Patent Abstract:
A voltage detecting device applied to a driving device of a light-emitting diode device is provided. The detecting device includes a voltage inspecting device, an isolating/connecting control device and a comparing device. The voltage inspecting device is coupled to an output terminal of the driving device to inspect a status of an output voltage of the driving device and outputs an inspecting signal. The isolating/connecting control device, coupled between the voltage inspecting device and the driving device, isolates or connects the output terminal of the driving device according to the inspecting signal. The comparing device, composed of low voltage elements, is coupled to the isolating/connecting control device, and compares the output voltage of the driving device with a reference voltage and generates a detecting signal according to the comparing result.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This Application claims priority of Taiwan Patent Application No.100207618, filed on Apr. 29, 2011, the entirety of which is incorporated by reference herein. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to detecting an output voltage of an LED driving device and more particularly to detecting an output voltage of a driving device that uses a constant current source to drive an LED. 
     2. Description of the Related Art 
     Ways to drive a light-emitting diode (LED) can be classified into three types: a constant voltage source type, a constant current source type and a pulse type. Advantages of the constant voltage source type driver are low costs and uncomplicated external circuitry; but, a disadvantage is luminance inconsistency of LEDs. Generally, the constant current source type driver can overcome the problem of luminance inconsistency of LEDs. For driving of the constant current source type driver, LEDs are connected in serial so as to assure the LEDs of luminance consistency. In addition, the constant current source type driver can avoid the condition where the reliability of the driving is influenced by the driving current when the driving current exceeds a largest rated value. 
     When an LED fails, the output voltage of the LED driving device is different from a normal operating voltage. Therefore, a voltage detecting device for detecting whether the output voltage of the LED driving device is normal or not is needed. Particularly, when the power of a system has to be lowered due to heat dissipating concerns, the voltage of power source is lowered. In the case, if the output voltage of the LED driving device is lower than a normal operating range, the output current may be abnormal, and the abnormal output current has to be detected. 
       FIG. 1  is a schematic diagram of an LED device showing a known way to detect an output voltage of a driving circuitry. The LEDs LED 1-N  are connected in serial and are coupled to an LED driving circuitry. The LED driving circuitry comprises a driving device  108  and a comparator CP 1 . In this way, an output terminal a of the driving device  108  of the LED driving circuitry is directly coupled to a negative terminal of the comparator CP 1 , so that the voltage of the output terminal a can be compared with a reference voltage VREF coupled to a positive terminal of the comparator CP 1 . When the voltage of the output terminal a is lower than the reference voltage VREF, a detecting signal Flag 1  output by the comparator CP 1  is at a high voltage level. The condition where the detecting signal Flag 1  is at a high voltage level means that the voltage of the output terminal a of the driving device is too low and that the output current lout may be abnormal. 
     In order to connect a plurality of LEDs in serial, a voltage VLED is usually much higher than voltage VDD. Therefore, when a pulse-width modulator PWM controls a switch Si to be off, the comparator CP 1  has to be able to receive a high-voltage-level voltage. However, the area of the circuitry of the comparator CP 1  is larger when using high voltage elements to realize the comparator CP 1 . Furthermore, an input offset voltage is higher. Therefore, the detecting way as shown in  FIG. 1  is not accurate when detecting the voltage of the output terminal a of the driving device. 
       FIG. 2  is a schematic diagram of an LED device showing another known way to detect an output voltage of a driving circuitry. In this case, in order to use low voltage elements to realize the comparator CP 2 , the resistances R 1  and R 2  are used to divide voltage so as to divide the voltage of an output terminal b of a driving device  208  by (VLED/VSS). Accordingly, when the pulse-width modulator PWM controls a switch S 2  to be off, voltage of a terminal c connected to a negative terminal of the comparator CP 2  is ensured to be lower than or equal to the voltage VDD. 
     In the way shown in  FIG. 2 , though low voltage elements can be used to realize the comparator CP 2  and thus the input offset voltage is smaller, the influence caused by the input offset is enlarged by (VLED/VSS) times. The greater the amount of LEDs, the higher the voltage VLED is, and thus the bigger the influence caused by the input offset is. In addition, there is also an error caused by mismatch of the resistances R 1  and R 2  in the way shown in  FIG. 2 . 
     BRIEF SUMMARY OF THE INVENTION 
     In view of this, the invention provides a voltage detecting device, applied to a driving device of a light-emitting diode device. The voltage detecting device uses a comparing device realized by low voltage elements to detect an output voltage of the driving device, so that a detecting error enlarged by voltage division of resistances is hindered. 
     The voltage detecting device comprises: a voltage inspecting device, coupled to an output terminal of the driving device to inspect a status of an output voltage of the driving device and outputting an inspecting signal; an isolating/connecting control device, coupled between the voltage inspecting device and the driving device, isolating or connecting the output terminal of the driving device according to the inspecting signal; and a comparing device, coupled to the isolating/connecting control device, comparing the output voltage of the driving device with a reference voltage and generating a detecting signal according to the comparing result. The comparing device is composed of low voltage elements. 
     Another embodiment of the invention provides a driving circuitry for light-emitting diode (LED) devices, comprising: a driving device, coupled to an LED device to drive the LED device; a voltage inspecting device, coupled to an output terminal of the driving device to inspect a status of an output voltage of the driving device and outputting an inspecting signal; an isolating/connecting control device, coupled between the voltage inspecting device and the driving device, isolating or connecting the output terminal of the driving device according to the inspecting signal; and a comparing device, coupled to the isolating/connecting control device, comparing the output voltage of the driving device with a reference voltage and generating a detecting signal according to the comparing result. The comparing device is composed of low voltage elements. 
     A detailed description is given in the following embodiments with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram of an LED device showing a known way to detect an output voltage of a driving circuitry; 
         FIG. 2  is a schematic diagram of an LED device showing another known way to detect an output voltage of a driving circuitry; 
         FIG. 3  shows a block diagram of a voltage detecting device applied to an LED driving device according to an embodiment of the invention; 
         FIG. 4  shows a circuitry of a voltage detecting device applied to an LED driving device according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
     As shown in  FIG. 3 , according to an embodiment of the invention, the voltage detecting device  300  comprises a voltage inspecting device  302  coupled to an output terminal of a driving device  308 , a comparing device  306  composed of low voltage elements and an isolating/connecting control device  304  coupled between the output terminal of the driving device  308  and the comparing device  306 . The voltage inspecting device  302  receives the voltage of the output terminal of the driving device  308  and outputs an inspecting signal. If the voltage of the output terminal of the driving device  308  is at a high voltage level, the inspecting signal is a second signal  SW . If the voltage of the output terminal of the driving device  308  is at a low voltage level, the inspecting signal is a first signal SW. The comparing device  306  compares the voltage of the output terminal of the driving device  308  with a reference voltage VREF and outputs a detecting signal Flag 3  to determine whether the voltage of the output terminal of the driving device  308  is normal or not. The isolating/connecting control device  304  electrically isolates the comparing device  306  from the voltage of the output terminal of the driving device  308  or electrically connects the comparing device  306  with the voltage of the output terminal of the driving device  308  according to the inspecting signal output by the voltage inspecting device  302 . If the inspecting signal is the second signal  SW , the isolating/connecting control device  304  electrically isolates the comparing device  306  from the voltage of the output terminal of the driving device  308  to prevent the comparing device  306  from being damaged by the high-voltage-level voltage of the output terminal. If the inspecting signal is the first signal SW, the isolating/connecting control device  304  electrically connects the comparing device  306  with the voltage of the output terminal of the driving device  308  to make the comparing device  306  compare the voltage of the output terminal with the reference voltage so as to determine whether the voltage of the output terminal is normal or not. When the voltage of the output terminal of the driving device  308  is at a high voltage level, since the isolating/connecting control device  304  electrically isolates the comparing device  306  from the voltage of the output terminal of the driving device  308 , the comparing device  306  can be realized by low voltage elements. In one embodiment of the invention, the driving device  308  and the voltage detecting device  300  are packaged together or manufactured in the same integrated circuit. 
     Referring  FIG. 4 , a driving device  408  of the LED device LED 1-N  comprises a switch S 1  controlled by a pulse-width modulator and a constant current source lout. The pulse-width modulator (not shown in the Fig.) transmits a PWN control signal to control the switch S 1  to be on or off. When the switch S 1  is on, the constant current source lout drives the LED device LED 1-N . 
     As shown in  FIG. 4 , in a voltage detecting device  400 , a voltage inspecting device  402  comprises a diode D 1  and transistors M 1  and M 2 . A cathode of the diode D 1  is coupled to an output terminal k of the driving device  408 . Sources of the transistors M 1  and M 2  are connected to a power supply source VDD. Gates of the transistors M 1  and M 2  are connected together. A drain of the transistor M 1  is coupled to the gate of the transistor M 1  and is further coupled to an anode of the diode D 1  through a resistance R 5 . A drain of the transistor M 2  is grounded through a resistance R 6 . The drain of the transistor M 2  is further coupled to an inspecting signal of the voltage inspecting device  402 . An isolating/connecting control device  404  comprises an isolating diode D 2  and a controllable current source I 1  and a controllable current source I 2 . A cathode of the isolating diode D 2  is coupled to a negative input terminal of a comparator CP 3  of a comparing device  406 . The voltage detecting device  400  further comprises a compensating diode D 3  and a controllable current source I 3 . An anode of the compensating diode D 3  is coupled to the controllable current source I 3  and a positive input terminal of the comparator CP 3  of the comparing device  406 . A cathode of the compensating diode D 3  is coupled to the reference voltage VREF. The compensating diode D 3  matches the isolating diode D 2 . The output current of the controllable current source I 3  is equal to the output current of the controllable current source I 1 . 
     When the switch S 1  of the driving device  408  is off, the LED device LED 1-N  is not driven, and the voltage of the output terminal k of the driving device  408  is pulled up to the voltage VLED. Therefore, the diode D 1  is off. Because the diode D 1  is off, the voltage of the drain terminal h of the transistor M 2  is pulled down. At the same time, the voltage inspecting device  402  outputs the inspecting signal  SW . The inspecting signal  SW  makes the controllable current source I 2  to be on and the controllable current source I 1  to be off. Since the controllable current source I 2  is on, the voltage of the anode of the isolating diode D 2  is pulled down to be grounded by the turned-on controllable current source I 2 , and thus the isolating diode D 2  is off. At the time, the output terminal k of the driving device  408  is isolated from the comparing device  406  by the turned-off isolating diode D 2 . Therefore, the comparing device  406  is not damaged by the high-voltage-level voltage of the output terminal k of the driving device  408 . Accordingly, the comparing device  406  can be realized by low voltage elements. 
     When the switch S 1  of the driving device  408  is on, the LED device LED 1-N  is driven, and the voltage of the output terminal k of the driving device  408  is pulled down. Therefore, the diode D 1  is on. Because the diode D 1  is on, the voltage of the drain terminal h of the transistor M 2  is pulled up. At the same time, the voltage inspecting device  402  outputs the inspecting signal SW. The inspecting signal SW makes the controllable current source I 1  to be on and the controllable current source I 2  to be off. Since the controllable current source I 1  is on, the voltage of the negative input terminal of the comparator CP 3  is equal to a sum of the voltage of the output terminal k of the driving device  408  and the voltage of the turned-on isolating diode D 2 . At the same time, the inspecting signal SW controls the controllable current source I 3  to make the controllable current source I 3  to be on. Therefore, the voltage of the positive input terminal of the comparator CP 3  is equal to a sum of the reference voltage VREF and the voltage of the turned-on compensating diode D 3 . Since the compensating diode D 3  matches the isolating diode D 2  and the output current of the controllable current source I 3  is equal to the output current of the controllable current source I 1 , the voltage of the turned-on isolating diode D 2  is equal to the voltage of the turned-on compensating diode D 3 . Therefore, the comparator CP 3  can compare the voltage of the output terminal k of the driving device  408  with the reference voltage VREF. When the voltage of the output terminal k of the driving device  408  is lower than the reference voltage VREF, the detecting signal Flag 3  output by the comparator CP 3  is at a high voltage level. In this case, the voltage of the output terminal k of the driving device  408  being too low is detected and the output current Iout may be abnormal. 
     In one embodiment of the invention, the driving device  408  and the voltage detecting device  400  are packaged together or manufactured in the same integrated circuit. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. To the contrary, it is intended to cover various modifications and similar arrangements (as would be apparent to those skilled in the art). Therefore, the scope of the appended claims should be accorded the broadest interpretation so as to encompass all such modifications and similar arrangements.

Technology Classification (CPC): 7