JP2012003861A - Constant current circuit - Google Patents

Abstract

PROBLEM TO BE SOLVED: To perform individual constant current driving of a plurality of series LED circuits where the same number of LEDs are connected in series in a common constant current circuit.SOLUTION: The constant current circuit comprises a first switch SW1 which sequentially selects two series LED circuits LED1 and LED2 where the same number of LEDs are connected in series, a second switch SW2 which controls the time of applying a current to the LED series circuit while any one of the series LED circuits is selected by the first switch, and an operational amplifier 12 which performs constant current driving of the series LED circuit while any one of the series LED circuits is selected by the first switch.

Description

  The present invention relates to a constant current circuit for supplying a constant current to an LED light source formed by connecting a plurality of LED series circuits in which the same number of LEDs are connected in series.

  As an illumination light source or a backlight light source of a liquid crystal display, one in which a plurality of LED series circuits in which the same number of LEDs are connected in series is connected in parallel to form one light source is known. Since the light emission luminance of the LED is determined by the drive current, in such an LED light source, a constant current circuit for supplying a constant current is provided in each LED series circuit so that the luminance of the light source is kept constant. However, there is a problem that the number of circuit elements increases because constant current circuits are required as many as the number of LED series circuits.

  Therefore, for example, Patent Document 1 proposes a common constant current circuit in which a constant current is simultaneously applied to a plurality of LED series circuits connected in parallel to each other.

JP 2005-122979 A

  However, in the common constant current circuit as disclosed in Patent Document 1, the current values flowing through a plurality of LED series circuits connected in parallel are averaged, and based on the averaged current values, the current values are averaged. Since the constant current drive is performed by one constant current circuit, it is impossible to deal with an application in which each LED series circuit is adjusted to have a different luminance.

  In particular, when the variation in the forward voltage of the LED is large, the conventional method is more than the method of Patent Document 1 in which the same constant current drive is performed for all the LED series circuits by averaging so as to absorb the variation. As described above, it goes without saying that the LED drive current is individually set for each LED series circuit and the constant current drive is performed to obtain more uniform luminance as a light source.

  SUMMARY OF THE INVENTION An object of the present invention is to provide a common constant current circuit capable of individually driving a plurality of LED series circuits in which the same number of LEDs are connected in series with each other.

The constant current circuit according to claim 1 includes: a first switching unit that sequentially selects a plurality of LED series circuits in which the same number of LEDs are connected in series; and any one LED series circuit by the first switching unit. A second switching means for controlling a time during which a current is applied to the LED series circuit during the selected period; and the LED during a period when any one LED series circuit is selected by the first switching means. Constant current driving means for driving the series circuit with constant current.
The constant current circuit according to claim 2 is the constant current circuit according to claim 1, wherein any one of the LED series circuits is selected by the first switching means. According to the desired light emission luminance, it is further characterized by further comprising control means for controlling the time for applying the current to the LED series circuit.
The constant current circuit according to claim 3 is the constant current circuit according to claim 2, wherein the control means responds to a desired light emission luminance of the LED series circuit and a time for applying a current to the LED series circuit. The magnitude of the current applied to the LED series circuit is controlled.

  According to the present invention, a plurality of LED series circuits in which the same number of LEDs are connected in series are sequentially switched and driven with constant current, so that the plurality of LED series circuits can be individually driven with constant current. A constant current circuit can be provided.

FIG. 1 is a diagram illustrating a configuration of a light source device including a constant current circuit according to an embodiment of the present invention. FIG. 2 is a timing chart for explaining the operation of the constant current circuit. FIG. 3 is a diagram showing a timing chart for explaining a method of changing the luminance.

  Hereinafter, an embodiment of the present invention will be described with reference to FIGS. 1 and 2. Here, FIG. 1 is a diagram illustrating a configuration of a light source device including a constant current circuit according to an embodiment of the present invention, and FIG. 2 is a timing chart for explaining the operation of the constant current circuit. FIG. FIG. 3 is a diagram showing a timing chart for explaining the method of changing the luminance.

  In the present embodiment, as shown in FIG. 1, two LED series circuits LED1 and LED2 in which the same number of LEDs are connected in series are individually driven with a constant current as an example. Of course, the number of series circuits is not limited to two, and may be three or four.

Here, a constant drive voltage _VLED is applied to the high potential side of the LED series circuits LED1 and LED2. The low potential side of the first LED series circuit LED1 is connected to the drain of the first field effect transistor FET1, and the low potential side of the second LED series circuit LED2 is the drain of the second field effect transistor FET2. It is connected to the. As these field effect transistors FET1 and FET2, power MOSFETs that are capable of high-voltage switching and high-speed switching are used. The sources of these field effect transistors FET1 and FET2 are connected in common and connected to the ground via a current limiting resistor R.

  The constant current circuit 10 according to the present embodiment includes a control unit 11, an operational amplifier 12, a first switch SW1, a second switch SW2 including two interlocked switches SW21 and 22, and resistors R1 and R2. .

  The control unit 11 receives a luminance setting operation by a user operation of an operation unit (not shown), outputs a current setting voltage and a pulse width control signal that cause the LED series circuits LED1 and LED2 to emit light with the luminance, and lights up the LED. A switching signal for switching the series circuit is output. The pulse width control signal is supplied to the control terminal of the switch SW2 for switching control of the second switch SW2, and the switching signal is supplied to the control terminal of the switch SW1 for switching control of the first switch SW1. .

  The operational amplifier 12 is supplied with the current setting voltage from the control unit 11 at one input terminal (+), and has the field effect transistors FET1 and FET2 and the current limiting resistor R at the other input terminal (−). The voltage value of the output voltage output from the output terminal is set so that the potential difference is eliminated, that is, the LED drive current flowing through the LED series circuits LED1 and LED2 is constant. change.

  The first switch SW1 has an input terminal connected to the output terminal of the operational amplifier 12, a first output terminal connected to the input terminal of one switch SW21 of the second switch SW2, and the second switch A second output terminal connected to the input terminal of the other switch SW22 of the switch SW2; and a control terminal to which a switching signal from the control unit 11 is input. That is, the first switch SW1 selectively supplies the output voltage of the operational amplifier 12 to the two switches SW21 and SW2 constituting the second switch SW2 according to the switching signal from the control unit 11.

  The output terminal X1 of one switch SW21 of the second switch SW2 is connected to the gate of the first field effect transistor FET1 connected to the first LED series circuit LED1, and via a resistor R1. Connected to ground. Similarly, the output terminal X2 of the other switch SW22 of the second switch SW2 is connected to the gate of the second field effect transistor FET2 connected to the second LED series circuit LED2, and has a resistance. It is connected to the ground via R2. Here, the resistors R1 and R2 have the same resistance value. These two switches SW21 and S22 further have a control terminal to which a pulse width control signal from the control unit 11 is input, and open and close together according to this pulse width control signal. However, since the output voltage of the operational amplifier 12 is applied to only one of the two switches SW21 and S22 by the first switch SW1, it is applied to one of the gates of the field effect transistors FET1 and FET2. However, the output voltage is not applied. That is, the field effect transistors FET1 and FET2 are selectively turned on. Moreover, the conduction time depends on the pulse width of the pulse width control signal.

  Thus, when one of the field effect transistors FET1 and FET2 is turned on, a current flows through the corresponding LED series circuit LED1 or LED2, the field effect transistor FET1 or FET2, and the current limiting resistor R. Since the current value is determined by the voltage value of the output voltage of the operational amplifier 12, the corresponding LED series circuit LED1 or LED2 emits light with the brightness according to the voltage value. At this time, the operational amplifier 12 changes the voltage value of the output voltage so that the current flowing through the LED series circuit LED1 or LED2 is constant, thereby driving the LED at a constant current.

  Next, the operation of the constant current circuit according to the present embodiment will be further described with reference to the timing chart of FIG.

When an electric current is passed, the LED is turned on with a luminance proportional to the amount of the electric current. Assuming that the current that is continuously passed through the _LED is I _LED , when the current is passed intermittently (ON / OFF duty 50% drive), the current value (2I _LED ) that is twice that current is passed through to the same brightness. Can be obtained. If this ON / OFF frequency is increased, it will appear to the human eye in the same way as continuous lighting that flows continuously through the LED.

  By utilizing such characteristics, in this embodiment, one constant current circuit is periodically switched to light up the two LED series circuits LED1 and LED2 with a constant luminance.

  That is, as shown by the top waveform in FIG. 2, the control unit 11 periodically switches between the switch SW21 side (field effect transistor FET1 side) and the switch SW22 side (field effect transistor FET2 side). A signal is generated and supplied to the control terminal of the first switch SW1.

On the other hand, as shown by the second waveform from the top in FIG. 2, ON / OFF (duty 50%: T1 _OFF = T1 _ON ) in the period when the switching signal is on the switch SW21 side (field effect transistor FET1 side). In addition, a pulse width control signal that is ON / OFF (duty 50%: T2 _OFF = T2 _ON ) is generated during the period when the switching signal is on the switch SW22 side (field effect transistor FET2 side) This is supplied to the control terminals of the switches SW21 and SW22.

  As a result, at the output terminal X1 of the switch SW21, as shown by the third waveform from the top in FIG. 2, the switching signal is on the switch SW21 side (field effect transistor FET1 side) and the pulse width control signal is on. The output voltage V1 of the operational amplifier 12 appears, and this is applied to the corresponding field effect transistor FET1. The field effect transistor FET1 is turned on, and the LED series circuit LED1 is turned on. The output voltage V1 at this time is four times as large as the current value when a current is continuously passed through the LED series circuit LED1 when each LED of the LED series circuit LED1 is lit at a predetermined brightness. It becomes a voltage value that allows the current value to flow.

  Similarly, at the output terminal X2 of the switch SW22, as shown by the bottom waveform in FIG. 2, the switching signal is on the switch SW22 side (field effect transistor FET2 side) and the pulse width control signal is ON. The output voltage V2 of the operational amplifier 12 appears, and this is applied to the corresponding field effect transistor FET2. The field effect transistor FET2 conducts, and the LED series circuit LED2 is lit. The output voltage V2 at this time is four times as large as the current value when a current is continuously passed through the LED series circuit LED2 when each LED of the LED series circuit LED2 is lit with a predetermined luminance. It becomes a voltage value that allows the current value to flow.

  When the two LED series circuits LED1 and LED2 are caused to emit light with the same luminance, the control unit 11 supplies the operational amplifier 12 with a current setting voltage so that the output voltages V1 and V2 of the operational amplifier 12 are the same. .

  There are two methods for changing the luminance: a method of changing the output voltage of the operational amplifier 12 and a method of changing the time (duty) during which a current is passed through the LED series circuit.

For example, as shown by the second waveform from the top in FIG. 3, if 3T1 _OFF '= T1 _ON ' in the period when the switching signal is on the switch SW21 side (field effect transistor FET1 side), FIG. As shown by the third waveform from the top, even if the output voltage V1 of the operational amplifier 12 remains unchanged, the current flows through the field effect transistor FET1 for a longer time, so that it is brighter to the human eye. Thus, although the physical light emission luminance does not change, the luminance can be visually increased.

Further, for example, as indicated by the second waveform from the top in FIG. 3, 50% duty drive of T2 _OFF = T2 _ON is performed during the period when the switching signal is on the switch SW22 side (field effect transistor FET2 side). If the output voltage of the operational amplifier 12 is increased from V2 to 2 ′ as shown by the bottom waveform in FIG. 3, the amount of current flowing through the field effect transistor FET2 increases, so that the physical brightness increases. At the same time, the visual brightness increases.

  In FIG. 3, two luminance changing methods are adopted at the same time, but it goes without saying that either one may be used.

  As described above, according to the constant current circuit 10 according to the present embodiment, two LED series circuits LED1 and LED2 in which the same number of LEDs are connected in series with each other by using a common constant current circuit. It becomes possible to drive at constant current.

  Further, by using the second switch SW2 for voltage switching to the field effect transistors FET1 and FET2, the ON duty of the field effect transistors FET1 and FET2 is controlled, so that the brightness of the LED series circuits LED1 and LED2 is increased. Can be controlled.

  Note that the present invention is not limited to the above-described embodiment as it is, and can be embodied by modifying the components without departing from the scope of the invention in the implementation stage. For example, the first and second switches SW1 and SW2 can be replaced with analog switches sold under the product name CD4053.

  In addition, various inventions can be formed by appropriately combining a plurality of components disclosed in the one embodiment. For example, even if some constituent elements are deleted from all the constituent elements shown in the embodiment, the problem described in the column of the problem to be solved by the invention can be solved and the effect of the invention can be obtained. In addition, a configuration from which this component is deleted can also be extracted as an invention.

FET1 ... first field effect transistor FET2 ... second field effect transistor LED1, LED2 ... LED series circuit R ... current limiting resistor R1, R2 ... resistor SW1 ... first switch SW2 ... second switch SW21, SW22 Switch V1, V2 Output voltage V _LED Drive voltage X1, X2 Output terminal 10 Constant current circuit 11 Control unit 12 Operational amplifier

Claims (3)

A first switching means for sequentially selecting a plurality of LED series circuits in which the same number of LEDs are connected in series;
A second switching means for controlling a time during which a current is applied to the LED series circuit during a period in which any one LED series circuit is selected by the first switching means;
Constant current driving means for driving the LED series circuit in constant current during a period when any one LED series circuit is selected by the first switching means;
A constant current circuit comprising:   Control means for controlling a time for applying a current to the LED series circuit in accordance with a desired light emission luminance of the LED series circuit during a period when any one LED series circuit is selected by the first switching means. The constant current circuit according to claim 1, further comprising:   The said control means controls the magnitude | size of the electric current applied to the said LED series circuit according to the desired light emission brightness | luminance of the said LED series circuit, and the time to apply an electric current to the said LED series circuit. 4. The constant current circuit according to 3.

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