panel.
To produce the above-mentioned gray level, the driving current sources 510-530 would be switched continuously, which would result in electromagnetic interference (EMI). To overcome the EMI problem, three resistors R31-R33 are respectively connected in series between the output terminal A2 of the pulse-width basic circuit 560 and each of the driving current sources 510-530, wherein the resistors R31-R33 have different resistances, so that the time point for disabling or enabling each the driving current source can be effectively delayed and thereby the EMI can be effectively reduced.
On the contrast, when the transmission-gate 570 is turned off according to the PWM signal, the voltage at the output terminal A1 of the amplifier 540 is unable to be smoothly delivered to the driving current sources 510-530, and the output terminal of the transmission-gate 570 outputs the grounded voltage due to the turned on transistor M2. Then, the driving current sources 510-530 are disabled and the set of LED strings 120 is turned off. In summary, the PWM 550 uses the duty cycle of the produced PWM signal for controlling the luminance of the set of LED strings 120 corresponding to a gray level of the display panel.
The present invention also provides the second embodiment for anyone skilled in the art to further understand the spirit of the present invention.
In terms of the implement of the current-adjusting apparatus 614, to avoid the driving current sources of the set of LED strings 620 from directly outputting a large driving current, the second embodiment uses a scheme of amplifying current stage by stage. That is, the current amplifier 616 produces a basic current according to the voltage at the positive terminal of the amplifier 640, wherein the basic current can be also adjusted by an adjustable resistor Rext. The current amplifier 616 amplifies the basic current and produces an amplified current at the output terminal thereof. The driving current sources 616-619 produce a driving current by mirroring the amplified current.
In addition, AND-gates AN1-AN3 are added in the pulse-width basic circuit 615. The AND-gates AN1-AN3 together receive an enabling signal NO so as to provide a path for entirely turning off the set of LED strings 620 (when the enabling signal NO takes the logic low-level voltage).
A more essential point is to employ a current-balancing device 630 connected in series onto the conduction path of the driving currents for balancing the driving currents and reducing the difference between the driving currents. The current-balancing device 630 includes an amplifier 631, transistors MB1-MB3 and feedback resistor Rf1-Rf3. When the set of LED strings 620 produces a voltage difference ΔV between the different second terminals S1-S3 of the LED strings due to a time factor or a temperature variation, the voltage difference ΔV would cause a driving current error.
Assuming the drain voltages at the transistors MB1 and MB2 have a variation and are expressed by the following equation (1):
wherein VD,MB1 and VD,MB1 respectively represent the drain voltages of the transistors MB1 and MB2 prior to having a variation; V′D,MB1 and V′D,MB1 respectively represent the drain voltages of the transistors MB1 and MB2 after having a variation.
In addition, it is assumed there is a micro-current IR flows through the feedback resistors Rf1 and Rf2 and the resistances of the two resistors are the same, R. The source voltages of the transistors MB1 and MB2 can be expressed by the following equation (2):
V′
S,MB1
=V
S,MB1
+I
R
×R and V′S,MB2=VS,MB2−IR×R
wherein VS,MB1 and VS,MB2 respectively represent the source voltages of the transistors MB1 and MB2 prior to having a variation; V′S,MB1 and V′S,MB1 respectively represent the source voltages of the transistors MB1 and MB2 after having a variation
The currents produced by the transistors MB1 and MB2 working within the saturation regions thereof are expressed in the following equation (3):
wherein ILED1 and ILED2 respectively represent the currents flowing through two LED strings, VG represents the voltage at the output terminal of the amplifier 631, Vref represents the reference voltage received by the amplifier 631, Vt represents the conduction voltage, Isink1 and Isink2 represent the driving currents produced by the driving current sources 617 and 618, and k and λ represent constants.
The current difference between the two LED strings and the average value thereof can be expressed by the following equations (4) and (5):
I
LED1
−I
LED2
=k(VGS−VTO)2λ(2IRR)+2IR (4)
(ILED1+ILED2)/2=k(VGS−VTO)2(1+λVREF2) (5)
wherein VGS is the voltage difference between the gate and the source of the driving current sources 617 and 618, and VREF is the second reference voltage VREF.
The equation (5) divides the equation (4), the current variation between the two LED strings is obtained as the following equation (6):
δ=2λIRR (6)
Since the feedback resistors Rf1 and Rf2 are disposed on the negative feedback path and one of the ends is coupled to the input terminal with a high impedance of the amplifier 631; therefore, only a tiny current (a level of μA) flows through the feedback resistors Rf1 and Rf2, and the voltage difference between the two ends is also subject to the negative feedback characteristic so that the voltage drop caused by the negative feedback takes also a tiny level of mV. The constant λ is a channel-length modulation parameter, roughly equal to 10 mV. Under the above-mentioned architecture, the current error between the two LED strings is estimated as 10−2% according to the equation (6).
In summary, the present invention uses a conduction voltage detecting apparatus for detecting the number of the LED strings in open-loop state, and thereby adjusts the driving voltage and the driving current so as to reduce unnecessary power consumption. the present invention further uses a current-balancing device to effectively reduce the current error between each of the LED strings. As a result, the set of LED strings provided by the present invention has good luminance uniformity.
The foregoing description of the preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form or to exemplary embodiments disclosed. Accordingly, the foregoing description should be regarded as illustrative rather than restrictive. Obviously, many modifications and variations will be apparent to practitioners skilled in this art. The embodiments are chosen and described in order to best explain the principles of the invention and its best mode practical application, thereby to enable persons skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use or implementation contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents in which all terms are meant in their broadest reasonable sense unless otherwise indicated. Therefore, the term “the invention”, “the present invention” or the like is not necessary limited the claim scope to a specific embodiment, and the reference to particularly preferred exemplary embodiments of the invention does not imply a limitation on the invention, and no such limitation is to be inferred. The invention is limited only by the spirit and scope of the appended claims. The abstract of the disclosure is provided to comply with the rules requiring an abstract, which will allow a searcher to quickly ascertain the subject matter of the technical disclosure of any patent issued from this disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. Any advantages and benefits described may not apply to all embodiments of the invention. It should be appreciated that variations may be made in the embodiments described by persons skilled in the art without departing from the scope of the present invention as defined by the following claims. Moreover, no element and component in the present disclosure is intended to be dedicated to the public regardless of whether the element or component is explicitly recited in the following claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 97114262 | Apr 2008 | TW | national |
