Patent Publication Number: US-8115422-B2

Title: LED drive circuit

Description:
RELATED APPLICATIONS 
     Pursuant to 35 U.S.C. §119(e), this application claims the benefit of the filing date of U.S. Provisional Application Ser. No. 61/043,261 filed on Apr. 8, 2008, which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an LED drive circuit for driving an LED, and more particularly, to an LED drive circuit formed of a constant current circuit using a switching regulator IC. 
     2. Description of the Related Art 
     LEDs are used in various electronic devices. There are various drive circuits as an LED drive circuit, and an LED drive circuit formed of a constant current circuit using a switching regulator IC is used as well. 
       FIG. 7  is a circuit diagram of an LED drive circuit using a conventional step-down switching regulator IC. An LED drive circuit  1  using the conventional step-down switching regulator IC includes a step-down switching regulator IC  2 A, a resistor circuit  3  for detecting a current of an LED  10  connected to an output terminal VOUT, and a switching transistor  4 , a coil  5 , a diode  6 , and capacitors  7  and  8  which are external elements of the switching regulator IC  2 A. In a case of this circuit, a configuration in which the current of the LED  10  is detected on a terminal VRET side is taken as an example. 
       FIG. 8  is a circuit diagram of an LED drive circuit for stepping down a power supply voltage to drive an LED using a conventional step-up switching regulator IC. An LED drive circuit  1  using the conventional step-up switching regulator IC includes a step-up switching regulator IC  2 B, a resistor circuit  3  for detecting a current of an LED  10  connected to an output terminal VOUT, and a switching transistor  4 , a coil  5 , a diode  6 , and capacitors  7  and  8  which are external elements of the switching regulator IC  2 B. In a case of this circuit, a configuration in which the current of the LED  10  is detected on the output terminal VOUT side is taken as an example. 
     The LED drive circuits  1  as described above cause the LED to emit light by the following operation. 
     For example, in the LED drive circuit  1  of  FIG. 7 , the switching regulator IC  2 A controls on/off of the switching transistor  4 , and causes a current generated in the coil  5  to flow from the output terminal VOUT to the LED  10 . An output current of the output terminal VOUT generates a voltage by flowing through the resistor circuit  3 . The voltage in proportion to the output current is input to an FB terminal of the switching regulator IC  2 A. The switching regulator IC  2 A controls on/off of the switching transistor  4  so that the voltage becomes constant. Accordingly, the LED  10  emits light with constant luminance. 
     The LED drive circuit  1  of  FIG. 8  also causes the LED  10  to emit light with constant luminance by the similar operation. 
     In a market, there is such a demand that the LED drive circuit is driven by power supply obtained by directly rectifying a commercial AC power supply, for example, by power supply of DC 100 V which is converted from AC 100 V. 
     However, the above-mentioned LED drive circuits  1  each have a circuit configuration in which a power supply voltage is directly applied to a power supply terminal Vdd of the switching regulator IC. Accordingly, the LED  10  cannot be driven by a power supply voltage which is equal to or larger than a breakdown voltage of an IC in the conventional LED drive circuits  1 . For example, in the case of a circuit driven by a power supply voltage of 100 V, a switching regulator IC having a breakdown voltage of 5 V cannot be used. In such a case, a switching regulator IC having high breakdown voltage is used or a power supply voltage is stepped down. However, there arises such a problem that the switching regulator IC is costly in the former case while power efficiency deteriorates in the latter case. 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above-mentioned problem, and aims to drive an LED without loss of power efficiency even when a power supply voltage exceeds a breakdown voltage of a switching regulator IC. 
     In order to solve the conventional problem, an LED drive circuit using a switching regulator IC according to the present invention has the following configuration. 
     The LED drive circuit includes: a voltage clamp circuit for clamping a power supply voltage to an operating voltage of the switching regulator IC; and a current detecting circuit for detecting a current flowing through an LED, and outputting a voltage obtained by converting the detected current into a voltage within a range of the operating voltage of the switching regulator IC. 
     With the above-mentioned configuration, the LED drive circuit according to the present invention is capable of setting a voltage within a range of an operating voltage of the switching regulator IC by the voltage clamp circuit even when a voltage of a power supply is 100 V, and setting the voltage output from the current detecting circuit within the range of the operating voltage of the switching regulator IC. Accordingly, a switching regulator IC having low breakdown voltage can be used for the LED drive circuit, and thus the LED drive circuit can be configured at low cost. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the accompanying drawings: 
         FIG. 1  is a circuit diagram of an LED drive circuit using a step-down switching regulator IC according to a first embodiment of the present invention; 
         FIG. 2  is a circuit diagram illustrating an example of a current detecting circuit used for the LED drive circuit of  FIG. 1  according to the first embodiment; 
         FIG. 3  is a circuit diagram illustrating a specific example of the LED drive circuit using the step-down switching regulator IC according to the first embodiment; 
         FIG. 4  is a circuit diagram of an LED drive circuit using a step-up switching regulator IC according to a second embodiment of the present invention; 
         FIG. 5  is a circuit diagram illustrating an example of a current detecting circuit used for the LED drive circuit of  FIG. 4  according to the second embodiment; 
         FIG. 6  is a circuit diagram illustrating a specific example of the LED drive circuit using the step-up switching regulator IC according to the second embodiment; 
         FIG. 7  is a circuit diagram of an LED drive circuit using a conventional step-down switching regulator IC; and 
         FIG. 8  is a circuit diagram of an LED drive circuit using a conventional step-up switching regulator IC. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       FIG. 1  is a circuit diagram of an LED drive circuit using a step-down switching regulator IC according to this embodiment. 
     An LED drive circuit  1  according to this embodiment includes: a power supply  9 ; a step-down switching regulator IC  2 A; a coil  5 , a diode  6 , capacitors  7  and  8  which are external elements of the switching regulator IC  2 A; a switching transistor  4 ; an LED  10 ; a voltage clamp circuit  1 ; and a current detecting circuit  13 A. 
     The power supply  9  is connected between a power supply input terminal Vin and a power supply input terminal GND. The switching regulator IC  2 A includes a power supply terminal Vdd connected to the power supply input terminal Vin, a power supply terminal Vss connected to an output terminal of the voltage clamp circuit  11 , an output terminal EXT connected to a gate of the switching transistor  4 , and an FB terminal connected to an output terminal of the current detecting circuit  13 A. The switching transistor  4  and the coil  5  are connected in series between the power supply input terminal Vin and an output terminal VOUT. The LED  10  is connected between the output terminal VOUT and a terminal VRET. The current detecting circuit  13 A is connected between the terminal VRET and the power supply input terminal GND. The voltage clamp circuit  11  is connected between the power supply input terminal Vin and the power supply input terminal GND. 
     The switching regulator IC  2 A and the external elements thereof input a feedback voltage based on an output current of the output terminal VOUT to the FB terminal, and control the switching transistor  4  so that a current flowing through the LED  10  becomes constant. The voltage clamp circuit  11  clamps a voltage applied between the power supply terminals Vdd and Vss of the switching regulator IC  2 A. That is, only a clamp voltage is applied by a zener diode between the power supply terminals Vdd and Vss. The current detecting circuit  13 A has a function of inputting a voltage for controlling the output current to the FB terminal of the switching regulator IC  2 A operated by the clamp voltage of the zener diode. 
     The voltage clamp circuit  11  includes the zener diode and a resistor which are connected in series between the power supply input terminals Vin and GND of the power supply  9  of the LED drive circuit  1 . For example, it is assumed that a voltage of the power supply  9  is 100 V and an operating voltage of the switching regulator IC  2 A is 5 V. In a case of the voltage clamp circuit  11  of  FIG. 1 , a voltage of 100 V is input to the power supply terminal Vdd of the switching regulator IC  2 A, while a voltage of 95 V which is clamped by the voltage clamp circuit  11  is input to the power supply terminal Vss. That is, a voltage of 5 V is applied between the power supply terminals Vdd and Vss of the switching regulator IC  2 A. Accordingly, even when the power supply voltage is 100 V, the LED drive circuit  1  can be configured by using the switching regulator IC  2 A which has an operating voltage of 5 V. 
     Here, the current detecting circuit  13 A is provided between the terminal VRET to which the current flowing through the LED  10  returns and the power supply input terminal GND. The switching regulator IC  2 A operates with a voltage of the power supply input terminal Vin as a reference, and thus the current detecting circuit  13 A detects the current flowing through the LED  10 , converts the detected current into a voltage with the voltage of the power supply input terminal Vin as the reference, and outputs the converted voltage. 
       FIG. 2  illustrates an example of the current detecting circuit used for the LED drive circuit of  FIG. 1 . The current detecting circuit  13 A includes a current detecting resistor  21 , an NPN bipolar transistor  22 A including an emitter and a base which are connected to both ends of the current detecting resistor  21 , and a resistor  23  connected between a collector of the bipolar transistor  22 A and the power supply input terminal Vin. 
     The current detecting circuit  13 A is configured in this manner, and hence a high power supply voltage can be applied between the collector and the emitter of the bipolar transistor  22 A. As a result, the current detecting circuit  13 A can convert a voltage to be output into a voltage within a range of the operating voltage of the switching regulator IC  2 A with 100 V as the reference, and output the converted voltage. 
       FIG. 3  illustrates a specific example of the LED drive circuit  1  using the step-down switching regulator IC  2 A according to this embodiment. The current detecting circuit  13 A of  FIG. 3  is an example in which the current detecting circuit  13 A of  FIG. 2  is illustrated more specifically. The current detecting circuit  13 A of  FIG. 3  includes a bias circuit  24  and a voltage setting circuit  25 . The bias circuit  24  includes a diode and a resistor, and applies a bias voltage to the base of the bipolar transistor  22 A. The voltage setting circuit  25  includes a diode and resistors. The diode includes an anode terminal connected to the power supply terminal Vss, and has a function of accurately setting a voltage range of the voltage output from the current detecting circuit  13 A. 
     The LED drive circuit  1  is configured in this manner, and thus, even when the voltage of the power supply  9  is 100 V, can set the voltage within a range of the operating voltage of the switching regulator IC  2 A by the voltage clamp circuit  11 . In addition, the LED drive circuit  1  can set the voltage output from the current detecting circuit  13 A within the range of the operating voltage of the switching regulator IC  2 A. Accordingly, the LED drive circuit  1  can use the switching regulator IC  2 A having low breakdown voltage, whereby the LED drive circuit  1  can be manufactured at low cost. 
     Second Embodiment 
       FIG. 4  is a circuit diagram of an LED drive circuit using a step-up switching regulator IC according to this embodiment. 
     An LED drive circuit  1  according to this embodiment includes: a power supply  9 ; a step-up switching regulator IC  2 B; a coil  5 , a diode  6 , capacitors  7  and  8  which are external elements of the switching regulator IC  2 B; a switching transistor  4 ; an LED  10 ; a voltage clamp circuit  11 ; and a current detecting circuit  13 B. 
     The power supply  9  is connected between a power supply input terminal Vin and a power supply input terminal GND. The switching regulator IC  2 B includes a power supply terminal Vdd connected to an output terminal of the voltage clamp circuit  11 , a power supply terminal Vss connected to the power supply input terminal GND, an output terminal EXT connected to a gate of the switching transistor  4 , and an FB terminal connected to an output terminal of the current detecting circuit  13 B. The diode  6  and the switching transistor  4  are connected in series between the power supply input terminal Vin and the power supply input terminal GND. The current detecting circuit  13 B is connected between the power supply input terminal Vin and an output terminal VOUT. The LED  10  is connected between the output terminal VOUT and a terminal VRET. The coil  5  is connected between a connection point of the switching transistor  4  and the diode  6 , and the terminal VRET. The voltage clamp circuit  11  is connected between the power supply input terminal Vin and the power supply input terminal GND. 
     The switching regulator IC  2 B and the external elements thereof input a feedback voltage based on an output current of the output terminal VOUT to the FB terminal, and control the switching transistor  4  so that a current flowing through the LED  10  becomes constant. The voltage clamp circuit  11  clamps a voltage applied between the power supply terminals Vdd and Vss of the switching regulator IC  2 B. That is, only a clamp voltage is applied by a zener diode between the power supply terminals Vdd and Vss. The current detecting circuit  13 B has a function of inputting a voltage for controlling the output current to the FB terminal of the switching regulator IC  2 B operated by the clamp voltage of the zener diode. 
     The voltage clamp circuit  11  includes the zener diode and a resistor which are connected in series between the power supply input terminals Vin and GND of the LED drive circuit  1 . For example, it is assumed that a voltage of the power supply  9  is 100 V and an operating voltage of the switching regulator IC  2 B is 5 V. In a case of the voltage clamp circuit  11  of  FIG. 4 , a voltage of 0 V is input to the power supply terminal Vss of the switching regulator IC  2 B, while a voltage of 5 V which is clamped by the voltage clamp circuit  11  is input to the power supply terminal Vdd. That is, a voltage of 5 V is applied between the power supply terminals Vdd and Vss of the switching regulator IC  2 B. Accordingly, even when the power supply voltage is 100 V, the LED drive circuit  1  can be configured by using the switching regulator IC  2 B which has an operating voltage of 5 V. 
     Here, the current detecting circuit  13 B is provided between the power supply input terminal Vin and the output terminal VOUT for outputting a current to the LED  10 . The switching regulator IC  2 B operates with a voltage of the power supply input terminal GND as a reference, whereby the current detecting circuit  13 B detects the current flowing through the LED  10 , converts the detected current into a voltage with the voltage of the power supply input terminal GND as the reference, and outputs the converted voltage. 
       FIG. 5  illustrates an example of the current detecting circuit  13 B used for the LED drive circuit of  FIG. 4 . The current detecting circuit  13 B includes a current detecting resistor  21 , a PNP bipolar transistor  22 B including an emitter and a base which are connected to both ends of the current detecting resistor  21 , and a resistor  23  connected between a collector of the bipolar transistor  22 B and the power supply input terminal GND. 
     The current detecting circuit  13 B is configured in this manner, and hence a high power supply voltage can be applied between the collector and the emitter of the bipolar transistor  22 B. As a result, the current detecting circuit  13 B can convert a voltage to be output into a voltage within a range of the operating voltage of the switching regulator IC  2 B with 0 V as the reference, and output the converted voltage. 
       FIG. 6  illustrates a specific example of the LED drive circuit  1  using the step-up switching regulator IC  2 B according to this embodiment. The current detecting circuit  13 B of  FIG. 6  is an example in which the current detecting circuit  13 B of  FIG. 5  is illustrated more specifically. The current detecting circuit  13 B of  FIG. 6  includes a bias circuit  24  and a voltage setting circuit  25 . The bias circuit  24  includes a diode and a resistor, and applies a bias voltage to the base of the bipolar transistor  22 B. The voltage setting circuit  25  includes a diode and resistors. The diode includes a cathode terminal connected to the power supply terminal Vdd, and has a function of accurately setting a voltage range of the voltage output from the current detecting circuit  13 B. 
     The LED drive circuit  1  is configured in this manner, and thus, even when the voltage of the power supply  9  is 100 V, can set the voltage within a range of the operating voltage of the switching regulator IC  2 B by the voltage clamp circuit  11 . In addition, the LED drive circuit  1  can set the voltage output from the current detecting circuit  13 B within the range of the operating voltage of the switching regulator IC  2 B. Accordingly, the LED drive circuit  1  can use the switching regulator IC  2 B having low breakdown voltage, whereby the LED drive circuit  1  can be manufactured at low cost. 
     Note that the first embodiment describes the example in which the current detecting resistor  21  is provided between the power supply input terminal GND and the terminal VRET. However, the similar effects as those of the first embodiment can be attained with the configuration in which the current detecting resistor  21  is provided between the power supply input terminal Vin and the output terminal VOUT as in the second embodiment. In such a case, the current detecting circuit  13 A may have the configuration of the second embodiment. Further, the same holds true for the case where the current detecting resistor  21  is provided between the power supply input terminal GND and the terminal VRET in the second embodiment.