JP2005116738A - Led driving circuit and power saving method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To achieve power saving in a driving circuit for controlling the luminescence of an LED. <P>SOLUTION: A monitor circuit is provided for controlling a voltage of a variable voltage power supply employing the power supply of an LED driving circuit as the variable voltage power supply, and a gate voltage impressed on an FET as an input. In the monitor circuit, the minimum value of a gate voltage coping with the part of voltage lowering caused by the FET is obtained previously from the total voltage value of the maximum forward direction voltage value permitted in the standard of a plurality of LEDs, is stored while corresponding to a controlling current value and the voltage value of the gate voltage, and monitored in the monitor circuit to be compared with the minimum value of the gate voltage in accordance with a control current value. When the voltage value of the gate voltage is smaller than the minimum value of the gate voltage, the value of a power supply voltage set in the variable voltage power supply is controlled to be reduced. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to an LED driving circuit that controls light emission of an LED (light emitting diode) and a power saving method that reduces wasteful power consumed by the LED driving circuit.

  In particular, the present invention is suitable for driving an LED as a light source applied to a backlight of a liquid crystal display device.

  The brightness or luminous flux of an LED changes depending on the current, but the forward voltage drop varies depending on the individual LED even if the same current flows due to manufacturing variations. Therefore, even if the same voltage is applied to a plurality of LEDs, the current varies, and the luminance does not become the same.

  Therefore, as an LED drive circuit, generally, a current control circuit is placed in series with the LED to control the current, and the luminance is controlled so that the current flowing through each LED is the same.

  FIG. 2 shows a typical circuit example of a conventional LED driving circuit. However, the LED drive circuit is not limited to the configuration of FIG.

  Hereinafter, a conventional LED driving circuit will be described with reference to FIG.

  In FIG. 2, a current control FET (field effect transistor) 5 and a sense resistor 6 that senses current are connected in series to a lighting section in which three LEDs 7 are connected in series. The gate voltage of the FET 5 is controlled by the operational amplifier 4 with reference to the sense voltage of the sense resistor 6.

  Here, FIG. 2 illustrates a case where the lighting section in which three LEDs 7 are connected in series is three blocks, but the present invention is not limited to this, and the number of blocks can be any number of one block or more. Also, the number of LEDs in series may be any number.

  The main symbols in FIG. 2 will be described. Block numbers 1 to 3 are assigned to each block of the lighting unit, the block number value is #, LED 7 is indicated as LED # 1 to # 3, and FET (Field effect transistor) 5 is indicated as FET #, sense resistor 6 is indicated as Rsense #, and operational amplifier 4 is indicated as OP #. Here, lighting of each block 1 to 3 is designated by a separate signal of Block #.

  In FIG. 2, VRref and OPref at the left end are volumes for designating the brightness of the LED, and OP # to FET so that the voltage generated at the current sense resistor Rsense # of each block is equal to the output voltage of OPref. Control the output voltage to #. In the LED controller 2, when the signal of the Block # is Hi (for example, 5V), the OPref voltage is passed to the OP # for lighting, and when it is Lo (for example, 0V), the OP # is supplied with the 0V (GND) voltage. I will turn it off.

  Reference numeral 3 denotes a constant voltage power source that is a power source for the LED, and includes a DC converter that generates a DC voltage for lighting the LED from an AC (alternating current) power source or some DC (direct current) power source.

  In general, the DC converter has a function of sensing and monitoring the output voltage in order to keep the output voltage constant. If the sensed voltage is changed, the output voltage can be changed within a certain range. it can.

A technique for driving a plurality of LEDs connected in series with a constant current transistor circuit is also disclosed in Patent Document 1, for example.
JP-A-5-191568

  By the way, the constant voltage power supply 3 which is a power supply for LED must be a voltage higher than the worst value of the forward voltage drop due to the variation in the LED characteristics. The remaining voltage obtained by subtracting the actual forward voltage drop from the LED 7 from the voltage of the constant voltage power supply 3 is borne by the FET 5 and the sense resistor 6 of the current control circuit, but the voltage borne by the FET 5 × current The electric power is wasteful electric power that only becomes heat.

  Further, in the conventional LED driving circuit, a constant voltage power source is used as the LED power source, and the LED power source voltage is determined in accordance with the worst case of the LED having a variation in characteristics. Therefore, it was forced to generate wasteful power.

  The present invention provides an LED drive circuit capable of reducing such wasteful power and a power saving method thereof.

  The present invention monitors the control signal used in the current control circuit, so that the LED forward voltage drop is small, that is, the LED performance is good and the burden voltage of the FET becomes large. By reducing the voltage of the power supply itself, wasteful power consumed by the FET is reduced and power saving is realized.

The present invention has solved the above problems by the LED driving circuit and the power saving method described in the following sections.
(1) A variable voltage power supply, a plurality of LEDs connected in series to the variable voltage power supply, a FET further connected in series to the plurality of LEDs, and a current value sensed between the FET and ground. A sense resistor, a current control operational amplifier for applying a gate voltage to the FET in accordance with a current to be controlled based on a voltage value applied to the sense resistor, and the variable voltage with the gate voltage applied to the FET as an input A monitor circuit for controlling the voltage of the power supply, and the monitor circuit corresponds to the voltage drop that the FET will bear in advance corresponding to the current value for driving the plurality of LEDs. And a minimum value of the gate voltage calculated including the maximum variation allowed by the standards of the plurality of LEDs is stored. A power supply voltage value to be set for the variable voltage power supply when the gate voltage is smaller than the minimum value of the gate voltage. The LED drive circuit is characterized in that control is performed in such a way that power consumption is reduced and power consumption is minimized.
(2) In the LED drive circuit according to (1), the plurality of LEDs are further arranged in parallel with a variable voltage power supply, and each of the plurality of LED blocks further includes an FET and a sense resistor. In the monitor circuit, when controlling the voltage of the variable voltage power supply with the gate voltage applied to the FET of each block as an input in the monitor circuit, among the voltage values of the gate voltage applied to the FET of each block, An LED driving circuit that controls a voltage value of the variable voltage power source with a highest voltage value as a reference.
(3) A FET and a sense resistor are connected in series to a plurality of LEDs connected in series to a power supply, and a current control operational amplifier that applies a gate voltage to the FET. The voltage value applied to the sense resistor is used as a reference. A power saving method for an LED driving circuit that drives a plurality of LEDs by controlling a gate voltage according to a current to be controlled, wherein the power source is a variable voltage power source and a gate voltage applied to the FET is input And a monitor circuit for controlling the voltage of the variable voltage power supply, and the monitor circuit corresponds in advance to a voltage drop that the FET bears corresponding to the current value for driving the plurality of LEDs. Storing the minimum value of the gate voltage calculated by including the maximum variation allowed in the standards of the plurality of LEDs, and in the monitor circuit, The gate voltage is monitored, compared with the minimum value of the gate voltage according to the current value to be controlled, and when the gate voltage is smaller than the minimum value of the gate voltage, the power supply voltage value set for the variable voltage power supply The method for reducing the power consumption of the LED driving circuit is characterized in that the power consumption is controlled to a minimum and the power consumption is minimized.
(4) The power saving method for the LED driving circuit according to (3) above, further including arranging the plurality of LEDs in parallel with a power supply in a plurality of blocks, wherein each of the plurality of LEDs has a current control circuit. When the FET and the sense resistor are further connected in series, in the monitor circuit, when the gate voltage applied to the FET of each block is input and the voltage of the variable voltage power source is controlled, the voltage is applied to the FET of each block. A power saving method for an LED drive circuit, wherein the voltage value of the variable voltage power supply is controlled with reference to the highest voltage value among the voltage values of the gate voltage.

  According to the present invention, it is possible to reduce the power in the FET that has been wasted in the past, and to realize the power saving of the LED driving circuit.

  The best mode for carrying out the LED driving circuit and the power saving method of the present invention will be described with reference to FIG. Here, the same numbers and symbols are assigned to the same members constituting the conventional LED driving circuit already described in FIG. 2, and the description thereof is omitted.

  In the present invention, first, the conventional constant voltage power supply 3 (see FIG. 2) is replaced with a variable voltage power supply 13 to enable control of the applied voltage. In each block, the gate voltage (TP #) applied to the FET 5 is input to the monitor circuit 11 and monitored, and the voltage of the variable voltage power supply 13 is controlled based on the monitored value.

  That is, the monitor circuit 11 applied to the present invention has a function of monitoring the gate voltage (TP #) as an input and controlling the applied voltage of the variable voltage power supply 13.

  Here, the high gate voltage (TP #) means that the resistance value of the FET # is lowered to reduce the voltage burden on the transistor. That is, it means that FET # is trying to cope with a large forward voltage drop of LED #. On the other hand, the low gate voltage (TP #) means that the forward voltage drop of the LED # is small, so that the resistance value of the FET # is increased to increase the voltage burden of the transistor.

  Details of the monitor circuit 11 will be described below.

  In the monitor circuit 11, the voltage drop that the FET bears is calculated in advance from the total voltage value of the maximum forward voltage value allowed by the standard of a plurality of LEDs (three in the example of FIG. 1). The corresponding minimum value of the gate voltage is stored corresponding to the current value to be controlled. This value may be stored in some storage means. If the monitor circuit is configured with a simple microcomputer or sequencer, it may be stored in the memory provided by the monitor circuit, or if it is configured in hardware. For example, it can be made with a ladder resistor appropriately from a power supply for an operational amplifier. In this case, if the operational amplifier power supply is set to 12 V, for example, the operational amplifier output is about several V to 11 V, so the accuracy of the comparator does not matter much.

  The monitor circuit 11 compares the monitored gate voltage with the minimum value of the gate voltage stored in the storage means corresponding to the current value to be controlled, and the voltage value of the gate voltage is the minimum gate voltage. When the value is smaller than the value, the power supply voltage value set to the variable voltage power supply is controlled to be lowered so that the consumed power is minimized. When driving a plurality of blocks of LEDs, the gate voltage (TP #) is used to control the voltage of the variable voltage power supply 13 with the gate voltage (TP #) applied to the FET5 (FET #) of each block as an input. Among them, the highest voltage value is selected as a reference, and the voltage value of the variable voltage power supply 13 is controlled.

  In the reverse case, that is, the gate voltage is greater than the maximum value of the gate voltage corresponding to the voltage drop borne by the FET, which is obtained from the total voltage value of the minimum forward voltage value allowed by the standard. When the voltage is high, it is necessary to perform control to maintain the LED drive by controlling the power supply voltage value set in the variable voltage power supply to be increased. However, although this control is necessary as a function of the monitor circuit, it is the control opposite to the power saving intended here, and is not an essential requirement, and therefore the description thereof is omitted here.

  When there are a plurality of blocks, a predetermined specified voltage (Vmid) lower than the maximum value (Vmax) of the upper limit gate voltage is determined as the voltage value to be compared with the gate voltage, and there is no voltage value higher than Vmid. If there is a gate voltage lower than the lower limit gate voltage (Vmin), the LED power supply voltage may be controlled to be lowered. And by controlling in this way, all the voltages are kept between Vmin and Vmax. Here, when the gate voltage is equal to or higher than Vmax, the LED current is insufficient, and when it is equal to or lower than Vmin, the LED current is excessive.

  It should be noted that it is necessary to select an FET that is a current control transistor so that a situation in which more than the upper limit and less than the lower limit occur simultaneously as a result of sensing a large number of blocks.

  By the way, the voltage change of the LED power source in the present invention aims to cope with the variation in the forward voltage of the LED. Here, looking at one LED, the voltage change range is about −0.4 V to +0.6 V from the standard value of the LED, and it is considered sufficient if it can be moved in units of about 0.2 V. In the example shown in FIG. 1, three LEDs are connected in series. In this case, about three times as much may be considered. When power saving is considered, adjustment on the negative voltage side may be unnecessary. Further, since the processing is simple, there is no need to control using a microcomputer, and a simple sequencer may be used. The simplest algorithm is to check only if there is something that exceeds Vmax at startup, and if it exceeds even one block, simply increase the voltage one step and check again. That's fine. In this case, the default voltage of the variable voltage power source 13 is set on the low voltage side of the LED variation, and it is only necessary to adjust only the direction of increasing the voltage, so the Vmin side check is unnecessary.

  In the LED drive circuit of the present invention, it is preferable to use the LED as backlight illumination for a liquid crystal display device, and the luminance is high, and the current flowing through the LED is targeted for about several tens mA to 100 mA. Further, a DC / DC converter or an AC / DC converter is assumed as an LED power source to be applied, that is, a variable voltage power source, and the power conversion efficiency of the converter hardly changes even when the power source voltage is lowered. That is, the power consumption in the circuit that is increased for power saving is sufficiently smaller than the power that can be reduced by lowering the LED power source. Therefore, the power consumption of the entire power saving system can be reduced by applying the present invention.

  The power saving method of the present invention will be described with an LED driving circuit applied to a backlight light source of a liquid crystal display device as an example.

  As shown in FIG. 1, the LED driving circuit according to the present invention includes three LEDs connected in series and three blocks. In the conventional example, the LED driving circuit described in FIG. 2 is applied.

  Here, the standard value of the forward voltage drop for each color of RGB (red, green, blue) has characteristics shown in Table 1, for example.

  In this embodiment, the red LED in Table 1 is taken as an example, and the case of driving the red LED will be described.

  When three red LEDs are connected in series, from Table 1, the standard value is 6.9 V at a current of 80 mA, and the maximum is 8.4 V in consideration of the manufacturing variation (maximum +1.5 V for three).

  Here, assuming that the light is dimmed up to a maximum of 100 mA, 1 V is generated with a sense resistance of 10Ω, so this value and a slight voltage generated because the transistor resistance value of the FET does not become zero can be reduced to 3 What is added to the forward voltage drop of the LED corresponds to the LED power supply voltage.

  Here, in the present invention, 80 mA is generated in series with three red LEDs, and 0.8 V is generated with a sense resistance of 10 Ω. Therefore, 6.9 + 0.8 + (slight voltage) ≈8 V is a standard voltage of the LED power supply as a standard. From the above, by applying the procedure of the present invention, a necessary change is performed in the range from a little less than 8V to 9.5V. Here, in the case of the present invention, depending on the performance of the LED, a voltage of less than 8V may be used. On the other hand, in the conventional example, it is necessary to provide a constant voltage power supply of 9.5 V as an LED power supply unconditionally.

  From the above, in the example of the present invention, it is possible to realize a power reduction of about 17% (over 1.5V ÷ 9.5V) at maximum as compared with the conventional example.

  In addition, when the current is reduced by reducing the brightness, the standard voltage also decreases. However, in the present invention, the LED power supply voltage can be decreased by automatically following the decrease in the standard voltage.

  For example, assuming that dimming is performed at 10 mA, from Table 1, the red LED is connected in series to 5.4V, and the standard voltage is 0.1V with the sense resistor 10Ω in the same manner as described above, so 5.4V + 0.1V + ( Some voltage) ≈ 6V. Even in this case, a constant voltage of 9.5 V is applied in the conventional example, but in the example of the present invention, the voltage of the variable voltage power source can be adjusted to be less than 6 V depending on the performance of the LED. Compared to the example, a power reduction of about 37% ((less than 9.5-6) V ÷ 9.5V) can be realized.

  That is, the power saving effect in the present invention is clear.

It is a schematic diagram which shows the structure of the LED drive circuit of this invention. It is a schematic diagram which shows the structure of the conventional LED drive circuit.

Explanation of symbols

2 LED controller 3 Constant voltage power supply 4 Operational amplifier 5 FET (field effect transistor)
6 Sense resistor 7 LED
11 Monitor circuit
13 Variable voltage power supply

Claims (4)

Variable voltage power supply,
A plurality of LEDs connected in series to the variable voltage power supply;
FET further connected in series to the plurality of LEDs;
A sense resistor which is arranged between the FET and ground and senses a current value;
Based on the voltage value applied to the sense resistor, an operational amplifier for current control that applies a gate voltage to the FET according to a current to be controlled;
A monitor circuit for controlling the voltage of the variable voltage power supply with a gate voltage applied to the FET as an input, and
The monitor circuit has in advance a gate voltage corresponding to a voltage drop that the FET bears corresponding to a current value for driving the plurality of LEDs, and is allowed in accordance with the specifications of the plurality of LEDs. The minimum value of the gate voltage calculated including the maximum variation is stored.
In the monitor circuit, the gate voltage is monitored and compared with a minimum value of the gate voltage according to a current value to be controlled. When the gate voltage is smaller than the minimum value of the gate voltage, the variable voltage power supply An LED driving circuit, wherein a power supply voltage value to be set is controlled to be lowered so that power consumption is minimized. The LED driving circuit according to claim 1,
Furthermore, a plurality of LEDs are arranged in parallel with the variable voltage power supply, and the FET and the sense resistor are further connected in series in each of the plurality of LED blocks,
In the monitor circuit, when controlling the voltage of the variable voltage power supply with the gate voltage applied to the FET of each block as an input, the highest voltage value among the voltage values of the gate voltage applied to the FET of each block is used as a reference. An LED driving circuit for controlling a voltage value of the variable voltage power source. An operational amplifier for current control that connects a FET and a sense resistor in series to a plurality of LEDs connected in series to a power supply, and applies a gate voltage to the FET, and is controlled based on the voltage value applied to the sense resistor. A power saving method of an LED drive circuit for driving the plurality of LEDs by controlling a gate voltage according to a current to be performed,
The power source is a variable voltage power source,
Comprising a monitor circuit for controlling the voltage of the variable voltage power supply with a gate voltage applied to the FET as an input;
The monitor circuit has in advance a gate voltage corresponding to a voltage drop that the FET bears corresponding to a current value for driving the plurality of LEDs, and is allowed in accordance with the specifications of the plurality of LEDs. Store the minimum value of the gate voltage calculated including the maximum variation.
In the monitor circuit, the gate voltage is monitored and compared with a minimum value of the gate voltage according to a current value to be controlled. When the gate voltage is smaller than the minimum value of the gate voltage, the variable voltage power supply A power-saving method for an LED drive circuit, characterized in that the power supply voltage value to be set is controlled to decrease to minimize power consumption. It is a power saving method of the LED drive circuit according to claim 3,
Furthermore, in the case where a plurality of LEDs are arranged in parallel with the power supply, and the FETs and sense resistors of the current control circuit are further connected in series in each of the plurality of LED blocks,
In the monitor circuit, when controlling the voltage of the variable voltage power supply with the gate voltage applied to the FET of each block as an input, the highest voltage value among the voltage values of the gate voltage applied to the FET of each block is used as a reference. A method for saving power in an LED drive circuit, wherein the voltage value of the variable voltage power supply is controlled.

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