Abstract:
The present invention discloses a constant-current charge pump, wherein a current detection circuit and a regulation circuit are arranged in the output of a pump circuit and used to control the current output by the pump circuit. When the load varies, the current variation is detected, and the regulation circuit pumps and regulates the current output by the pump circuit to stabilize the output current. Thereby, the output current will vary very slightly for different loads and input voltages.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a charge pump, particularly to a charge pump applying to an LED driver circuit. 
       BACKGROUND OF THE INVENTION 
       [0002]    Recently, LCD (Liquid Crystal Display) of mobile phones, PDA (Personal Digital Assistant), etc., has evolved from a monochromatic display to a colored one. As liquid crystal itself cannot emit light, LCD needs a backlight unit. Due to the consideration of power consumption, white-light LEDs have replaced traditional electric bulbs to function as the light source of a backlight unit. A white-light LED needs a high forward voltage and a high forward current. The current control of a backlight unit has a dominating influence on the running time of the batteries of a portable electronic device. A charge pump is usually used in a backlight unit to achieve high energy efficiency and is often designed to be a 1.5× power supply or a 2× one. 
         [0003]    With using a common white-light LED driver IC, a white-light LED usually receives an input voltage of only 2.6-3.5V. However, a white-light LED needs to be driven by a current of 20 mA. Therefore, a white-light LED needs a forward voltage of about 3.6-4.0V, which is higher than the input voltage. Thus, a charge pump is used to raise the input voltage. 
         [0004]    Refer to  FIG. 1  a diagram schematically showing the architecture of a conventional charge pump circuit. An external pump capacitor Cp is coupled to a conventional 2× charge pump circuit  10 , and the output of the charge pump  10  is coupled to an output capacitor Co, which is grounded. The 2× charge pump circuit  10  is simply to double the input voltage. Therefore, a current regulator  30  is added to behind the load  20  (white-light LED) to obtain a stable 20 mA current. 
         [0005]    Refer to  FIG. 2  a diagram schematically showing the circuit of a conventional charge pump circuit. The conventional charge pump circuit  10  has a charge transistor  11  and a discharge transistor  12 . Two external pads P C1  and P C2  are respectively coupled to two sides of a pump capacitor Cp; the output terminal of the charge pump circuit  10  is coupled to a load  20  via a pad P OUT ; the charge pump circuit  10  is grounded via a pad P GND . If the charge pump IC is incorporated with a current regulator  30 , the IC needs additional pins to connect with the load  20  so that the load  20  can connect with the current regulator  30 . As light-white LEDs need a great current, a plurality of pads P LED  is used to supply power in parallel. In  FIG. 2 , ten pieces of pads P LED  are used. However, increasing the number of pads increases IC area. Further, it is hard to layout many pads. 
       SUMMARY OF THE INVENTION 
       [0006]    The objective of the present invention is to eliminate the current regulator, which is originally added behind the load in the application of a charge pump, and simplify the design of pins, wherein a detection circuit and a regulation circuit are added to between the output of a pump circuit and the load (white-light LED) to form a constant-current charge pump, which bumps current to achieve a stable output current according to the detected variation of the output current. Thereby, the output current will vary very slightly for different input voltages and loads. 
         [0007]    The present invention proposes a constant-current charge pump, which comprises: a pump circuit used to boost voltage; an inverter used to control an external pump capacitor coupled to the pump circuit; a current detection circuit arranged in the output of the pump circuit and used to detect output current and convert current into voltage; and a regulation circuit receiving an input voltage and the voltage output by the current detection circuit and functioning as a negative feedback mechanism to regulate the current output by the pump circuit. 
         [0008]    The current detection circuit further comprises: a resistor coupled to the output of the pump circuit and used to detect an output current; and a voltage amplifier receiving voltage difference between two ends of the resistor, amplifying the voltage difference and transmitting the voltage difference amplified to the regulation circuit. 
         [0009]    The regulation circuit further comprises: an error amplifier receiving a voltage sent out by the current detection circuit and comparing the voltage with a reference voltage; and a regulation transistor receiving an input voltage and controlled by the error amplifier to regulate a current output by the pump circuit. 
         [0010]    The regulation circuit is coupled to the inverter and providing a boost voltage required by the pump capacitor via the inverter. Alternatively, the regulation circuit is coupled to the input of the pump circuit and determines how much current the regulation transistor should pump to a charge transistor of the pump circuit. 
         [0011]    In the present invention, a current detection circuit and a regulation circuit are arranged in the output of a pump circuit and used to control the current output by the pump circuit. When the load varies, the circuit changes the current amount of the pump circuit to regulate the output current. The design of the present invention doesn&#39;t need to add a current regulator to behind the load. Further, in the present invention, the output current will vary very slightly (about below 0.5%) for different loads and input voltages. The performance of the constant-current charge pump of the present invention is satisfactory and very close to that of a charge pump with an external current regulator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a diagram schematically showing the architecture of a conventional charge pump circuit and additional current regulator. 
           [0013]      FIG. 2  is a diagram schematically showing the circuit of a conventional charge pump circuit and additional current regulator. 
           [0014]      FIG. 3  is a diagram schematically showing the architecture of a constant-current charge pump according to the present invention. 
           [0015]      FIG. 4  is a diagram schematically showing the circuit of a constant-current charge pump according to the present invention. 
           [0016]      FIG. 5  is a diagram showing the detailed circuit of  FIG. 4 . 
           [0017]      FIG. 6  is a diagram showing the detailed circuit of another embodiment of a constant-current charge pump according to the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0018]    Below, the technical contents of the present invention are to be described in detail with the embodiments. However, it should be understood that the embodiments are only to exemplify the present invention but not to limit the scope of the present invention. 
         [0019]    Refer to  FIG. 3  a diagram schematically showing the architecture of a constant-current charge pump according to the present invention, wherein a 2× charge pump is used to exemplify the present invention. Similar to a common charge pump, the constant-current charge pump  100  of the present invention is coupled to an external pump capacitor Cp, and the output of the charge pump  100  is coupled to an output capacitor Co, which is grounded. The constant-current charge pump  100  is to double the input voltage and provide a constant current 200 mA for a load  200  (white-light LED). 
         [0020]    Refer to  FIG. 4  a diagram schematically showing the circuit of a constant-current charge pump according to the present invention. The constant-current charge pump  100  comprises a pump circuit  110 , and the pump circuit  110  includes a charge transistor  111  and a discharge transistor  112  sequentially arranged in between the input voltage VDD and the output OUT. One side of the external pump capacitor Cp is coupled to between the charge transistor  111  and the discharge transistor  112 ; the other side of the external pump capacitor Cp is coupled to the output of an inverter  10   1 . The output OUT of the constant-current charge pump  100  is coupled to a grounded output capacitor Co and a load  200  (such as a white-light LED). 
         [0021]    In the present invention, a current detection circuit  120  and a regulation circuit  130  are arranged in before the output OUT to control the output current of the constant-current charge pump  100 . The current detection circuit  120  detects the variation of the output current. The regulation circuit  130  utilizes a negative feedback mechanism and an appropriate current-regulating mechanism to pump a voltage and regulate an output current for achieving fast load response and current stabilization. When the load  200  varies, the current output by the charge pump  100  is regulated to achieve a stable current. 
         [0022]    The current detection circuit  120  includes a resistor  121  and a voltage amplifier  122 . The resistor  121  is arranged in between the pump circuit  110  and the output OUT and used to detect the output current. The voltage amplifier  122  is coupled to two ends of the resistor  121  and transfers the voltage difference between two ends of the resistor  121  to the regulation circuit  130 . The regulation circuit  130  includes an error amplifier  131  and a regulation transistor  132 . The error amplifier  131  receives the voltage output by the voltage amplifier  122  and functions as a negative feedback mechanism to regulate output current. The positive input terminal of the error amplifier  131  receives a reference voltage Vref (such as 1.2V), and the negative input terminal of the error amplifier  131  receives the voltage transformed from the output current. The error amplifier  131  determines how much current the regulation transistor  132  will pump to the pump capacitor Cp via the inverter  101 . 
         [0023]    When the current required by the load  200  increases, the feedback voltage received by the error amplifier  131  also increases. Once the error amplifier  131  receives the error voltage, it will controls the regulation transistor  132  to release less current and stabilize the feedback voltage at the reference voltage Vref (such as 1.2V). The output current is a multiple of the product of the feedback voltage, the resistance of the resistor  121  (10Ω) and the magnification (6×) of the voltage amplifier  122 ; therefore, the output current will be stabilized at 20 mA(1.2V/6=0.2V and 0.2V/10Ω=20 mA). 
         [0024]    The constant-current charge pump  100  of the present invention can provide a constant current for the load  200  and regulate output current without using any current regulator added to behind the pump circuit  110  or the load  200 . Therefore, the present invention can simplify the design of IC pins and reduce numbers of pins. 
         [0025]    Refer to  FIG. 5  a diagram showing the detailed circuit of  FIG. 4 . Suppose the constant-current charge pump  100  is a 2× pump circuit. The charge transistor  111  and the discharge transistor  112  of the pump circuit  110  are respectively controlled by a first clock CK 1  and a second clock CK 2 , wherein the first clock CK 1  and the second clock CK 2  are out of phase. The inverter  101  includes a p-type transistor and an n-type transistor, and a third clock CLK of the inverter  101  is synchronous with the second clock CK 2 . The input terminal receives an input voltage (generally about 2.6-3.5V). The pump capacitor Cp is arranged in between the output of the inverter  101  and the junction of the charge transistor  111  and the discharge transistor  112 . 
         [0026]    When the load  200  needs a greater current, the voltage difference between two ends of the resistor  121  also increases. The voltage amplifier  122  converts the voltage difference into an error voltage and sends the error voltage to the error amplifier  131 . Once the error amplifier  131  detects the error voltage, it will control the regulation transistor  132  to release less current and stabilize the feedback voltage at the reference voltage Vref (such as 1.2V). Then, the error amplifier  131  can determine how much current the regulation transistor  132  should pump to make the inverter  101  able to provide a boost voltage Vjmp for the pump capacitor Cp. Thus, the output voltage Vout of the constant-current charge pump  100  is the sum of the boost voltage Vjmp and the input voltage VDD received by the charge transistor  111  of the pump circuit  110 . 
         [0027]    Refer to  FIG. 6  a diagram showing the detailed circuit of another embodiment of a constant-current charge pump according to the present invention.  FIG. 6  is different from  FIG. 4  and  FIG. 5  in that the regulation transistor  132  of the regulation circuit  130  is arranged in before the charge transistor  111  of the pump circuit  110 . When the load  200  needs a greater current, the voltage difference between two ends of the resistor  121  also increases. The voltage amplifier  122  converts the voltage difference into an error voltage and sends the error voltage to the error amplifier  131 . Once the error amplifier  131  detects the error voltage, it will control the regulation transistor  132  to release less current and stabilize the feedback voltage at the reference voltage Vref (such as 1.2V). Then, the error amplifier  131  can determine how much current the regulation transistor  132  should pump so that the charge transistor  111  of the pump circuit  110  can obtain the required input voltage Vin. Thus, the output voltage Vout of the constant-current charge pump  100  is the sum of the input voltage Vin and the source voltage VDD received by the inverter  101 . 
         [0028]    The spirit of the present invention is to obtain a stable current seemingly generated by a current regulator without using any current regulator, which will increase the numbers of pins and pads. In the present invention, a control circuit (the current detection circuit  120  and regulation circuit  130 ) is added to between the pump circuit  110  and the load  200 . The control circuit functions like a water pipe. When the control circuit detects the variation of the load resistance or the input voltage, the water pipe of the control circuit varies its diameter to stabilize the current. Thus, the output current is not decided by the load resistance or the input voltage but by the control circuit. Thereby, a stable current, which does not vary with the load or the input voltage, is achieved. 
         [0029]    Those described above are only the preferred embodiments to exemplify the present invention but not to limit the scope of the present invention. Any equivalent modification or variation according to the spirit of the present invention is to be also included within the scope of the present invention.