Patent Publication Number: US-2010109559-A1

Title: Method and circuit of controlling an led charge pump driving circuit

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of driving a Light Emitting Device (LED), and more particularly, to a method of controlling an LED charge pump driving circuit. 
     2. Description of the Prior Art 
     A conventional method of driving an LED includes using driving circuits based on a charge pump or an inductor. A charge pump driving circuit is also called a switched-capacitor driving circuit, and mainly utilizes a capacitor to transmit power from an input end to an output end without involving any inductors. Besides, the charge pump has a small size and a simple circuit. In this way, selection of circuit components of the charge pump driving circuit usually includes suitable capacitors according to related component specifications. Therefore, the charge pump driving circuit is the most popular method for driving the LED. 
     Please refer to  FIG. 1 .  FIG. 1  is a diagram illustrating a conventional LED charge pump driving circuit  10 . When an input voltage V in  of the charge pump driving circuit  10  is too high or too low, or under heavy disturbance conditions, the input voltage V in  is not suitable for driving the LED  18  directly. Consequently, the charge pump driving circuit  10  is required for generating a suitable and stable output voltage V out . The charge pump driving circuit  10  comprises a charge pump  12 , a control circuit  14  and a current sink  16 . The charge pump  12  stores charges of an input end and transmits the charges to an output end with a capacitor and a switch so as to generate the output voltage V out  at a level greater than the input voltage V in  for driving the LED  18 . The current sink  16  is used to provide a constant current to each of the LEDs  18 . The control circuit  14  is capable of controlling the charge pump  12  and the current sink  16  is capable of adjusting magnitude of the current passed to the LED  18 . 
     Although the conventional charge pump driving circuit  10  is capable of providing a constant output voltage V out  to the LED  18 , the charge pump driving circuit  10  is unable to control the luminance of the LED  18  effectively, since the luminance of the LED  18  is determined by the driving current instead of the driving voltage. Additionally, the charge pump driving circuit  10  usually generates excessively large transient output voltage V out  in order to ensure that the LED  18  turns on. However, this lowers driving efficiency of the charge pump driving circuit  10 . 
     SUMMARY OF THE INVENTION 
     The present invention provides a method of controlling the LED charge pump driving circuit. The method comprises inputting an input voltage to a charge pump for generating an output voltage; using a driver for driving an LED for generating a loading voltage according to the output voltage. When the loading voltage is greater than a first predetermined voltage, the charge pump is turned on. When the loading voltage is smaller than a second predetermined voltage, the charge pump is turned off. When the loading voltage is greater than a third predetermined voltage, the driver is locked. 
     The present invention further provides a method of controlling the LED charge pump driving circuit. The method comprises inputting an input voltage to a charge pump for generating an output voltage; using a plurality of driver for driving a plurality of LEDs according to the output voltage and detecting a plurality of loading voltages of the plurality of drivers. When one of the plurality of loading voltages is greater than the first predetermined voltage, the charge pump is turned on. When all the plurality of loading voltages are smaller than the second predetermined voltage, the charge pump is turned off. When the loading voltage of the driver is greater than the third predetermined voltage, the driver is locked. 
     The present invention further provides an LED charge pump driving circuit. The charge pump driving circuit comprises a charge pump, a driver, a current mirror, a first resistor, a second resistor, a third resistor, a first comparator, a second comparator, and a third comparator. The charge pump comprises an input end and an output end. The driver is electrically connected to the output end for generating a loading voltage for driving an LED. The current mirror provides a reference current. The first resistor is electrically connected to the input end for generating a first predetermined voltage according to the reference current. The second resistor is electrically connected to the first resistor for generating a second predetermined voltage according to the reference current. The third resistor is electrically connected to output end for generating a third predetermined voltage according to the reference current. The first comparator compares the loading voltage and the first predetermined voltage for generating a first controlling signal. The second comparator compares the loading voltage and the second predetermined voltage for generating a second controlling signal. The third comparator compares the loading voltage and the third predetermined voltage for generating a third controlling signal. 
     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 diagram illustrating a conventional LED charge pump driving circuit. 
         FIG. 2  is a block diagram illustrating the LED charge pump driving circuit of the present invention. 
         FIG. 3  is a circuit diagram illustrating a driver of  FIG. 2 . 
         FIG. 4  is a flow chart illustrating a method of utilizing a single driver for controlling the LED charge pump driving circuit of the present invention. 
         FIG. 5  is a flow chart illustrating a method of utilizing a plurality of drivers for controlling the LED charge pump driving circuit of the present invention. 
         FIG. 6  is a circuit diagram of the control circuit embodying  FIG. 4  and  FIG. 5  of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Please refer to  FIG. 2  and  FIG. 3 .  FIG. 2  is a block diagram illustrating an LED charge pump driving circuit  20  of the present invention, and  FIG. 3  is a circuit diagram illustrating a driver of  FIG. 2 . As shown in  FIG. 2 , the charge pump driving circuit  20  comprises a charge pump  22 , a control circuit  24 , a plurality of drivers  26 , and a plurality of LEDs  28 . In this embodiment of the present invention, the control circuit  24  turns the charge pump on/off according to the loading voltage Vx of a plurality of LEDs  28 . Further, the control circuit  24  determines whether the driver is open-circuited or not according to the loading voltage Vx of a plurality of LEDs  28 , namely whether the driver  26  is not electrically connected to the LED or whether the LED electrically connected to the driver is burned out. When the charge pump  24  determines that the driver  26  is open-circuited, the driver  26  is locked. After the driver  26  is locked, the control circuit  24  discontinues performing operations based on the loading voltage Vx of the driver  26 . When the charge pump  22  is turned on, the charge pump  22  can generate an output voltage V out  which is greater than the input voltage V in  (V out =M*V in ). As shown in  FIG. 3 , each of the drivers  26  comprises an operational amplifier  261 , a PMOS transistor  262 , a first resistor  263 , and a second resistor  264 . A drain of the PMOS transistor  262  is electrically connected to the LED  28 , a source of the PMOS transistor  262  is electrically connected to a negative input end of the operational amplifier  261 , and a gate of the PMOS transistor  262  is electrically connected to the output end of the operational amplifier  261 . The first resistor  263  is electrically connected between the output end of the charge pump  22  and the positive input end of the operational amplifier  261 . The second resistor  264  is electrically connected between the output end of the charge pump  22  and the negative input end of the operational amplifier  261 . The driver  26  generates a set voltage V set  and a driving current I d  with the first resistor  263  and the second resistor  264 , and the operational amplifier  261  controls the PMOS transistor  262  for outputting the driving current Id for driving the LED  28 . 
     Please refer to  FIG. 4 .  FIG. 4  is a flow chart illustrating a method of utilizing a single driver for controlling the LED charge pump driving circuit of the present invention. In this embodiment of the present invention, the LED charge pump driving circuit is shown in  FIG. 2 , and the circuit of the driver is shown in  FIG. 3 . When the LED charge pump driving circuit  20  has only one driver  26 , the control circuit  24  controls the charge pump  22  and the driver  26  according to the following steps: 
     Step  400 : Start. The control circuit  24  determines whether to turn the charge pump  22  on or off according to the loading voltage Vx of the driver  26 , or locks the driver  26 . 
     Step  410 : Determine whether the loading voltage Vx is greater than a first predetermined voltage VA. When the loading voltage Vx is greater than the first predetermined voltage VA, go to Step  420 . Else, go to Step  460 . 
     Step  420 : Turn the charge pump  22  on. The output voltage Vout of the charge pump  22  is greater than the input voltage Vin of charge pump  22 . When the charge pump  22  reaches a stable state, go to Step  430  and Step  440 . 
     Step  430 : Determine whether the loading voltage Vx is smaller than a second predetermined voltage VB. When the loading voltage Vx is smaller than the second predetermined voltage VB over a predetermined duration t, go to Step  460 . Else, go to Step  420 . 
     Step  440 : Determine whether the loading voltage Vx is greater than a third predetermined voltage VC. When the loading voltage Vx is greater than the third predetermined voltage VC, go to Step  450 . Else, go to Step  420 . 
     Step  450 : Lock the driver  26 ; go to Step  460 . 
     Step  460 : Turn the charge pump  22  off. 
     Please refer to  FIG. 5 .  FIG. 5  is a flow chart illustrating a method of utilizing a plurality of drivers for controlling the LED charge pump driving circuit of the present invention. In an embodiment of the present invention, the LED charge pump driving circuit is shown in  FIG. 2 , and the circuit of the driver is shown in  FIG. 3 . When the LED charge pump driving circuit  20  has a plurality of drivers, the control circuit  24  controls the charge pump  22  and the driver  26  according to the following steps: 
     Step  500 : Start. The control circuit  24  determines whether to turn the charge pump  22  on or off according to the loading voltage Vx of the plurality of drivers  26 , or locks the driver  26 . 
     Step  510 : Determine whether the loading voltage Vx is greater than the first predetermined voltage VA. When the loading voltage Vx is greater than the first predetermined voltage VA, go to Step  520 . Else, go to Step  560 . 
     Step  520 : Turn the charge pump  22  on. The output voltage Vout of the charge pump  22  is greater than the input voltage of the charge pump  22 . When the charge pump  22  reaches the stable state, go to Step  530  and Step  540 . 
     Step  530 : Determine whether the loading voltage Vx is smaller than the second predetermined voltage VB. When the loading voltage Vx is smaller than the second predetermined voltage VB over a predetermined duration t, go to Step  560 . Else, go to Step  520 . 
     Step  540 : Determine whether the loading voltage Vx is greater than the third predetermined voltage VC. When the loading voltage Vx is greater than the third predetermined voltage VC, go to Step  550 . Else, go to Step  520 . 
     Step  550 : Lock the driver  26 ; go to Step  560 . 
     Step  560 : Determine whether the loading voltages Vx of other drivers  26  are in the same condition. If so, go to Step  570 . If not, go to Step  520 . Step  570 : Turn the charge pump  22  off. 
     When the LED charge pump driving circuit  20  has the plurality of drivers  26 , before turning off the charge pump  22 , it is essential to confirm the charge pump  22  is not being used by other drivers  26 . Therefore, as one of the loading voltages Vx of the drivers  26  is greater than the first predetermined voltage VA, the charge pump  22  is turned on. However, it is essential that all of the loading voltages Vx of the drivers  26  be smaller than the first predetermined voltage VA before the charge pump  22  is turned off. When all the loading voltages Vx of the drivers  26  are smaller than the second predetermined voltage VB over the predetermined duration t, the charge pump  22  is turned off. Additionally, when the driver  26  is locked, the control circuit  24  does not continue to make determinations according to the loading voltage Vx of the driver  26 . 
     Please refer to  FIG. 6 .  FIG. 6  is a circuit diagram of the control circuit  24  embodying  FIG. 4  and  FIG. 5  of the present invention. The control circuit  24  comprises a first resistor  61 , a second transistor  62 , a third resistor  63 , a fourth resistor  64 , an operational amplifier  65 , an NMOS transistor  66 , a current mirror  67 , a first comparator  71 , a second comparator  72 , and a third comparator  73 . A positive input end of the operational amplifier  65  receives a reference voltage Vbg. A negative input end of the operational amplifier  65  is electrically connected to the source of the NMOS transistor. An output end of the operational amplifier  65  is electrically connected to a gate of the NMOS transistor  66 . A source of the NMOS transistor  66  is electrically connected to the first resistor  61 . A drain of the NMOS transistor  66  is electrically connected to the current mirror  67 . The operational amplifier  65  can control the NMOS transistor  66  for generating a reference current Ir according to the reference voltage Vbg and the first resistor  61 . The third resistor  63  and the fourth resistor  64  are connected in series between the input voltage V in  and the current mirror  67 . The current mirror  67  provides the reference current Ir to the third resistor  63  and the fourth resistor  64  connected in series for generating the first reference voltage VA at the node A and generating the second reference voltage VB at the node B. The reference current Ir generates a voltage Vsw on the resistor  63  and generates a voltage Vh on the resistor  64 . Therefore, equations of the first reference voltage VA and the second reference voltage VB respectively are represented as follows: 
         VA=V in− V sw 
         VB=V in− Vsw−Vh    
     The first comparator  71  compares the first reference voltage VA and the loading voltage Vx for generating a first controlling signal S 1 . The second comparator  72  compares the second reference voltage VB and the loading voltage Vx for generating a second controlling signal S 2 . Consequently, the control circuit  24  turns the charge pump  22  on/off according to the first controlling signal S 1  and the second controlling signal S 2 . The second resistor  62  is electrically connected between the output voltage and the current mirror  67 . The current mirror  67  provides the reference current Ir to the second resistor  62  for generating the third reference voltage VC at the node C. The third comparator  73  compares the third reference voltage and the loading voltage Vx for generating a third controlling signal S 3 . Therefore, the control circuit  24  determines whether the driver  26  is open-circuited for locking the driver  26  according to the third controlling signal S 3 . 
     In conclusion, the present invention provides a method of controlling the LED charge pump driving circuit. The charge pump driving circuit comprises a charge pump, a control circuit, a driver, and an LED. The control circuit determines whether to turn the charge pump on or off according to the loading voltage of the driver and determines whether the driver is open-circuited. The charge pump generates an output voltage according to an input voltage. The driver drives the LED for generating a loading voltage according to the output voltage. When the loading voltage is greater than a first predetermined voltage, the charge pump is turned on. When the loading voltage is smaller than a second predetermined voltage over a predetermined duration, the charge pump is turned off. When the loading voltage is greater than a third predetermined voltage, the driver is locked. In another embodiment, the charge pump driving circuit comprises a plurality of drivers for driving a plurality of LEDs. When one of the loading voltages of the plurality of drivers is greater than the first predetermined voltage, the charge pump is turned on. When all of the loading voltages of the plurality of drivers are smaller than the second predetermined voltage over the predetermined duration, the charge pump is turned off. 
     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.