Patent Publication Number: US-7224128-B2

Title: Device for driving light emitting diode strings

Description:
This application claims the benefit of Taiwan application Serial No. 093123030, filed Jul. 30, 2004, the subject matter of which is incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates in general to a device for driving a light emitting diode string, and more particularly to a device for driving a light emitting diode string for applying in a backlight module. 
   2. Description of the Related Art 
   Conventionally, backlight modules are provided as the light sources for LCD panels, where the light can be produced by LEDs. LEDs are solid state semiconductor light sources, and have the following advantages: extra-long lifetime, low power, low operating voltage, low operating temperature, and quick response time. These are advantages that can not be matched by cold cathode fluoresce lamps (CCFL), and are the reasons to the wide use of LEDs in various illuminations and small scale backlight modules of cellular phones. It is becoming apparent that LEDs will gradually replace CCFLs in many applications. 
     FIG. 1  (Prior Art) shows circuit diagram of a conventional driving device for LEDs. The driving device  100  includes a DC voltage source  102 , a DC chopper  104 , a filtering device  106 , and a LED string  108 . The DC chopper  104  is used for controlling the electrical connection between DC voltage source  102  and LED string  108 , and the LED string  108  is controlled to turn on or turn off accordingly, i.e. to light up or shut off. Since filtering circuit  106  has an inductance, the waveform of current I of LED string  108  forms triangular waves, as shown in  FIG. 1B . As a result, the LED string  108  can not operate with a fixed conducting current. Even if a voltage-stabilizing capacitor is connected to the LED string in parallel to stabilize current I, the problem of long capacitor charging and discharging time prevents LED string  108  from able to be quickly turned on or off. 
   SUMMARY OF THE INVENTION 
   It is therefore an object of the invention to provide a device for driving LED strings capable of operating with fixed conducting currents and quickly turning on or off the LED string. 
   The invention achieves the above-identified object by providing a driving device for LED strings, including a DC-to-DC converter, a LED string, a switch and a feedback circuit. DC-to-DC converter has a first DC-to-DC converter end, for outputting a DC voltage according to a feedback signal outputted by the feedback circuit. The LED string is coupled to the first DC-to-DC converter end. The switch and the LED string are serially connected. When the switch is turned on, the DC voltage drives the LED string, and the DC current flows through the LED string. The feedback circuit outputs the feedback signal according to the DC current. When the switch is turned on, the LED string is quickly turned on to reach a predetermined brightness level, and when the switch is turned off, the LED string is quickly turned off. 
   Other objects, features, and advantages of the invention will become apparent from the following detailed description of the preferred but non-limiting embodiments. The following description is made with reference to the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1A–1B  (Prior Art) shows a circuit diagram of a conventional driving device for LED strings and its related driving waveform. 
       FIG. 2  shows a circuit diagram of a driving device for LED strings according to a first embodiment of the invention. 
       FIG. 3  shows a circuit diagram of a driving device for LED strings according to a second embodiment of the invention. 
       FIG. 4  shows a circuit diagram of a driving device for LED strings according to a third embodiment of the invention. 
       FIG. 5  shows a circuit diagram of a driving device for LED strings having a Boost converter. 
       FIG. 6  shows a circuit diagram of a driving device for LED strings having a Buck-Boost converter. 
       FIG. 7  shows a circuit diagram of a driving device for LED strings having a Flyback converter. 
       FIG. 8  shows a circuit diagram of a driving device for LED strings having a Full-Bridge converter. 
       FIG. 9  shows relative waveforms of the control signal of switch  204 , the first reference voltage V 1 ″, the second reference voltage V 2 , and the output voltage Vo from the DC-to-DC converter  208 . 
       FIG. 10  shows a circuit diagram of a driving device for LED strings according to a fourth embodiment of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   First Embodiment 
     FIG. 2  shows a circuit diagram of a driving device  200  for LED strings according to a first embodiment of the invention. Driving device  200  can be applied in backlight modules of LCD panels, and comprises a DC-to-DC converter  208 , a LED string  202 , a switch  204 , and a feedback circuit  206 . For illustration, DC-to-DC converter  208  in this embodiment is supposed as a buck converter, and the LED string  202  is provided as the light source required to light a LCD panel. 
   DC-to-DC converter  208  has a first DC-to-DC converter end X 1  and a second DC-to-DC converter end X 2 . The second DC-to-DC converter end X 2  is coupled to a fixed voltage, such as the fixed voltage being a ground voltage. DC-to-DC converter  208  outputs a DC voltage VDC from the first DC-to-DC converter end X 1  according to a feedback signal fs. LED string  202  is coupled to the first DC-to-DC converter end X 1 . Switch  204  and LED string  202  are serially connected. When switch  204  is turned on, LED string  202  is driven by the DC voltage VDC, and a DC current I′ flows through LED string  202 , causing the LEDs to light up. Feedback circuit  206  then outputs the feedback signal fs according to the DC current I′. 
   To achieve the object of quickly turning on or turning off LED string  202 , i.e. to quickly light up or shut off LED string  202 , switch  204  and LED string  202  are connected in series in this embodiment. That is, when switch  204  is turned on, a fixed conducting current flows through LED string  202 , and LED string  202  is quickly lit up to reach a predetermined brightness level; when switch  202  is turned off, the fixed conducting current immediately stops flowing through LED string  202 , and LED string  202  is quickly shut off. Thus, the problem of slow response time of LED string  202  caused by the slow change of current I′ due to energy storing elements in DC-to-DC converter  208  can be prevented. Also, through the use of switch  204  to control the LED string  202  to be quickly turned on or turned off, the value of the energy storing elements of DC converter  208 , such as the inductances and capacitances, can be increased, and thereby causing the current I′ outputted to be more stable. 
   DC-to-DC converter  208  further includes a pulse width modulator  210 . The feedback circuit  206  generates feedback signal fs according to DC current I′, and the pulse width modulator  210  adjusts the output signal according to feedback signal fs so that DC-to-DC converter  208  can output stable DC voltage VDC. Furthermore, feedback circuit  206  includes a current-voltage converter  214 . Current-voltage converter  214  has a first end and a second end. The first end of the current-voltage converter  214  is coupled to the switch  204 , while the second end of the current-voltage converter  214  is coupled to the second DC-to-DC converter end X 2  of DC-to-DC converter  208 . Current-voltage converter  214  is for example a resistor Rs. When switch  204  is turned on to allow conduction, according to the DC current I′ that flowed through, current-voltage converter  214  generates a first reference voltage V 1  to be used as the feedback signal fs. The magnitude of current I′ can be controlled by DC-to-DC converter  208  according to feedback signal fs so that the light output of LED string is maintained. 
   In addition, in another embodiment derived from this embodiment, when each of multiple LED strings is being driven by a corresponding DC-to-DC converter, the magnitude of current I′ flowing through each LED string can be individually controlled. And the magnitude of I′ is being individually controlled by the feedback circuit associated with each LED string, so the currents flowing through LEDs of different characteristics which are disposed on different LED strings can still have the same magnitude so that same brightness can be produced by different LED strings, allowing the brightness of backlight module formed by multiple LED strings to be more even. 
   Second Embodiment 
   Referring to  FIG. 3 , a circuit diagram of a driving device for LED strings according to a second embodiment of the invention is shown. The difference between this embodiment and the first embodiment is that the driving device  200  further includes an amplifier  216 , which is connected to the first end of the current-voltage converter  214  and the pulse width modulator  210 . In addition, the resistor Rs′ can have a lower resistance than resistor Rs of the first embodiment in order to reduce power consumption in Rs′. After a smaller reference voltage V 1 ′ is being amplified by amplifier  216 , the feedback signal fs′ is generated and outputted to pulse width modulator  210 . By doing so, the driving device  200  of this embodiment can still produce a feedback signal fs′ of voltage magnitude close to that of feedback signal fs of the first embodiment, in order that DC-to-DC converter  208  can still control the magnitude of DC current I′ according to feedback signal fs′. Hence, the light output of LED string can remain constant. In addition, in another embodiment derived from this embodiment, when each of multiple LED strings is being driven by a corresponding DC-to-DC converter, the magnitude of current I′ flowing through each LED string can be individually controlled. And the magnitude of I′ is being individually controlled by the feedback circuit associated with each LED string, so the currents flowing through LEDs of different characteristics, which are disposed on different LED strings, can still have the same magnitude so that same brightness can be produced by different LED strings, allowing the brightness of backlight module to be more even. 
   Third Embodiment 
     FIG. 4  shows a circuit diagram of a driving device for LED strings according to a third embodiment of the invention. In the first and second embodiments, when the switch  204  is turned off, DC current I′ will not be generated, thus, feedback circuit  206  can not output feedback signal fs′ according to the first reference voltage V 1 ″, and without knowing the value of current DC voltage VDC, the DC-to-DC converter  208  can not effectively control DC voltage VDC, which may cause level shifting of DC voltage VDC. Hence, LED string  202  can not be quickly lit up to reach the predetermined brightness level the next time being turned on. 
   Therefore, this embodiment is different from the first and second embodiments in that the feedback circuit  206  further includes a voltage feedback circuit  218 . When switch  204  is turned off, voltage feedback circuit  218  outputs a second reference voltage V 2  according to DC voltage VDC to be used as the feedback signal Fs″. 
   Moreover, voltage feedback circuit  218  includes a first impedance element R 1 , a second impedance element R 2  and a diode D. The first impedance element R 1  has a first end of the first impedance element and a second end of the first impedance element. The first end of the first impedance element is coupled to DC voltage VDC, and the second end of the first impedance element is coupled to a node N. Node N is in turn coupled to the pulse width modulator  210 . R 2  also has two ends. The first end of the second impedance element R 2  is coupled to node N, and the second end of the second impedance element R 2  is coupled to the fixed voltage. The negative end of the diode D is coupled to node N, while the positive end of the diode D is coupled to the first end of current-voltage converter  214 . The voltage at node N is taken as the second reference voltage V 2 . In other words, when switch  204  is turned off, diode D is reverse-biased, and the second reference voltage V 2  at this time is determined by the first and second impedance elements. At this time, feedback circuit  206  is to use second reference voltage V 2  as the feedback signal fs″ to be fed back to the pulse width modulator  210 . Therefore, when LED string  202  is turned off due to switch  204  being turned off, DC-to-DC converter  208  can maintain the magnitude of DC voltage VDC according to the second reference voltage V 2  being fed back. Thus, when switch  204  is subsequently turned on, the problem of level shifting in VDC voltage level due to the switch being turned off can be prevented. Hence, the next time when LED string  202  is lit up again, a current I′ close to the predetermined magnitude of DC current will quickly flow through LED string  202 , thereby allowing LED string  202  to quickly light up to the predetermined brightness level. 
   Similarly, when switch  204  is turned on, most of DC current I′ flows into current-voltage converter  214 , so that current-voltage converter  214  can generate a reference voltage V 1 ″ according to DC current I′. In this embodiment, diode D is forward-biased and the second reference voltage is determined by the first reference voltage V 1 ″. Feedback circuit  206  at this time uses second reference voltage V 2  as feedback signal Fs″. That is, when switch  204  is turned on, first reference voltage V 1 ″ must be greater than the voltage at node N to make sure that diode D is forward-biased and second reference voltage V 2  can be determined by first reference voltage V 1 ″. 
   Next, how the second reference voltage V 2  is determined through the first reference voltage V 1 ″ turning on diode D is further discussed. Referring to  FIG. 9 , in which the relative waveforms of the control signal of switch  204 , the first reference voltage V 1 ″, the second reference voltage V 2 , and the DC voltage VDC from the DC-to-DC converter  208  are shown. In the figure, the effects from the forward-biased voltage drop across the diode D is ignored. When switch  204  is turned off, i.e. the control signal of switch  204  is low, the voltage V 2  at node N, determined from the voltage drop across the first and the second impedance elements R 1  and R 2  via VDC, is at a lower voltage level than that of Vref. Vref is a reference signal of the comparator in the pulse width modulator  210 , and is being compared with feedback signal Fs″. Thus, through the control of the pulse width modulator  210 , the output voltage Vo is increased so as to allow voltage V 2  to be substantially equal to the reference voltage Vref. The voltage bias at node N is higher than the voltage at the positive terminal of the diode D; thus, the diode is reverse-biased. Therefore, the first reference voltage V 1 ″ does not contribute to the voltage V 2  at node N. When switch  204  is turned on, the instant voltage of VDC (V(on)) is larger than the VDC (Vo(off)) before the switching. Thus, the DC current I′ is also higher than a predetermined value and V 1 ″ higher than the reference voltage Vref. Consequently, V 1 ″ is higher than V 2 , and the diode D is now forward-biased. Thus, the voltage V 2  at node N is determined by V 1 ″. Then, through the pulse width modulator  210 , the output voltage Vo is reduced such that the DC current I′ decreases the predetermined value, so that V 2  can be substantially equal to Vref. Thus, in design considerations, since the voltage V 2  is derived from the voltage across R 1  and R 2  via VDC when the switch  204  is turned off, the ratio of the first impedance element to the second impedance element R 1 /R 2  is arranged such that when the switch  204  is turned on, V 1 ″ is always higher than the voltage V 2 . 
   The feedback circuit  206  as described in the third embodiment can also adopt the method of the second embodiment, where an amplifier  216  can be connected between the first end of current-voltage converter  214  and the positive end of the diode D so that Rs can be selected a smaller resistance value in order to reduce the power consumed by Rs. 
   Fourth Embodiment 
     FIG. 10  shows a circuit diagram of a driving device for LED strings according to a fourth embodiment of the invention. The layout in this embodiment, as distinguishable from the third embodiment, is that a third impedance element R 3  is connected to the second end of the first impedance element R 1  and the first end of the second impedance element R 2  at node N, and in place of the diode D of the voltage feedback circuit  218 . Additionally, the control signal for switch  204  is connected to the first end of R 1  through an inverter INV. The second end of R 2  is connected to a fixed voltage, and is preferably at a non-zero voltage VCC. Like the above-mentioned embodiments, switch  204  is controlled by the control signal of switch  204 , and is indicated on the figure as “CS”. When the control signal CS is high, the switch  204  is turned on and the voltage level at the second end of R 1  is equal to 0V since the control signal CS is inverted by inverter INV. The pulse width modulator  210  then controls DC-to-DC converter  208  to keep the voltage at node N to substantially approach the reference voltage Vref. By applying this embodiment, the voltage V 1 ″ required to be generated by the current-voltage converter  214  and fed back to the pulse width modulator  210  can be reduced, and the equivalent impedance of the current-voltage converter  214  can also be reduced, thereby effectively reducing energy dissipation. For instance, the fixed voltage at VCC is at 12V, and the reference voltage Vref is at 2.5V, and with an impedance ratio of R 1 :R 2 :R 3  of 3:6:2, where R 1 , R 2  and R 3  are significantly greater than the equivalent impedance of current-voltage converter  214 , i.e. impedance of Rs, then a feedback voltage V 1 ″ of only 1V is required to make the bias level at node N equal to Vref. When the control signal CS of switch  204  is low, the switch  204  is turned off and the voltage level at the second end of R 1  is equal to VCC since the control signal CS is inverted by inverter INV. The feedback voltage V 1 ″ is at 0V due to the switch  204  being turned off. As a result, the voltage V 2  at node N is now greater than Vref, such as V 2 =6V. The pulse width modulator  210  then controls the DC-to-DC converter  208  to stop generating power, in response to the voltage at node N being greater than Vref. The DC voltage VDC then is maintained by the output capacitor of the DC-to-DC converter  208 , so that when the switch  204  is turned on again, the LED string  202  can be quickly lit up. 
   In addition, the DC-to-DC converter  208  under the four embodiments can also be replaced by a Buck converter, a Boost converter, a Buck-Boost converter, a Flyback converter, or a Full-Bridge converter to achieve the same effects in quickly lighting up and turning off LED string  202 , and the use of the respective converters in the driving device for LED strings are shown in  FIGS. 5–8 . 
   The driving device for LED strings as mentioned above achieves the effects of quickly lighting up and turning off the LED strings. Also, the driving device for LED strings has the advantages of allowing the current flowing through the LED strings to remain stable while the LED strings are lit up, so that the LED string can maintain a constant light output despite different characteristics of LED strings, thus effectively reducing brightness variations across different LED strings. 
   While the invention has been described by way of example and in terms of a preferred embodiment, it is to be understood that the invention is not limited thereto. On the contrary, it is intended to cover various modifications and similar arrangements and procedures, and the scope of the appended claims therefore should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements and procedures.