LED driver circuit, lighting circuit, and backlight device

Abstract

To provide an LED driver capable of generating an accurate drive current.SOLUTION: When the PWM dimming signal SPWM is at the first level, the error amplifier EA1 generates a signal obtained by amplifying a difference between the voltage of the first node M1 and the voltage of the second node IN1, and when the PWM dimming signal SPWM is at the second level, the error amplifier IN2 generates a signal for turning off the transistor M1. The error amplifier EA1 is configured so that the polarities (+, -) of the first and second nodes IN1 and IN2 can be inverted. The selector circuit 220 switches the reference voltage VREF and the current detection signal VCS between the first state φ 1 and the second state φ 2, and supplies the switched voltage to the error amplifier EA1.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to a light emitting diode (LED) driver circuit. [Background technology]

[0002] A constant current circuit is used to light an LED at the desired brightness. The constant current circuit is composed of a transistor, a sense resistor, and an error amplifier. A transistor for adjusting the current and a sense resistor for detecting the current are connected in series on the path of the drive current flowing through the LED. The sense resistor generates a current detection signal proportional to the drive current. The error amplifier feedback controls the gate voltage of the transistor so that the current detection signal approaches the reference voltage. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-44081

[0004] [overview] The present disclosure has been made in this context, and one exemplary purpose of an embodiment thereof is to provide an LED driver capable of generating an accurate drive current.

[0005] An LED driver circuit according to an embodiment of the present disclosure includes an output terminal to be connected to a light-emitting diode, an output transistor having one end connected to the output terminal, a sense resistor connected between the other end of the output transistor and a fixed voltage line, an output node connected to a control terminal of the output transistor, and is configured to generate, at the output node, a signal obtained by amplifying an error between a voltage at a first input node and a voltage at a second input node when a pulse-width modulation signal is at a first level, and to generate, at the output node, a signal that turns the output transistor off when the pulse-width modulation signal is at a second level, and is configured such that the polarities of the first input node and the second input node are reversible. the selector circuit is switchable between a first state and a second state, and in the first state, supplies a current detection signal corresponding to a voltage drop across the sense resistor to a first input node of the error amplifier and a reference voltage to a second input node of the error amplifier, and in the second state, supplies the current detection signal to the second input node of the error amplifier and the reference voltage to the first input node of the error amplifier; and a controller that inverts the polarities of the first input node and the second input node of the error amplifier every n cycles (n≧1) of the pulse width modulation signal and switches the selector circuit between the first state and the second state.

[0006] Any combination of the above elements, or mutual substitution of elements or expressions between methods, devices, systems, etc., are also valid aspects of the present invention or the present disclosure. Furthermore, the description in this section (Means for Solving the Problems) does not explain all essential features of the present invention, and therefore, subcombinations of the described features may also constitute the present invention. [Brief explanation of the drawings]

[0007] [Figure 1] FIG. 1 is a circuit diagram of a light emitting device including an LED driver circuit according to an embodiment. [Figure 2] FIG. 2 is a circuit diagram of a light emitting device according to a comparative technique. [Figure 3] FIG. 3 is a waveform diagram illustrating the operation of the light emitting device of FIG. [Figure 4]FIG. 4 is a waveform diagram illustrating the operation of the light emitting device of FIG. [Figure 5] FIG. 5 is a circuit diagram showing an example of the configuration of the error amplifier. [Figure 6] FIG. 6 is a diagram illustrating an example of the configuration of the controller. [Figure 7] FIG. 7 is a block diagram of a backlight device. [Figure 8] FIG. 8 is a circuit diagram of a light emitting device according to a modified example.

[0008] [Detailed explanation] (Outline of the embodiment) A summary of some exemplary embodiments of the present disclosure is provided. This summary is intended to provide a simplified overview of some concepts of one or more embodiments in order to provide a basic understanding of the embodiments as a prelude to the more detailed description that follows. It is not intended to limit the scope of the invention or disclosure. This summary is not an exhaustive overview of all possible embodiments, and is not intended to identify key elements of all embodiments or to delineate the scope of some or all aspects. For convenience, the term "one embodiment" may refer to one embodiment (example or variant) or multiple embodiments (examples or variants) disclosed herein.

[0009] An LED driver circuit according to one embodiment includes an output terminal to be connected to a light emitting diode, an output transistor having one end connected to the output terminal, a sense resistor connected between the other end of the output transistor and a fixed voltage line, an output node connected to a control terminal of the output transistor, and is configured to generate, at the output node, a signal obtained by amplifying an error between a voltage at a first input node and a voltage at a second input node when a pulse width modulation signal is at a first level, and to generate, at the output node, a signal that turns off the output transistor when the pulse width modulation signal is at a second level, and is configured such that the polarities of the first input node and the second input node are reversible. the selector circuit is switchable between a first state and a second state, and in the first state, supplies a current detection signal corresponding to a voltage drop across the sense resistor to a first input node of the error amplifier and a reference voltage to a second input node of the error amplifier, and in the second state, supplies the current detection signal to the second input node of the error amplifier and the reference voltage to the first input node of the error amplifier; and a controller that inverts the polarities of the first input node and the second input node of the error amplifier every n cycles (n≧1) of the pulse width modulation signal and switches the selector circuit between the first state and the second state.

[0010] With this configuration, the effect of the input offset voltage of the error amplifier is canceled out between the first and second states, so the time average of the drive current is an accurate current amount proportional to the reference voltage without being affected by the input offset voltage.

[0011] In one embodiment, the error amplifier may include a differential pair including a first input transistor and a second input transistor, a load circuit having a first node and a second node that receive a differential current flowing through the differential pair, and a group of analog switches connected between the differential pair and the load circuit, the analog switches connecting the first input transistor to the first node and the second input transistor to the second node in a first state, and connecting the first input transistor to the second node and the second input transistor to the first node in a second state.

[0012] In one embodiment, the controller may include a D flip-flop that receives the pulse width modulated signal at a clock terminal and has an inverting output terminal connected to the input terminal.

[0013] In one embodiment, the LED driver circuit may be a current sink.

[0014] In one embodiment, the LED driver circuit may be a current source.

[0015] (Embodiment) Preferred embodiments will be described below with reference to the drawings. The same or equivalent components, parts, and processes shown in each drawing will be given the same reference numerals, and redundant explanations will be omitted as appropriate. Furthermore, the embodiments are examples and do not limit the disclosure and invention, and all features and combinations thereof described in the embodiments are not necessarily essential to the disclosure and invention.

[0016] In this specification, "a state in which component A is connected to component B" includes a case in which component A and component B are directly physically connected, and a case in which component A and component B are indirectly connected via other components that do not substantially affect the electrical connection state between them or impair the function or effect achieved by their combination.

[0017] Similarly, "a state in which component C is provided between component A and component B" includes not only cases in which components A and C, or components B and C, are directly connected, but also cases in which they are indirectly connected via other components that do not substantially affect the electrical connection state between them or impair the functions or effects achieved by their combination.

[0018] 1 is a circuit diagram of a light emitting device 100 including an LED driver circuit 200 according to an embodiment. The light emitting device 100 includes the LED driver circuit 200 and an LED 102.

[0019] The LED driver circuit 200 is a current sink type, and the output terminal OUT is connected to the cathode of the LED 102. The LED driver circuit 200 supplies a driving current I LED is supplied to the LED 102.

[0020] The LED driver circuit 200 is capable of adjusting the brightness of the LED 102 by PWM (pulse width modulation) dimming. The LED driver circuit 200 receives a PWM dimming signal S PWM Receives the PWM dimming signal S PWM When is at a first level (e.g., high), the LED 102 is supplied with a driving current I LED is supplied, and the PWM dimming signal S PWM When is at the second level (e.g., low), the drive current I LED becomes 0. PWM dimming signal S PWM When the duty cycle of the LED is d, the driving current I LED Time average of I LED(AVE) teeth, I LED(AVE) =d×I LED This becomes:

[0021] The LED driver circuit 200 includes an output transistor M1, a sense resistor R1, an error amplifier EA1, a voltage source 210, a selector circuit 220, and a controller 230.

[0022] One end of the output transistor M1 is connected to the output terminal OUT. For example, the output transistor M1 is an N-channel MOSFET (Metal Oxide Semiconductor Field Effect Transistor), and its drain is connected to the output terminal OUT. Note that the output transistor M1 may be another element such as a bipolar transistor, or may be a P-type transistor.

[0023] The sense resistor R1 is provided between the other end (source) of the output transistor M1 and the ground line GND, which is a fixed voltage line. A drive current I LED flows through the sense resistor R1, the drive current I LED A voltage drop proportional to the current is generated. This voltage is used as the current detection signal V CS It is called. V CS =I LED ×R1

[0024] The voltage source 210 generates a drive current I LED The reference voltage V that defines the target value (amplitude) of the instantaneous value of REF Generate.

[0025] The output node of the error amplifier EA1 is connected to the control terminal (gate) of the output transistor M1. PWM When is at a first level (high), a signal obtained by amplifying the difference between the voltage at the first input node IN1 and the voltage at the second input node IN2 is generated at the output node.

[0026] The error amplifier EA1 outputs the PWM dimming signal S PWM When the signal is at a second level (low), it generates a signal at the output node that turns off the output transistor M1.

[0027] The error amplifier EA1 is configured so that the polarities of the first input node IN1 and the second input node IN2 can be reversed. Specifically, in a first state φ1, the first input node IN1 is the inverting input terminal (-) and the second input node IN2 is the non-inverting input terminal (+), and in a second state φ2, the second input node IN2 is the inverting input terminal (-) and the first input node IN1 is the non-inverting input terminal (+).

[0028] The selector circuit 220 can switch between a first state φ1 and a second state φ2. In the first state φ1, the selector circuit 220 outputs a current detection signal V CSis supplied to the first input node IN1 of the error amplifier EA1, and the reference voltage V REF to the second input node IN2. In the second state φ2, the selector circuit 220 supplies the current detection signal V CS is supplied to the second input node IN2 of the error amplifier EA1, and the reference voltage V REF is supplied to a first input node IN1 of the error amplifier EA1.

[0029] The controller 230 generates a PWM dimming signal S PWM Based on this, the state of the selector circuit 220 and the polarity of the error amplifier EA1 are controlled.

[0030] The controller 230 inverts the polarities of the first input node IN1 and the second input node IN2 of the error amplifier OA every n cycles (n≧1) of the PWM signal, and switches the selector circuit 220 between the first state φ1 and the second state φ2.

[0031] The above is the configuration of the LED driver circuit 200. The advantages of the LED driver circuit 200 become clear when compared with a comparative technology. Therefore, the comparative technology will be described.

[0032] 2 is a circuit diagram of a light emitting device 100R according to a comparative example. The LED driver circuit 200R includes an output transistor M1, a sense resistor R1, a voltage source 210, and an operational amplifier OA2. The operational amplifier OA2 corresponds to the error amplifier EA1 in FIG. 1, and has a reference voltage V REF The inverting input terminal (-) is connected to the current detection signal V CS receive.

[0033] PWM dimming signal S PWM When is at the first level (high), the reference voltage V of the non-inverting input terminal (+) REF and the inverting input terminal (-) current detection signal V CS The operational amplifier OA2 generates a signal that amplifies the error of the PWM dimming signal S PWM When the signal is at a second level (low), it generates a signal at the output node that turns off the output transistor M1.

[0034] The above is the configuration of the light emitting device 100R. Next, the operation of the light emitting device 100R will be described.

[0035] 3 is a waveform diagram illustrating the operation of the light emitting device 100R of FIG. OFS The PWM dimming signal S PWM is the first level (high) on period T ON , the current I flowing through the output transistor M1 LED is expressed by the following formula: I LED =(V REF -V OFS ) / R1

[0036] Here, the input offset voltage V OFS is ideally zero, but in an actual circuit, it is affected by process variations and becomes non-zero. Therefore, the input offset voltage V OFS The variation in the drive current I LED But the real target amount is REF (=V REF / R1), and the light emission brightness of the LED 102 deviates from the design value.

[0037] Furthermore, when the LED driver circuit 200 drives LEDs on multiple channels, the brightness of the LEDs varies from channel to channel.

[0038] In the comparison technology, in order to suppress variations in LED brightness, the input offset voltage V OFS It was necessary to take measures such as adjusting the

[0039] Returning to the embodiment, its operation and advantages will be explained.

[0040] FIG. 4 is a waveform diagram illustrating the operation of the light emitting device 100 of FIG.

[0041] In this example, the PWM dimming signal S PWMIt is assumed that the first state φ1 and the second state φ2 are switched every n=1 period.

[0042] In the cycle of the first state φ1, the PWM dimming signal S PWM is the first level (high) on period T ON , the current I flowing through the output transistor M1 LED is expressed by the following formula: I LED1 =(V REF +V OFS ) / R1

[0043] In the cycle of the second state φ2, the PWM dimming signal S PWM is the first level (high) on period T ON , the current I flowing through the output transistor M1 LED is expressed by the following formula: I LED2 =(V REF -V OFS ) / R1

[0044] Therefore, taking the time average of multiple cycles, the on-period T ON The driving current I LED The average value of I REF The off period T OFF Drive current I LED Time average of I LED(AVE) is the PWM dimming signal S PWM When the duty cycle of is d, I LED(AVE) =d×I REF This becomes:

[0045] The operation of the LED driver circuit 200 is as described above.

[0046] According to this LED driver circuit 200, the input offset voltage V OFS This reduces the influence of the light source 104, and allows the LED 102 to emit light at an accurate brightness.

[0047] The present disclosure covers various devices and methods that can be understood as the block diagram or circuit diagram of Figure 1 or derived from the above description, and is not limited to a specific configuration. Below, more specific configuration examples and examples will be described not to narrow the scope of the present disclosure, but to aid in understanding and clarify the essence and operation of the present disclosure and the present invention.

[0048] 5 is a circuit diagram showing an example configuration of the error amplifier EA1. The error amplifier EA1 includes an input stage 250, a gain stage 256, and an output stage 258. The input stage 250 is a differential amplification stage, and includes an input differential pair 252 including a first input transistor M11 and a second input transistor M12, a tail current source CS2, and a group of analog switches 254.

[0049] The gain stage 256 is a load circuit connected to the input differential pair 252. The gain stage 256 has a first node N1 and a second node N2, and receives the differential currents I1 and I2 flowing through the input differential pair 252. The gain stage 256 is a folded cascode circuit and includes transistors M21 to M24. The configuration of the gain stage 256 is not particularly limited, and may be another type including a current mirror load.

[0050] The output stage 258 generates a signal at an output node out that is connected to the gate of the output transistor M1 according to the output of the gain stage 256. The output stage 258 may be of a push-pull type that includes transistors M31 and M32.

[0051] The analog switch group 254 is connected between the input differential pair 252 and its load circuit, a gain stage 256. In the first state φ1, the analog switch group 254 connects the first input transistor M11 to the first node N1 and connects the second input transistor M12 to the second node N2. In the second state φ2, the analog switch group 254 connects the first input transistor M11 to the second node N2 and connects the second input transistor M12 to the first node N1.

[0052] The error amplifier EA1 outputs the PWM dimming signal S PWMWhen is at the second level (low), the power supply voltage V DD Alternatively, the error amplifier EA1 may include a switch SW31 that cuts off the PWM dimming signal S PWM may include a switch SW32 that fixes the output node out to the ground voltage when is at a second level (low).

[0053] 6 is a diagram showing an example of the configuration of the controller 230. The controller 230 switches between a first state φ1 and a second state φ2 every n=1 cycles. The controller 230 includes a D flip-flop 232.

[0054] The D flip-flop 232 receives the PWM dimming signal S at the clock terminal. PWM , and the inverting output terminal / Q is connected to the input terminal D. In this configuration, the PWM dimming signal S PWM That is, at every positive edge of the PWM dimming signal S PWM For each period of n, the output Q of the D flip-flop 232 alternates between high and low. When the non-inverted output Q is high, it can be assigned to one of the first state φ1 and the second state φ2, and when the inverted output / Q is high, it can be assigned to the other of the first state φ1 and the second state φ2. Note that the configuration of the controller 230 is not limited to that shown in FIG. 6 and can be designed according to n.

[0055] The following describes the application of the LED driver circuit 200. The light emitting device 100 can be used as a variety of lighting devices, but its preferred application is as a backlight device for a liquid crystal display.

[0056] 7 is a block diagram of a backlight device 100A. The backlight device 100A includes m LEDs 102_1 to 102_m, a power supply circuit 110, and a lighting circuit 300. The anodes of the m LEDs 102_1 to 102_m are connected in common and are connected to an output line 112 of the power supply circuit 110.

[0057] The lighting circuit 300 includes a plurality of LED driver circuits 200_1 to 200_m corresponding to m LEDs 102_1 to 102_m. The i-th (i=1, 2 . . . m) LED driver circuit 200_i is connected to the cathode of the corresponding LED 102_i via the corresponding output terminal OUTi.

[0058] The output voltage V of the power supply circuit 110 OUT is maintained at a voltage level higher than Vf+Vsat, where Vf is the forward voltage of the LED 102 and Vsat is the minimum operating voltage of the LED driver circuit 200.

[0059] The above is the configuration of the lighting circuit 300. According to this lighting circuit 300, the plurality of LEDs 102_1 to 102_m can be made to emit light with equal luminance, thereby making it possible to suppress uneven luminance on the liquid crystal display.

[0060] Next, a modification of the light emitting device 100 will be described.

[0061] Fig. 8 is a circuit diagram of a light emitting device 100B according to a modified example. The LED driver circuit 200 is configured as a current source type and has a configuration in which the LED driver circuit 200 in Fig. 1 is inverted, that is, the power supply line and the ground line are swapped.

[0062] (Addendum) The present specification discloses the following techniques.

[0063] (Item 1) an output terminal to be connected to a light emitting diode; an output transistor having one end connected to the output terminal; a sense resistor connected between the other end of the output transistor and a fixed voltage line; an error amplifier having an output node connected to a control terminal of the output transistor, configured to generate at the output node a signal obtained by amplifying an error between a voltage at a first input node and a voltage at a second input node when the pulse width modulation signal is at a first level, and to generate at the output node a signal that turns the output transistor off when the pulse width modulation signal is at a second level, the polarities of the first input node and the second input node being reversible; a selector circuit switchable between a first state and a second state, in the first state, supplying a current detection signal corresponding to a voltage drop across the sense resistor to the first input node of the error amplifier and a reference voltage to the second input node of the error amplifier, and in the second state, supplying the current detection signal to the second input node of the error amplifier and the reference voltage to the first input node of the error amplifier; a controller that inverts polarities of the first input node and the second input node of the error amplifier and switches the selector circuit between the first state and the second state every n cycles (n≧1) of the pulse width modulation signal; 1. An LED driver circuit comprising:

[0064] (Item 2) The error amplifier a differential pair including a first input transistor and a second input transistor; a load circuit having a first node and a second node that receive a differential current flowing through the differential pair; an analog switch group connected between the differential pair and the load circuit, the analog switch group connecting the first input transistor to the first node and the second input transistor to the second node in the first state, and connecting the first input transistor to the second node and the second input transistor to the first node in the second state; Item 2. The LED driver circuit of item 1, comprising:

[0065] (Item 3) The controller 3. The LED driver circuit according to item 1 or 2, further comprising a D flip-flop having a clock terminal receiving the pulse width modulation signal and an inverting output terminal connected to an input terminal.

[0066] (Item 4) 4. The LED driver circuit of any one of items 1 to 3, wherein the LED driver circuit is a current sink type.

[0067] (Item 5) 4. The LED driver circuit of any one of items 1 to 3, wherein the LED driver circuit is a current source type.

[0068] (Item 6) A lighting circuit comprising a plurality of LED driver circuits according to any one of items 1 to 5.

[0069] (Item 7) the plurality of light emitting diodes; Item 6. A lighting circuit for driving the plurality of light-emitting diodes; A backlight device comprising: [Explanation of symbols]

[0070] 100 Light-emitting device 102 LED 200 LED driver circuit M1 Output transistor R1 Sense resistor EA1 Error Amplifier 210 Voltage Source 220 Selector Circuit 230 Controller 250 input stage 252 input differential pairs 254 Analog Switches 256 gain stages 258 output stage 100A backlight unit 110 Power supply circuit 300 Lighting circuit

Claims

1. an output terminal to be connected to a light emitting diode; an output transistor having one end connected to the output terminal; a sense resistor connected between the other end of the output transistor and a fixed voltage line; an error amplifier having an output node connected to a control terminal of the output transistor, configured to generate at the output node a signal obtained by amplifying an error between a voltage at a first input node and a voltage at a second input node when the pulse width modulation signal is at a first level, and to generate at the output node a signal that turns the output transistor off when the pulse width modulation signal is at a second level, the polarities of the first input node and the second input node being reversible; a selector circuit switchable between a first state and a second state, in the first state, supplying a current detection signal corresponding to a voltage drop across the sense resistor to the first input node of the error amplifier and a reference voltage to the second input node of the error amplifier, and in the second state, supplying the current detection signal to the second input node of the error amplifier and the reference voltage to the first input node of the error amplifier; a controller that inverts polarities of the first input node and the second input node of the error amplifier and switches the selector circuit between the first state and the second state every n cycles (n≧1) of the pulse width modulation signal; 1. An LED driver circuit comprising:

2. The error amplifier a differential pair including a first input transistor and a second input transistor; a load circuit having a first node and a second node that receive a differential current flowing through the differential pair; an analog switch group connected between the differential pair and the load circuit, the analog switch group connecting the first input transistor to the first node and the second input transistor to the second node in the first state, and connecting the first input transistor to the second node and the second input transistor to the first node in the second state; 2. The LED driver circuit of claim 1, comprising:

3. The controller 3. The LED driver circuit according to claim 1, further comprising a D flip-flop having a clock terminal receiving the pulse width modulated signal and an inverting output terminal connected to an input terminal.

4. 3. The LED driver circuit of claim 1, wherein the LED driver circuit is a current sink type.

5. 3. The LED driver circuit of claim 1, wherein the LED driver circuit is a current source type.

6. A lighting circuit comprising a plurality of LED driver circuits according to claim 1 or 2.

7. A plurality of the light emitting diodes; a lighting circuit according to claim 6 that drives the plurality of light-emitting diodes; A backlight device comprising:

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