JP2014154328A - Control circuit for light-emitting device, light-emitting device using the same, and electronic apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce a withstand voltage of a control circuit of a light-emitting device and/or its circuit area.SOLUTION: A switching power supply 4 supplies a driving voltage Vto an anode of a light-emitting element 6. A power transistor Q1 is a NPN-type bipolar transistor, and its collector is connected to a cathode of the light-emitting element 6. A detection resistor Rs is provided between a ground terminal and an emitter of the power transistor Q1. A first transistor M1 generates a base current Ib of the power transistor Q1. A feedback circuit 30 generates a second error signal VERR2 in which its magnitude is adjusted so that a current flowing to the first transistor M1, comes close to a predetermined target current Ib. A pulse modulator 10 generates a pulse modulation signal Sin which its duty factor is adjusted according to the second error signal V. A driver 20 switches a switching transistor M11 according to the pulse modulation signal S.

Description

  The present invention relates to a driving technique for a light emitting element.

  In recent years, light-emitting devices using light-emitting elements such as LEDs (light-emitting diodes) have been used as backlights and lighting devices for liquid crystal panels. FIG. 1 is a circuit diagram showing a configuration example of a light emitting device 1r according to a comparative technique examined by the present inventors. The light emitting device 1r shown in FIG. 1 should not be certified as a conventional technique. The light emitting device 1 includes an LED string (light emitting element) 6, a switching power supply 4, a power transistor Q1, a detection resistor Rs, and a control circuit 2r.

The light emitting element 6 includes a plurality of LEDs connected in series. The switching power supply 4 boosts the input voltage _VIN input to the input terminal P1, and supplies the drive voltage _VOUT to the first terminal (anode) of the light emitting element 6 connected to the output terminal P2.

  The power transistor Q <b> 1 is an NPN bipolar transistor, and its collector is connected to the second terminal (cathode) of the light emitting element 6. The detection resistor Rs is provided between the emitter of the power transistor Q1 and the ground terminal.

The power transistor Q1, the detection resistor Rs, the first transistor M1 of the control circuit 2r, and the first error amplifier EA1 form a current driver 8r that generates the drive current I _LED . The control circuit 2r stabilizes the drive current I _LED generated by the power transistor Q1 at a level corresponding to the target luminance of the light emitting element 6 by adjusting the base current Ib of the power transistor Q1.

The detection resistor Rs, a voltage drop Vs proportional to the drive current _{I LED} is generated. The first error amplifier EA1 adjusts the gate voltage of the first transistor M1 so that the voltage drop Vs matches the first reference voltage _VREF indicating the target brightness. As a result, the first transistor M1 to the power transistor The base current Ib supplied to Q1 is feedback controlled. Thereby, the drive current I _LED is stabilized to the following current amount.
I _LED = V _REF1 / Rs

In order for the current driver 8r to generate the drive current I _LED proportional to the first reference voltage _VREF1 , it is desirable that the power transistor Q1 operates in the active region. That is, the collector voltage of the power transistor Q1, that is, the cathode voltage V _LED of the light emitting element 6 needs to be maintained higher than a certain threshold voltage. Therefore the control circuit 2r, the collector voltage _{V LED} of the power transistor Q1, to match the predetermined second reference voltage _{V REF2,} the feedback control of the drive voltage _{V OUT.} The second reference voltage V _REF2 is set to a voltage level at which the power transistor Q1 can operate in the active region.

The second error amplifier EA2 amplifies an error between the collector voltage V _LED and the second reference voltage V _REF2 and generates an error signal V _ERR2 . Pulse modulator 10 generates a pulse modulation signal _{S PM} whose duty ratio in response to the error signal _{V ERR2} is adjusted. Driver 20, in response to the pulse modulation signal _{S PM,} drives the switching transistor M11 of the switching power supply 4.

JP 2012-243821 A

Here, the driving voltage _{V OUT} of the switching power supply 4 generates, the collector voltage _{V LED,} using the forward voltage _{V F} of the light-emitting element 6, the following relationship holds.
V _LED = V _OUT -V _F
That is, when the forward voltage _{V F} of the light-emitting element 6 is reduced, collector voltage _{V LED} is increased. In the control circuit 2r of FIG. 1, since the collector voltage V _LED is input to the second error amplifier EA2, it is necessary to design the withstand voltage of the circuit element connected to the LED terminal of the control circuit 2r so that the cost is high. Become.

  When the control circuit 2r is integrated on a single semiconductor substrate and packaged, an LED terminal connected to the cathode of the light emitting element 6 is required. In particular, when the light emitting elements 6 having a plurality of channels are driven in parallel, LED terminals are required by the number of channels of the light emitting elements 6, so that the circuit area is significantly increased.

  This problem is uniquely recognized by the present inventors and should not be regarded as a general technical recognition of those skilled in the art.

  The present invention has been made in view of these problems, and one of exemplary purposes of an embodiment thereof is to reduce the withstand voltage of the control circuit of the light-emitting device and / or to reduce the circuit area. .

  One embodiment of the present invention relates to a control circuit of a light-emitting device. A light emitting device includes a light emitting element, a switching power supply that supplies a driving voltage to a first terminal of the light emitting element, an NPN bipolar transistor power transistor having a collector connected to the second terminal of the light emitting element, and an emitter of the power transistor. And a detection resistor provided between the ground terminal and the grounding terminal. The control circuit is connected to the base of the power transistor and amplifies an error between the first transistor for supplying a base current to the power transistor and the first detection voltage corresponding to the voltage drop of the detection resistor and a predetermined first reference voltage. To generate a first error signal and output it to the control terminal of the first transistor, and a feedback circuit for generating a second error signal whose magnitude is adjusted so that the base current approaches a predetermined target current And a pulse modulator that generates a pulse modulation signal whose duty ratio is adjusted according to the second error signal, and a driver that drives the switching transistor of the switching power supply according to the pulse modulation signal.

  According to this aspect, in the light emitting device using the NPN type bipolar transistor as the power transistor of the current driver, the base current of the power transistor is stabilized at the target value instead of detecting the collector voltage and stabilizing it by feedback. Thus, the collector-emitter voltage of the power transistor, that is, the collector voltage can be stabilized at a target level corresponding to the base current. Thereby, since the control circuit does not need to detect the collector voltage of the power transistor, the breakdown voltage can be lowered and / or the circuit area can be reduced.

The light emitting device may have a plurality of channels. A set of the light emitting element, the power transistor, the detection resistor, the first transistor, and the first error amplifier may be provided for each channel. The feedback circuit may use a channel having the largest base current among the plurality of channels as a feedback channel, and generate the second error signal based on the base current of the feedback channel.
Thereby, the collector voltage of the power transistor of the feedback channel can be stabilized to the target level by feedback, and the collector voltage of the power transistor of the other channel can be maintained at a level higher than the target level.

  The feedback circuit is provided for each channel, and a plurality of base current detection units that convert the base current into a second detection voltage corresponding to the feedback circuit, and amplifies an error between the second detection voltage of the feedback channel and a predetermined second reference voltage. And a second error amplifier for generating a second error signal.

  The base current detection unit may be configured such that the second detection voltage decreases as the base current increases. The second error amplifier may amplify an error between the lowest voltage among the second detection voltages of the plurality of channels and the second reference voltage. Thereby, the channel with the largest base current can be selected as the feedback channel.

The base current detection unit is a constant current source that generates a predetermined bias current, a current amplifier circuit that generates a detection current obtained by multiplying the base current by a predetermined coefficient, and a potential at one end thereof is fixed to synthesize the bias current and the detection current. A conversion resistor provided on the current path, and a voltage drop of the conversion resistor may be output as the second detection voltage.
According to this configuration, the second detection voltage can be reduced as the base current increases. Further, the feedback gain can be determined according to the predetermined coefficient.

  The current amplification circuit includes: a first current converter that generates a first current obtained by multiplying a base current by a first coefficient; a second current converter that generates a detection current by multiplying the first current by a second coefficient; May be included.

  The first current conversion unit has a fixed potential at one end, a first resistor provided on the path of the base current, and a first voltage / a voltage that converts a voltage drop of the first resistor into a first current proportional thereto. And a current conversion circuit.

  The first voltage / current conversion circuit has a potential at one end fixed, a second resistor paired with the first resistor, a second transistor connected to the other end of the second resistor, and a first input terminal thereof The voltage at the other end of the second resistor is input, the potential at the connection point of the second resistor and the second transistor is input to the second input terminal, and the output terminal is connected to the control terminal of the second transistor. A third error amplifier, and the current flowing through the second transistor may be the first current.

  The second current conversion unit has a fixed potential at one end thereof, a third resistor provided on the path of the first current, and a second voltage / And a current conversion circuit.

  The second voltage / current conversion circuit has a potential fixed at one end thereof, a fourth resistor paired with the third resistor, a third transistor connected to the other end of the fourth resistor, and a first input terminal thereof. The voltage at the other end of the third resistor is input, the potential at the connection point of the fourth resistor and the third transistor is input to the second input terminal, and the output terminal is connected to the control terminal of the third transistor. And a fourth error amplifier, and the current flowing through the third transistor may be a detection current.

  The first current conversion unit and the second current conversion unit may have a symmetrical configuration that is inverted from each other.

  The first current converter may include a first current mirror circuit that generates a first current by multiplying the base current by a predetermined coefficient.

  The second current converter may include a second current mirror circuit that folds the first current by a predetermined coefficient and generates a detection current.

  The first transistor, the first error amplifier, the base current detection unit, the second error amplifier, and the pulse modulator may be integrated on a single semiconductor substrate.

  The light emitting element may be an LED string including a plurality of light emitting diodes connected in series.

  Another embodiment of the present invention relates to a light emitting device. A light emitting device includes a light emitting element, a switching power supply that supplies a driving voltage to a first terminal of the light emitting element, an NPN bipolar transistor power transistor having a collector connected to the second terminal of the light emitting element, and an emitter of the power transistor. And a detection resistor provided between the ground terminal and one of the control circuits described above.

  Another embodiment of the present invention relates to an electronic device. The electronic apparatus includes a liquid crystal panel and the above-described light emitting device provided as a backlight of the liquid crystal panel.

  Note that any combination of the above-described constituent elements and the constituent elements and expressions of the present invention replaced with each other among methods, apparatuses, systems, and the like are also effective as an aspect of the present invention.

  According to an aspect of the present invention, the breakdown voltage of the control circuit of the light emitting device can be reduced and / or the circuit area can be reduced.

It is a circuit diagram which shows the structural example of the light-emitting device which concerns on the comparison technique which the present inventors examined. It is a circuit diagram which shows a light-emitting device provided with the control circuit which concerns on embodiment. 3A and 3B are diagrams illustrating the operation of the control circuit of FIG. It is a circuit diagram which shows the structural example of a base current detection part. FIG. 5 is a circuit diagram illustrating a more specific configuration example of the base current detection unit of FIG. It is a figure which shows the example of an electronic device provided with the light-emitting device of FIG.

  The present invention will be described below based on preferred embodiments with reference to the drawings. The same or equivalent components, members, and processes shown in the drawings are denoted by the same reference numerals, and repeated descriptions are omitted as appropriate. The embodiments do not limit the invention but are exemplifications, and all features and combinations thereof described in the embodiments are not necessarily essential to the invention.

In this specification, “the state in which the member A is connected to the member B” means that the member A and the member B are electrically connected in addition to the case where the member A and the member B are physically directly connected. It includes the case of being indirectly connected through another member that does not affect the connection state.
Similarly, “the state in which the member C is provided between the member A and the member B” refers to the case where the member A and the member C or the member B and the member C are directly connected, as well as an electrical condition. It includes the case of being indirectly connected through another member that does not affect the connection state.

  FIG. 2 is a circuit diagram showing the light emitting device 1 including the control circuit 2 according to the embodiment. The light emitting device 1 includes a plurality of channels CH1 to CHN, a plurality of light emitting elements 6_1 to 6_N provided for each of the plurality of channels CH1 to CHN, a switching power supply 4, and a plurality of power transistors provided for each channel. Q1_1 to Q1_N, a plurality of detection resistors Rs1 to RsN provided for each channel, and a control circuit 2.

The light emitting element 6 is an LED string including a plurality of LEDs connected in series. The first terminals (anodes) of the light emitting elements 6_1 to 6_N of the plurality of channels are connected in common. The switching power supply 4 is a step-up DC / DC converter, boosts the input voltage _VIN input to the input terminal P1, and first terminals (anodes) of the plurality of light emitting elements 6 connected to the output terminal P2. _Is supplied with a driving voltage _VOUT .

  The switching power supply 4 includes an inductor L1, a rectifier diode D1, a switching transistor M11, and an output capacitor C1. Since the topology of the output circuit 102 is general, description thereof is omitted.

The switching terminal SW of the control circuit 2 is connected to the gate of the switching transistor M11. The control circuit 2 generates a gate pulse signal G1 whose duty ratio is adjusted by feedback so that the output voltage _VOUT required for lighting the light emitting element 6 is obtained and the light emitting element 6 emits light at a target luminance, The switching operation of the switching transistor M11 is controlled.

  The control circuit 2 is a functional IC integrated on a single semiconductor substrate. Note that “integrated integration” includes the case where all the circuit components are formed on a semiconductor substrate and the case where the main components of the circuit are integrated, and is used for adjusting circuit constants. Part of the resistors, capacitors, and the like may be provided outside the semiconductor substrate. The switching transistor M11 may be built in the control circuit 2.

The plurality of power transistors Q1_1 to Q1_N are NPN bipolar transistors, and their collectors are connected to the second terminals (cathodes) of the corresponding light emitting elements 6_1 to 6_N.
The plurality of detection resistors Rs1 to RsN are provided between the emitters of the corresponding power transistors Q1_1 to Q1_N and the ground terminal, respectively.

  The control circuit 2 includes a plurality of first transistors M1_1 to M1_N provided for each of the plurality of channels CH1 to CHN, a plurality of first error amplifiers EA1_1 to EA1_N provided for each of the plurality of channels CH1 to CHN, a feedback circuit 30, A pulse modulator 10 and a driver 20 are provided.

In each channel CH, the first transistor M1, the first error amplifier EA1, the power transistor Q1, and the detection resistor Rs form a current driver 8 that supplies a drive current I _LED to the corresponding light emitting element 6.

The first transistor M1_i of the i-th channel CHi (1 ≦ i ≦ N) is connected to the base of the corresponding power transistor Q1_i, and supplies the base current Ibi to the corresponding power transistor Q1_i.
The first error amplifier EA1 of the i-th channel CHi is first detected voltage Vsi and a first error signal by amplifying an error of a predetermined first reference voltage _{V REF1} V corresponding to the voltage drop Vsi corresponding sense resistor Rsi _ERR1 is generated and output to the control terminal (gate) of the corresponding first transistor M1. The first transistor M1 may be a bipolar transistor.

With this configuration, in each channel, the drive current I _LED flowing in the power transistor Q1 is stabilized to the target amount I _{LED_REF} = V _REF1 / Rs.

Feedback circuit 30 generates a second error signal _{V ERR2} current Ib flowing through the first transistor M1 is adjusted so that the magnitude approaches a predetermined target current _{Ib REF.} More specifically, among the plurality of channels, the channel having the largest current Ib flowing through the first transistor M1 is defined as a feedback channel CHj, and the second error is determined based on the current Ibj flowing through the first transistor M1 of the feedback channel CHj. A signal V _ERR2 is generated.

For example, the feedback circuit 30 includes a plurality of base current detection units 31_1 to 31_N and a second error amplifier EA2.
The i-th channel base current detection unit 31_i converts the current flowing through the corresponding first transistor M1_i, that is, the base current Ibi of the corresponding power transistor Q1_i, into a second detection voltage V _DET i corresponding thereto.

The second error amplifier EA2 is possible to amplify the second detection voltage _{V DET} j feedback channels among the plurality of channels second detection voltage _V DET _{1 to V DET} N, an error of a predetermined second reference voltage _{V REF2} Thus, the second error signal V _ERR2 is generated.

The base current detection unit 31_i is configured such that the second detection voltage V _DET2 decreases as the current Ibi flowing through the corresponding first transistor M1_i increases. The second error amplifier EA2 amplifies an error between the lowest voltage among the second detection voltages V _DET 1 to V _DET N of each of the plurality of channels and the second reference voltage V _REF2 . As a result, the channel having the lowest second detection voltage V _DET among the plurality of channels can be set as a feedback channel.

Pulse modulator 10 generates a pulse modulation signal _{S PM} whose duty ratio in response to the second error signal _{V ERR2} is adjusted. The configuration of the pulse modulator 10 is not particularly limited, and a known or future usable circuit may be used. For example, the pulse modulator 10 can use various types of modulators including a voltage mode, a peak current mode, and an average current mode. As a pulse modulation method, pulse width modulation or pulse frequency modulation can be used.

Driver 20, in response to the pulse modulation signal _{S PM,} drives the switching transistor M11 of the switching power supply 4.

The above is the configuration of the control circuit 2.
3A and 3B are diagrams illustrating the operation of the control circuit 2 in FIG.

FIG. 3A shows the relationship between the base current Ib and the second detection voltage V _DET . Black circles in the figure indicate operating points of a plurality of channels (N = 4). As described above, the channel in which the current Ib flowing through the first transistor M1 is the largest, in other words, the channel CHj in which the second detection voltage V _DET is the lowest is the feedback channel, and its voltage level matches the second reference voltage V _REF2. Take feedback. As a result, the base current Ibj in the feedback channel is stabilized at the target level Ib _REF corresponding to the second reference voltage V _REF2 . In other channels, the base current Ib is smaller than the target level Ib _REF . That is, in all channels, the base current Ib is controlled so as not to exceed the target level Ib _REF .

FIG. 3B shows the voltage-current characteristic of the power transistor Q1 (collector-emitter voltage V _CE -collector current Ic).
In all channels, when the collector current Ic, that is, the drive current I _LED is stabilized at the same target level I _{LED_REF} (= V _REF1 / Rs), the collector-emitter voltage V _CE decreases as the base current Ib increases. Become. Therefore, the collector-emitter voltage V _CE becomes the minimum value V _MIN in the feedback channel CHj.

According to the control circuit 2 according to the embodiment, in the light emitting device 1 using the NPN bipolar transistor as the power transistor Q1 of the current driver 8, instead of detecting the collector voltage V _LED and stabilizing it by feedback, The base current Ib of the power transistor Q1 can be stabilized at the target value Ib _REF . As a result, the collector-emitter voltage V _CE of the power transistor Q1, that is, the collector voltage V _LED can be stabilized at the target level V _MIN according to the base current Ib _REF .

Since the control circuit 2 does not need to directly detect the collector voltage V _LED of the power transistor Q1, the breakdown voltage can be lowered and / or the circuit area can be reduced.

In particular, when the control circuit 2 is integrated on a semiconductor substrate and packaged, an LED terminal for detecting the collector voltage V _LED is not necessary, so that the circuit area can be greatly reduced.

In addition, according to the control circuit 2, in the channels other than the feedback channel, the collector-emitter voltage _VCE is maintained higher than the collector-emitter voltage _VMIN of the feedback channel.

Here, the collector voltage _{V LED} of the power transistor Q1 of each channel is the sum of the collector-emitter voltage _{V CE} of the voltage drop Vs and the power transistor Q1 of the detection resistor Rs, a voltage drop Vs of the detection resistor Rs is the first reference It is equal to the voltage V _REF1 .
V _LED = V _CE + Vs = V _CE + V _REF1

Therefore, according to the light-emitting device 1, the feedback channel, the collector voltage _{V LED} is a minimum voltage _V MIN _{+ V REF1} will, in the other channels, keeping the collector voltage _{V LED} to higher than the minimum voltage _V MIN _{+ V REF1} region Can do. Thereby, the light emitting element 6 can emit light stably in all channels.

Next, a specific configuration example of the control circuit 2 will be described.
FIG. 4 is a circuit diagram illustrating a configuration example of the base current detection unit 31. Only a single channel CHi is shown in FIG. The base current detection unit 31_i includes a constant current source 32, a current amplification circuit 34, and a conversion resistor 36.
The constant current source 32 generates a predetermined bias current Ic. Current amplifying circuit 34 generates a detection current _{I DET} i which the base current Ibi supplied from the first transistor M1_i to the power transistor Q1_i predetermined factor multiplication (× K).
One end of the conversion resistor 36 is grounded and its potential is fixed. The conversion resistor 36 is provided on a path of a current Id obtained by combining the bias current Ic and the detection current _IDET i. The combined current Id is given by the following formula (1).
Id = Ic−I _DET i (1)

A voltage drop proportional to the combined current Id occurs in the conversion resistor 36. The base current detection unit 31_i outputs the voltage drop of the conversion resistor 36 as the second detection voltage V _DET i. When the resistance value of the conversion resistor 36 is Rt, the second detection voltage V _DETi is given by the following equation (2).
V _DET i = Rt × Id = Rt × (Ic−I _DET i)
= Rt × (Ic−K × Ibi) (2)

According to the base current detector 31, as shown in FIG. 3A, when the base current Ibi is zero, the second detection voltage V _DET i is set to the maximum value V _MAX = Rt × Ic, and the base current Ibi is It can be reduced as it increases.

FIG. 5 is a circuit diagram showing a more specific configuration example of the base current detection unit 31 of FIG. The current amplifier circuit 34 includes a first current converter 38 and a second current converter 40. The first current converter 38 generates a first current I1 obtained by multiplying the current Ibi flowing through the first transistor M1_i by the first coefficient K1. The second current converter 40 multiplies the first current I1 by the second coefficient K2 to generate the detection current _IDET i.
_IDET i = K1 × K2 × Ibi

The first current converter 38 includes a first resistor R1 and a first voltage / current conversion circuit 42. The potential of one end of the first resistor R1 is fixed and provided on the path of the base current Ibi. The first voltage / current converting circuit 42, the voltage drop _{V R1} of the first resistor R1, to convert the first current I1 proportional to it.

  The first voltage / current conversion circuit 42 includes a second resistor R2, a second transistor M2, and a third error amplifier EA3. The potential of one end of the second resistor R2 is fixed and paired with the first resistor R1. The second transistor M2 is connected to the other end of the second resistor R2. The voltage at the other end of the second resistor R2 is input to the first input terminal of the third error amplifier EA3, and the potential at the connection point between the second resistor R2 and the second transistor M2 is input to the second input terminal. The output terminal is connected to the control terminal (gate) of the second transistor M2. The current flowing through the second transistor M2 becomes the first current I1.

According to this configuration, Ibi × R1 = I1 × R2 holds. Therefore, the first coefficient K1 is
K1 = I1 / Ibi = R1 / R2
It becomes.

The first current conversion unit 38 and the second current conversion unit 40 have symmetrical configurations that are inverted from each other. The second current conversion unit 40 includes a third resistor R3 and a second voltage / current conversion circuit 44. The potential of one end of the third resistor R3 is fixed and is provided on the path of the first current I1. The second voltage / current conversion circuit 44 converts the voltage drop VR3 of the third resistor R3 into a detection current _IDET i proportional to the voltage drop _VR3 .

The second voltage / current conversion circuit 44 includes a fourth resistor R4, a third transistor M3, and a fourth error amplifier EA4. The fourth resistor R4 is paired with the third resistor R3, and the potential at one end thereof is fixed. The third transistor M3 is connected to the other end of the fourth resistor R4. The voltage at the other end of the third resistor R3 is input to the first input terminal of the fourth error amplifier EA4, and the potential at the connection point between the fourth resistor R4 and the third transistor M3 is input to the second input terminal. The output terminal is connected to the control terminal of the third transistor M3. The current flowing through the third transistor M3 is the detection current _IDET i.

According to this configuration, I1 × R3 = _IDET i × R4 holds. Therefore, the second coefficient K2 is
K2 = I _DET i / I1 = R3 / R4
It becomes.

According to the current amplification circuit 34 of FIG. 5, the detection current _IDET i proportional to the base current Ibi can be generated.

  Then, the use of the light-emitting device 1 is demonstrated. FIG. 6 is a diagram illustrating an example of an electronic device 500 including the light emitting device 1 of FIG. The electronic device 500 is, for example, a liquid crystal display device, a television receiver, a car navigation display, a mobile phone terminal having a liquid crystal panel, a tablet PC, an audio player, or the like.

  The electronic device 500 includes an LCD (Liquid Crystal Display) panel 5. The plurality of light emitting elements 6_1 to 6_N are provided on the back surface of the LCD panel 5 as a backlight. A switching power supply 4, a control circuit 2, a power transistor Q 1, and a detection resistor Rs (not shown) are built in the housing of the electronic device 500.

According to the current amplifying circuit 34, the base current Ib increases, the voltage drop V _R1 of the first resistor R1 is increased, the drain-source voltage of the first transistor M1 is reduced, in the direction of the base current Ib decreases Feedback is required. Thereby, circuit protection can be applied.

  The present invention has been described based on the embodiments. This embodiment is an exemplification, and various modifications may exist in each of those constituent elements, each processing process, and a combination thereof. Hereinafter, such modifications will be described.

(Modification 1)
The configuration of the feedback circuit 30 is not limited to that of FIG. A person skilled in the art understands that various modifications exist in the circuit that can generate the signal V _ERR2 corresponding to the error between the current Ib flowing through the first transistor M1 and the target value Ib _REF .

  For example, the base current detection unit 31 may include a transistor that forms a current mirror circuit together with the first transistor M1, and the current flowing through the transistor may be converted into a voltage.

Further, in the base current detection unit 31 of FIG. 5, the first current conversion unit 38 may be configured using a first current mirror circuit that folds the base current Ibi by a predetermined coefficient and generates the first current I1. Good. Similarly, the second current conversion unit 40 may be configured using a second current mirror circuit that folds the first current I1 by a predetermined coefficient and generates the detection current _IDET i.

(Modification 2)
Although the multi-channel configuration has been described in the embodiment, the present invention is effective even when the number of channels N = 1.

(Modification 3)
The light emitting element 6 is not limited to the LED string, and may be another light emitting element that can be used now or in the future.

(Modification 4)
In the embodiment, the backlight of the liquid crystal panel has been described as an application of the light emitting device 1, but the present invention is not limited thereto. For example, the light emitting device 1 can be used for lighting equipment and the like.

(Modification 5)
In the embodiment, the non-insulated switching power supply using the inductor L1 has been described. However, the present invention can also be applied to an insulating switching power supply using a transformer.

(Modification 6)
The setting of the high level and the low level of each signal described in this embodiment is an example, and can be freely changed by appropriately inverting it with an inverter or the like.

  Although the present invention has been described using specific terms based on the embodiments, the embodiments only illustrate the principles and applications of the present invention, and the embodiments are defined in the claims. Many variations and modifications of the arrangement are permitted without departing from the spirit of the present invention.

DESCRIPTION OF SYMBOLS 2 ... Control circuit, EA1 ... 1st error amplifier, EA2 ... 2nd error amplifier, 10 ... Pulse modulator, 20 ... Driver, 30 ... Feedback circuit, 31 ... Base current detection part, 32 ... Constant current source, 34 ... Current Amplifier circuit 36 ... Conversion resistor 38 ... First current conversion unit 40 ... Second current conversion unit 42 ... First voltage / current conversion circuit 44 ... Second voltage / current conversion circuit M1 ... First transistor M2 ... second transistor, M3 ... third transistor, M4 ... fourth transistor, R1 ... first resistor, R2 ... second resistor, R3 ... third resistor, R4 ... fourth resistor, EA3 ... third error amplifier, EA4 ... 4th error amplifier, 1 ... light emitting device, 4 ... switching power supply, 6 ... light emitting element, 8 ... current driver, 500 ... electronic device, 5 ... LCD panel, Q1 ... power transistor, M1 ... first transistor, s ... detection resistor, L1 ... inductor, C1 ... output capacitor, D1 ... rectifier diode, M11 ... switching transistor.

Claims (17)

A control circuit for a light emitting device,
The light emitting device
A light emitting element;
A switching power supply for supplying a driving voltage to the first terminal of the light emitting element;
A power transistor of an NPN bipolar transistor, the collector of which is connected to the second terminal of the light emitting element;
A detection resistor provided between an emitter of the power transistor and a ground terminal;
With
The control circuit includes:
A first transistor connected to a base of the power transistor and supplying a base current to the power transistor;
A first error amplifier that generates a first error signal by amplifying an error between a first detection voltage corresponding to a voltage drop of the detection resistor and a predetermined first reference voltage, and outputs the first error signal to a control terminal of the first transistor; ,
A feedback circuit for generating a second error signal whose magnitude is adjusted such that the base current approaches a predetermined target current;
A pulse modulator that generates a pulse modulation signal in which a duty ratio is adjusted according to the second error signal;
A driver for driving a switching transistor of the switching power supply according to the pulse modulation signal;
A control circuit comprising: The light emitting device has a plurality of channels,
A set of the light emitting element, the power transistor, the detection resistor, the first transistor, and the first error amplifier is provided for each channel.
2. The feedback circuit according to claim 1, wherein a channel having the largest base current among a plurality of channels is set as a feedback channel, and the second error signal is generated based on the base current of the feedback channel. Control circuit according to. The feedback circuit includes:
A plurality of base current detection units provided for each channel and converting the base current into a second detection voltage according to the base current;
A second error amplifier that generates the second error signal by amplifying an error between the second detection voltage of the feedback channel and a predetermined second reference voltage;
The control circuit according to claim 2, comprising: The base current detection unit is configured such that the second detection voltage decreases as the base current increases.
The control circuit according to claim 3, wherein the second error amplifier amplifies an error between a lowest voltage among the second detection voltages of each of a plurality of channels and the second reference voltage. The base current detector is
A constant current source for generating a predetermined bias current;
A current amplification circuit for generating a detection current obtained by multiplying the base current by a predetermined coefficient;
A potential of one end thereof is fixed, and a conversion resistor provided on a current path obtained by combining the bias current and the detection current;
The control circuit according to claim 3, further comprising: a voltage drop of the conversion resistor as the second WA2 detection voltage. The current amplifier circuit is:
A first current converter that generates a first current obtained by multiplying the base current by a first coefficient;
A second current converter for generating the detected current by multiplying the first current by a second coefficient;
The control circuit according to claim 5, comprising: The first current converter is
A potential of one end thereof is fixed, and a first resistor provided on the path of the base current;
A first voltage / current conversion circuit for converting a voltage drop of the first resistor into the first current proportional thereto;
The control circuit according to claim 6, further comprising: The first voltage / current conversion circuit includes:
A second resistor having a potential at one end fixed and paired with the first resistor;
A second transistor connected to the other end of the second resistor;
The voltage at the other end of the second resistor is input to the first input terminal, the potential at the connection point of the second resistor and the second transistor is input to the second input terminal, and the output terminal is A third error amplifier connected to the control terminal of the second transistor;
The control circuit according to claim 7, wherein a current flowing through the second transistor is the first current. The second current converter is
A third resistor provided on the path of the first current, the potential of one end of which is fixed;
A second voltage / current conversion circuit for converting a voltage drop of the third resistor into the detection current proportional thereto;
The control circuit according to claim 6, further comprising: The second voltage / current conversion circuit includes:
A fourth resistor having a fixed potential at one end and paired with the third resistor;
A third transistor connected to the other end of the fourth resistor;
The voltage at the other end of the third resistor is input to the first input terminal, the potential at the connection point of the fourth resistor and the third transistor is input to the second input terminal, and the output terminal is A fourth error amplifier connected to the control terminal of the third transistor;
The control circuit according to claim 9, wherein a current flowing through the third transistor is the detection current.   The control circuit according to claim 6, wherein the first current conversion unit and the second current conversion unit have symmetrical configurations that are inverted from each other.   The control circuit according to claim 6, wherein the first current conversion unit includes a first current mirror circuit that folds the base current by a predetermined coefficient to generate the first current.   8. The control circuit according to claim 6, wherein the second current conversion unit includes a second current mirror circuit that folds the first current by a predetermined coefficient and generates the detection current. 9.   14. The first transistor, the first error amplifier, the feedback circuit, the pulse modulator, and the driver are integrated on a single semiconductor substrate. Control circuit.   The control circuit according to claim 1, wherein the light emitting element is an LED string including a plurality of light emitting diodes connected in series. A light emitting element;
A switching power supply for supplying a driving voltage to the first terminal of the light emitting element;
A power transistor of an NPN bipolar transistor whose collector is connected to the second terminal of the light emitting element;
A detection resistor provided between an emitter of the power transistor and a ground terminal;
A control circuit according to any of claims 1 to 15;
A light emitting device comprising: LCD panel,
The light emitting device according to claim 16 provided as a backlight of the liquid crystal panel;
An electronic device comprising:

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