Abstract:
An LED drive circuit having reduced power consumption has a constant current generating circuit for driving a plurality of LEDs and at least one switch connected to a respective LED for periodically turning on and off the respective LED at a rate higher than a visual perception rate to reduce power consumption.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an LED drive circuit which causes a light emitting diode (LED) to blink periodically to reduce power consumed by the LED. 
   2. Description of the Related Art 
   A conventional LED drive circuit such as shown in the circuit diagram of  FIG. 15  is known. That is, a power supply of a voltage VDD [V] is connected to a power supply terminal  10 , and a constant current generation circuit  15  operates in such a manner that a voltage difference between an output voltage Vref [V] of a reference voltage circuit  11  and a voltage Va [V] across a resistor  13  is amplified by an error amplifier  12  to control a gate voltage Verr for a transistor  14  so that Vref−Va=0. 
   In this drive circuit, LEDs  19  and  20  are respectively connected to two output terminals  1  and  2 . 
   If the resistance value of the resistor  13  is R 13  [Ω], a current I=Va/R 13  [A] flows through the resistor R 13 . The same current as that flowing through the resistor R 13  also flows through transistors  14  and  16 . If all of transistors  16  to  18  are identical in characteristics, a current mirror circuit  21  causes the same current as that flowing through the transistor  16  to flow through each of the transistors  17  and  18 , thereby lighting the LEDs  19  and  20 . 
   That is, currents Iout 1  and Iout 2  flowing through the LEDs  19  and  20  are given by the following equation (1):
 
 I out 1 = I out 2 = Va/R   13 [ A]   (1)
 
   Therefore the currents caused to flow through the LEDs  19  and  20  can be set to a desired current value by adjusting the value of the resistor  13  or the output voltage value of the reference voltage circuit  11 . 
   If power consumed by the reference voltage circuit  11  and the error amplifier circuit  12  is negligibly small in comparison with power consumed by the LEDS, power Pd consumed by the LED drive circuit shown in  FIG. 15  is given by the following equation (2):
 
 Pd=VDD×Va/R   13 ×3 [A]   (2)
 
   To reduce power consumption in the conventional LED drive circuit, however, it is necessary to reduce the LED current. If the LED current is reduced, a problem of reduction in luminance of the LED arises. 
   SUMMARY OF THE INVENTION 
   In view of the problem of the conventional art, an object of the present invention is to provide an LED drive circuit designed to reduce power consumption while maintaining the same luminance of LEDs observed with the eye as that obtained by the conventional LED drive circuit. 
   To achieve the above-described object, the present invention provides an LED drive circuit arranged to light LEDs in a time-division manner different from a continuous-lighting manner to reduce power consumption in the LED drive circuit. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawings: 
       FIG. 1  is a diagram showing an LED drive circuit which represents Embodiment 1 of the present invention; 
       FIG. 2  is a diagram showing switch drive voltages in Embodiment 1 of the present invention; 
       FIG. 3  is a diagram showing another LED drive circuit in Embodiment 1 of the present invention; 
       FIG. 4  is a diagram showing an LED drive circuit which represents Embodiment 2 of the present invention; 
       FIGS. 5A and 5B  are diagrams showing an example of switch drive voltages in Embodiment 2 of the present invention; 
       FIGS. 6A and 6B  are diagrams showing another example of switch drive voltages in Embodiment 2 of the present invention; 
       FIG. 7  is a diagram showing an example of a switch control circuit in Embodiment 2 of the present invention; 
       FIGS. 8A and 8B  are diagrams showing another example of switch drive voltages in Embodiment 2 of the present invention; 
       FIGS. 9A and 9B  are diagrams showing an example of switch drive voltages in Embodiment 3 of the present invention; 
       FIG. 10  is a diagram showing an LED drive circuit which represents Embodiment 4 of the present invention; 
       FIG. 11  is a diagram showing an LED drive circuit which represents Embodiment 5 of the present invention; 
       FIG. 12  is a diagram showing another LED drive circuit in Embodiment 5 of the present invention; 
       FIG. 13  is a diagram showing another LED drive circuit in Embodiment 5 of the present invention; 
       FIG. 14  is a diagram showing an LED drive circuit which represents Embodiment 6 of the present invention; and 
       FIG. 15  is a diagram showing a conventional LED drive circuit. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   (Embodiment 1) 
   Embodiments of the present invention will be described with reference to the accompanying drawings.  FIG. 1  shows an LED drive circuit which represents Embodiment 1 of the present invention. A constant current generation circuit  15 , a current mirror circuit  21 , and LEDs  19  and  20  shown in  FIG. 1  are the same as those in the conventional arrangement. 
   Switches  4  and  5  are respectively inserted between transistors  17  and  18  in the current mirror circuit and terminals  1  and  2  to which the LEDs are connected. ON/OFF control of the switches  4  and  5  is performed by means of signal voltages V 1  and V 2  from a switch control circuit  3 . 
     FIG. 2  shows an example of signal voltages V 1  and V 2  from the switch control circuit  3 . The abscissa represents time and the ordinate comprises two components respectively representing voltages V 1  and V 2 . In the example shown in  FIG. 2 , voltages V 1  and V 2  change in a complementary relationship with each other. When V 1  is high level (hereinafter referred to as H), V 2  is low level (hereinafter referred to as L). If the switches  4  and  5  are turned on when both V 1  and V 2  are H, the LEDs  19  and  20  repeat blinking by being alternately lighted. 
   If power consumed by the reference voltage circuit  11  and the error amplifier circuit  12  and power consumed by the switch control circuit  3  during this operation are negligibly small in comparison with power consumed by the LEDs, power Pd consumed by the LED drive circuit shown in  FIG. 1  is given by the following equation (3):
 
 Pd=VDD×Va/R   13 ×(1+2×½)[ W]   (3)
 
   The total of time periods during which a current is fed through each LED is ½ of that in the conventional arrangement, so that power consumption in this embodiment can be limited to ⅔ of that in the conventional arrangement (power consumption in the LED section only is ½ of that in the conventional arrangement). 
   For example, in a case where LEDS are used as a backlight for a liquid crystal panel, the LEDs can be used by being lighted in the same time-division manner as in this embodiment instead of being continuously lighted in the conventional manner, thereby reducing power consumption while ensuring substantially the same display performance as that based on the conventional art thanks to persistence of vision. 
   While the LEDs  19  and  20  are alternately lighted in a blinking manner in the method shown in  FIG. 2 , a period during which both the LEDs  19  and  20  are lighted or a period during which neither of the LEDs  19  and  20  is lighted may be set. If only a period during which the LED  19  or  20  is not lighted is set, power consumption can be reduced by a corresponding amount from that in the case of conventional continuous lighting. 
   In a case where LEDs are lighted as a backlight for a liquid crystal panel, it is necessary to light the LEDs for time-division lighting in such a cycle that visual perceptibility of flicker is sufficiently low. That is, it is necessary that the frequency at which each LED is turning in time-division lighting be set to 5 Hz or higher. 
   While in the circuit shown in  FIG. 1 , the switches  4  and  5  are inserted in the output lines from the transistors  17  and  18 , the same effect can also be achieved in such a manner that, as shown in  FIG. 3 , the voltages applied to the gates of the transistors  17  and  18  are changed by switch circuits  40  and  50  on the basis of the signals from the switch control circuit  3 . That is, when the signal V 1  from the switch circuit  3  is H, the gate of the transistor  17  is connected to the gate of the transistor  16  to cause a current to flow through the LED  19 . When the signal V 1  is L, the gate of the transistor  17  is connected to VDD to shut off the current to the LED  19 . Also, when the signal V 2  from the switch circuit  3  is H, the gate of the transistor  18  is connected to the gate of the transistor  16  to cause a current to flow through the LED  20 . When the signal V 2  is L, the gate of the transistor  18  is connected to VDD to shut off the current to the LED  20 . 
   A white-light LED may be used as a backlight for a liquid crystal panel. It is necessary to cause a current of 5 to 30 mA to flow through the LED, the current being selected by considering the light emitting efficiency of the LED. If the LEDs are lighted in time-division manner, it is possible to instantaneously feed a current larger than the rated current used in ordinary continuous energization. Thus, the effect of increasing the luminance can also be achieved. 
   (Embodiment 2) 
     FIG. 4  shows an LED drive circuit which represents Embodiment 2 of the present invention. The same constant current generation circuit  15 , current mirror circuit  21 , and LEDs  19  and  20  as those in the conventional arrangement are used. Switches  4  and  5  are respectively inserted between transistors  17  and  18  in the current mirror circuit and terminals  1  and  2  to which the LEDs are connected. ON/OFF control of the switches  4  and  5  is performed by means of signal voltages V 1  and V 2  from a switch control circuit  6 . A control terminal  7  to which a signal is externally supplied is connected to the switch control circuit  6 . The cycle in which V 1  and V 2  change or the lighting time is controlled on the basis of signal V 7  supplied through the control terminal  7 . 
     FIGS. 5A and 5B  show an example of a change in cycle.  FIG. 5A  shows a case where the voltage V 7  on the control terminal  7  is low, and  FIG. 5B  shows a case where the voltage V 7  on the control terminal  7  is high. The frequency of an internal oscillation circuit of the switch control circuit  6  is changed through the voltage V 7  on the control terminal  7 . When the voltage V 7  on the control terminal  7  is reduced, the frequency of the internal oscillation circuit of the switch control circuit  6  is lowered and the LED blinking cycle is increased. Conversely, when the voltage V 7  on the control terminal  7  is increased, the LED blinking cycle is reduced. 
   In Embodiment 2, the cycle of blinking of the LEDs can be adjusted according to the size and a characteristic of a liquid crystal panel. 
     FIGS. 6A and 6B  show an example of control of the LED on/off time on the basis of the signal supplied to the control terminal  7  in the arrangement shown in  FIG. 4 .  FIG. 6A  shows a case where the voltage V 7  on the control terminal  7  is low, and  FIG. 6B  shows a case where the voltage V 7  on the control terminal  7  is high. The time of a monostable multivibrator in the switch control circuit  6  is controlled in such a manner that when the voltage V 7  on the control terminal  7  is low, the ratio of the on times for the LEDs  19  and  20  is an even ratio, 50:50, and, when the voltage V 7  on the control terminal  7  is high, the LED  19  on time is reduced while the LED  20  on time is increased. 
   While the LEDS  19  and  20  are lighted in a complementary relationship with each other in the method shown in  FIGS. 6A and 6B , a period during which both the LEDs  19  and  20  are lighted or a period during which neither of the LEDS  19  and  20  is lighted may be set. 
     FIG. 7  shows an example of the control circuit  6  shown in  FIG. 4  in a case where the LEDs  19  and  20  are caused to blink in a certain cycle. An oscillation circuit  51  oscillates in a certain cycle. An output OSC 1  of the oscillation circuit is connected to a second monostable multivibrator  54  through a first monostable multivibrator  53  and an inverter  52 . The monostable multivibrator  53  is triggered by a rise of the voltage of the OSC 1  to output as voltage V 1  a pulse with a duration determined by the voltage on the control terminal  7 , while the monostable multivibrator  54  is triggered by a rise of the voltage of the inverter  52  to output as voltage V 2  a pulse with a duration determined by the voltage on the control terminal  7 . 
     FIGS. 8A and 8B  show an example of changes in outputs V 1  and V 2  from the monostable multivibrators  53  and  54  caused through selection of the voltage on the control terminal  7 . 
     FIG. 8A  shows voltages V 1  and V 2  when the voltage V 7  on the control terminal  7  is low, and  FIG. 8B  shows voltages V 1  and V 2  when the voltage V 7  on the control terminal  7  is high.  FIGS. 8A and 8B  show a case where the width of the pulse generated by each monostable multivibrator is small when the voltage V 7  on the control terminal  7  is low, and is long when the voltage V 7  on the control terminal  7  is high. 
   In Embodiment 2, the on/off time ratio and cycle of blinking of the LEDs can be adjusted according to the size of a liquid crystal panel, the temperature, and a characteristic such as display speed of the liquid crystal panel. 
   (Embodiment 3) 
     FIGS. 9A and 9B  show Embodiment 3 of the present invention in which an LED is selected as an object of blinking control through a signal supplied to the control terminal  7  in the circuit shown in  FIG. 3 . 
     FIG. 9A  shows voltages V 1  and V 2  when the voltage V 7  on the control terminal  7  is low, and  FIG. 9B  shows voltages V 1  and V 2  when the voltage V 7  on the control terminal  7  is high. When the voltage V 7  on the control terminal  7  is low, the LED  19  is continuously lighted by maintaining V 1  at H and control of blinking of the LED  20  is performed. On the other hand, when the voltage V 7  on the control terminal  7  is high, the LED  20  is continuously lighted by maintaining V 2  at H and control of blinking of the LED  19  is performed. 
   In Embodiment 3, one of a plurality of LEDs is continuously lighted while at least one of the other LEDs is controlled so as to blink, thus enabling LED drive for a backlight under a requirement of low power consumption according to use of a liquid crystal panel. 
   (Embodiment 4) 
     FIG. 10  shows an LED drive circuit which represents Embodiment 4 of the present invention. The circuit shown in  FIG. 10  differs from that shown in  FIG. 1  in that a variable resistor  30  is used in place of the resistor  13  in the constant current generation circuit  31 . The variable resistor  30  changes according to a signal voltage from an external terminal  31 . It is apparent from the equation (1) that each of the currents flowing through the LEDs  19  and  20  can be changed by changing the value of the variable resistor  30 . 
   While in the arrangement shown in  FIG. 10  the value of the variable resistor  30  is changed by an external signal, it is apparent from the equation (1) that each of the currents flowing through the LEDs  19  and  20  can also be changed by changing the value of output voltage Vref [V] of the reference voltage circuit  11 . 
   The circuit shown in  FIG. 10  may be modified in such a manner that the value of the variable resistor  30  is controlled not through a signal from the external terminal  31  but through an output from a temperature sensor which is provided in an integration manner in the LED drive circuit, thereby enabling the current caused to flow through each LED to be adjusted according to a characteristic of a liquid crystal which varies with temperature. 
   While the embodiments in which the number of LEDs to be controlled is two have been described, it is apparent that the same or more complicated LED drive method may be used to control three LEDs or more. Also, the switches  4  and  5  may be replaced with transistors which can easily used as a switch. 
   (Embodiment 5) 
     FIG. 11  shows an LED drive circuit which represents Embodiment 5 of the present invention. The same constant current generation circuit  15  as that in the conventional arrangement is used. The reference voltage circuit  11  in the constant current generation circuit  15  is supplied with power through the power supply terminal  10  connected thereto. A boosting circuit  101  boosts the voltage Vdd [V] applied to the power supply terminal  10  to a higher voltage VDDU [V] obtained through a terminal  100 . The boosting circuit  101  may be realize as any type of circuit, e.g., a charge pump type using a capacitance or a switching regulator type using a coil if it can perform a boosting function. An output of a comparator  60  is connected to the boosting circuit  101 . ON/OFF control of the operation of the boosting circuit  101  is performed on the basis of the output voltage of the comparator  60 . The plus terminal input voltage Vref [V] of the error amplifier circuit  12  in the constant current generation circuit  15  is applied to the plus terminal of the comparator  60 , while the minus terminal input voltage Va [V] of the error amplifier circuit  12  is applied to the minus terminal of the comparator  60 . 
   Referring to  FIG. 11 , the boosting circuit  101  performs boosting when the output voltage of the comparator  60  is high, i.e., when Vref [V]&gt;Va [V], and stops boosting when the output voltage of the comparator  60  is low, i.e., when Vref [V]&lt;Va [V]. This control enables the LEDs to be driven at the optimum boosted voltage VDDU [V] at which the current flowing through the resistor  13  is I=Vref/R 13  [A]. 
   A transistor  61  in a source follower circuit is driven by a constant current source  63  to generate at its source a voltage which is lower approximately by the threshold voltage than the voltage on the terminal  1  to which the LED  19  is connected. A transistor  62  also in a source follower circuit generates at its source, i.e., the gate and drain of the transistor  16 , a voltage which is higher approximately by the threshold voltage than the source voltage of the transistor  61 . If the absolute values of the threshold voltage of the transistors  61  and  62  are equal to each other, a voltage approximately equal to the voltage on the terminal  1  is generated at the gate and drain of the transistor  16  and, therefore, the current mirror circuit formed by the transistors  16  and  17  can operate accurately. 
   For example, a lithium-ion secondary battery may be used to obtain the power supply voltage VDD [V] at the terminal  10 . Its voltage is about 3.6 V. On the other hand, the forward ON voltage of a white LED is about 4.0 V at the maximum. It is necessary to boost the voltage of the lithium-ion secondary battery to the voltage at which the white LED can be lighted. 
   Generally speaking, if a constant current circuit is added after a stage for boosting by a boosting circuit, control is performed so that the voltage boosted by the boosting circuit has a certain constant value, e.g., 5 V. Therefore an excessively high voltage is applied between the drain and the source of the transistor  17  to cause loss or heat generation. If the boosted voltage is controlled so as to constantly maintain the LED current as in Embodiment 5, the drain-source voltage of the transistor  17  can be limited to a lower value to improve the characteristics in terms of loss and heat generation. 
   The arrangement shown in  FIG. 12  differs from that shown in  FIG. 11  in that an offsetting power supply  64  is inserted in the line to the minus input terminal of the comparator  60 . In the circuit shown in  FIG. 11 , there is a possibility of failure to normally perform the operation, depending on the offset voltage of the comparator  60 . The offsetting power supply  64  is inserted as shown in  FIG. 12  to stabilize the operation. If the voltage value of the offsetting power supply is Vof 1  [V], ON/OFF control of the boosting circuit  101  is such that when Vref &gt;VA +Vof 1 , the output of the comparator  60  is increased and the circuit  101  performs boosting and, when Vref&lt;VA+Vof 1 , the output of the comparator  60  is reduced and the circuit  101  stops boosting. The current flowing through the resistor  13  is thereby controlled so that I=(Vref−Vof 1 )/R 13  [A]. 
   In this case, Vof 1  [V] is set to a value higher than the offset voltage of the comparator  60 . 
     FIG. 13  shows another arrangement which differs from that shown in  FIG. 11  in that a comparator  70  which performs ON/OFF control of the boosting circuit  101  is supplied at its plus terminal with the output voltage Verr [V] from the error amplifier  12  and at its minus terminal with a voltage obtained by subtracting a voltage Vof 2  [V] of an offsetting power supply  71  from the boosted voltage VDDU [V]. In this case, ON/OFF control of the boosting circuit  101  is such that when Verr&gt;VDDU−Vof 2 , the output of the comparator  70  is increased and the circuit  101  performs boosting and, when Verr &lt;VDDU−Vof 2 , the output of the comparator  70  is reduced and the circuit  101  stops boosting. When the current I flowing through the resistor R 13  is smaller than Vref/R 13 , the output Verr of the error amplifier  12  is increased. Conversely, when the current I flowing through the resistor R 13  is larger than Vref/R 13 , the output Verr of the error amplifier  12  is reduced. Accordingly, when the current I flowing through the resistor R 13  is smaller than Vref/R 13 , the output Verr of the error amplifier  12  is increased to the same level as VDDU. In this time, the output of the comparator  70  is high and the boosting circuit  101  performs boosting. Thereafter, when the value of the voltage VDDU is increased to the level high enough to enable the constant current circuit  15  to cause a current to flow, the output voltage Verr of the error amplifier  12  decreases gradually. When Verr&lt;VDDU−Vof 2 , the output of the comparator  70  is reduced to stop the boosting operation of the boosting circuit  101 . This control enable prevention of an excessive increase in boosted voltage VDDU, thereby improving the characteristics in terms of loss and heat generation, as described above. 
   The comparator  60  shown in  FIG. 11  or  12  and the comparator  70  shown in  FIG. 13  may be arranged to have a certain amount of hysteresis to improve the stability of the circuit. 
   (Embodiment 6) 
     FIG. 14  shows Embodiment 6 of the present invention. The circuit shown in  FIG. 14  is formed, compared to that shown in  FIG. 12 , by adding a switching control circuit  3 , switches  4  and  5 , and an LED  20 . All of these components are equivalent to those shown in  FIG. 1 . Switches  74  and  75  are further added. The switches  4  and  74  operate in synchronization with each other and the switches  5  and  75  also operate in synchronization with each other. When the switch  4  is closed, the switch  74  is also closed. When the switch  4  is opened, the switch  74  is also opened. The switches  5  and  75  are also in the same relationship. 
   Since blinking-of the LEDs  19  and  20  is controlled by the switch control circuit  3 , ON/OFF control of the boosting circuit  101  is performed by using the anode voltage of the lighted LED. 
   However, the switches  74  and  75  are controlled on such a logic that one of them is operated with priority over the other and they are thereby prevented from being turned on simultaneously with each other when both the LEDs  19  and  20  are ON. 
   The arrangement may be such that, to eliminate occurrence of instability of operation when both the LEDs  19  and  20  are OFF, an OR output is obtained from the outputs V 1  and V 2  from the switch control circuit  3  and the boosting operation of the boosting circuit  101  is stopped when this output is low (L). 
   Further, lighting of the LEDs  19  and  20  may be controlled so that they are lighted in a complementary relationship with each other to optimize the LED drive circuit including the boosting circuit, because the boosting ability of the boosting circuit  101  may be reduced by half in comparison with that required in the case of continuous lighting. 
   It is not always necessary to light the LEDs  19  and  20  in a complementary relationship. Various drive methods, including those in Embodiments 1 to 4, are conceivable and any number of LEDs equal to or greater than 2 may be used. 
   The LED drive circuit of the present invention has the advantage of reducing power consumption during drive of LEDs by lighting the LEDs in a way most suitable for characteristics of a liquid crystal.