Abstract:
A Light Emitting Diode (LED) driving circuit includes a power supply circuit, an LED circuit, and a switch control circuit. The power supply circuit includes an energy storage device and a switch component connected to the energy storage device. The energy storage device stores electric power when the switch component is switched on. The LED circuit is connected to the energy storage device. The switch control circuit is connected to the switch component, and is adapted to control the on/off states of the switch component, thereby adjusting a current flowing through the LED circuit.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a Light Emitting Diode (LED) driving circuit capable of adjusting a current flowing through LEDS. 
     2. Description of Related Art 
     A typical LED driving circuit includes a bridge rectifier circuit, a filter circuit, a buck converter (output voltage less than input voltage). The bridge rectifier circuit can convert alternating current voltage to direct current (DC) voltage. The buck converter outputs a low voltage to the LEDS. Then the LEDS are lit. However, the voltage and current supplied to the LEDS is not adjustable, and sometimes less or more than the normal range. The LEDS can be easily damaged in the case of over current or undercurrent. 
     Therefore, there is room for improvement within the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Many aspects of the embodiments can be better understood with references to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
         FIG. 1  is a block diagram of a LED driving circuit according to an embodiment. 
         FIG. 2  is a detailed circuit of the LED driving circuit of  FIG. 1 , showing a rechargeable battery connected to a DC power source in a first manner. 
     
    
    
     DETAILED DESCRIPTION 
     The disclosure is illustrated by way of example and not by way of limitation. In the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
     Referring to  FIGS. 1 and 2 , an embodiment of an LED driving circuit includes a voltage comparing circuit  10 , a switch control circuit  20 , a power supply circuit  30 , and an indicating circuit  40 . 
     The voltage comparing circuit  10  includes a comparator U 1 . The comparator U 1  includes a positive input terminal connected to a reference voltage Vref, a negative input terminal connected to a first node P, and an output terminal connected to the switch control circuit  20 . 
     The switch control circuit  20  includes an oscillator  22 , a NAND gate U 2 , and an AND gate U 3 . The NAND gate U 2  includes a first input terminal A 1  connected to the oscillator  22 , a second input terminal B 1  connected to the comparator  10  output terminal, and a first output terminal T 1 . The AND gate U 3  includes a third input terminal A 2  connected to the first output terminal T 1 , a fourth input terminal B 2  connected to the comparator  10  output terminal, and a second output terminal T 2  connected to the power supply circuit  30 . 
     The power supply circuit  30  includes a power supply  32 , an inductor L, a Field Effect Transistor (FET) Q, a diode D, and a capacitor C. The power supply  32  can provide a direct current (DC) voltage of 5 volts, or 12 volts. A first terminal of the inductor L is connected to the power supply  32 . A second terminal of the inductor L is connected to a second node M. The FET Q is an N-channel enhancement FET. The FET Q includes a gate terminal connected to the second output terminal T 2 , a drain terminal connected to the second node M, and a source terminal connected to ground. When a voltage at the gate terminal is at a high level (e.g., ≧5V), the FET Q is rendered conductive (switched on). When the voltage at the gate terminal is at a low level (e.g., 0V), the FET Q is rendered non-conductive (switched off). The diode D includes an anode connected to the second node M, and a cathode connected to a third node N. The capacitor C includes a first terminal connected to the third node N, and a second terminal connected to ground. 
     The LED circuit  40  includes LED 1 -LED 3 , which are connected in series, and a resistor R. LED 1  includes a first anode connected to the third node N, and a first cathode connected to a second anode of the LED 2 . A second cathode of the LED 2  is connected to a third anode of the LED 3 . A third cathode of the LED 3  is connected to the first node P. The resistor R includes a first terminal connected to the first node P, and a second terminal connected to ground. 
     When the LED driving circuit starts, the power supply  32  is switched on. A current flowing through the inductor L increases. When the current flowing through the inductor L does not reach a predetermined value, a voltage at the first node P is less than the reference voltage Vref. Thus, the comparator U 1  output terminal outputs a high level signal to the second input terminal B 1  and the fourth input terminal B 2 . The oscillator  22  output a square wave signal to the first input terminal A 1 . Because a square wave signal repeats itself and will go, say, from a low level signal to a high level signal and vice versa, the first output terminal T 1  goes from the high level to the low level and vice versa. The second output terminal T 2  will follow the first output terminal T 1  and go from the high level to the low level and vice versa. That is, a voltage level of each of the NAND gate U 2  and the AND gate U 3  is opposite to that of the square wave signal. The FET Q is switched on or off with a frequency equal to the frequency of the square wave signal. The inductor L stores electric power when the FET Q is switched on, and discharges the electric power to the capacitor C when the FET Q is switched off. As the electric power of the capacitor C increases gradually, the voltage at the third node N increases correspondingly. A current flowing through the LED circuit  40  increases gradually. When the current flowing through the LED circuit  40  exceeds a predetermined value, the voltage at the first node P exceeds the reference voltage Vref. Thus, the comparator U 1  output terminal outputs a low level signal to the second input terminal B 1  and the fourth input terminal B 2 . Because of the characteristics of the NAND gate U 2 , the first output terminal T 1  will be maintained at a high level irrespective of the input at A 1 . And because of the characteristic of the AND gate U 3 , the second output terminal T 2  will be maintained at a low level. The FET Q is switched off. A current flowing to the inductor L decreases. The current flowing to the LED circuit  40  also decreases. When the current flowing to the LED circuit  40  becomes less than the predetermined value, the voltage at the first node P becomes less than the reference voltage Vref. The comparator U 1  output terminal returns to the high level. The second output terminal T 2  returns output regular high/low signals to the FET Q. The FET Q returns to be switched on or off periodically. 
     In one embodiment, the LED driving circuit can automatically decrease the current flowing through the LED circuit  40  when a over-current is detected, and increase the current when a undercurrent is detected. Therefore, the current flowing through each of the LED 1 -LED 3  can be maintained in a normal range. 
     While the present disclosure has been illustrated by the description of preferred embodiments thereof, and while the preferred embodiments have been described in considerable detail, it is not intended to restrict or in any way limit the scope of the appended claims to such details. Additional advantages and modifications within the spirit and scope of the present disclosure will readily appear to those skilled in the art. Therefore, the present disclosure is not limited to the specific details and illustrative examples shown and described.