JP5034557B2 - Light emitting diode driving circuit and imaging apparatus using the same - Google Patents

Description

The present invention relates to an LED driving circuit that drives and controls a light emitting diode (LED; hereinafter abbreviated as LED) and an imaging apparatus using the LED driving circuit, and in particular, provides high accuracy by supplying a stable current to the LED. The present invention relates to an LED drive circuit that realizes the light emission control and an imaging device using the same.

  2. Description of the Related Art Conventionally, LEDs have been widely used as light sources for various devices. For example, in imaging devices such as digital cameras, they are used for auxiliary light of an autofocus system that measures subject distance, or for operation display of a self-timer. For this purpose, the LED drive circuit side that drives the LED is required to realize high-precision drive control. In order to stabilize the light emission amount of the LED, the current flowing through the LED is stabilized. It is necessary to plan.

  In order to satisfy this requirement, for example, a constant current circuit disclosed in Patent Document 1 may be adopted. The configuration of this constant current circuit is as shown in FIG. That is, in this constant current circuit, the operational amplifier 102 controls the transistor 103 so that the voltage drop of the reference resistor 104 caused by the current flowing through the output terminal 105 is equal to the voltage of the reference power supply 100 (internal resistor 101). Realize stable operation. In addition, what is necessary is just to connect LED to the output terminal 105, when applying this circuit to drive control of LED.

JP-A-4-135219

  However, the constant current circuit disclosed in Patent Document 1 is expensive as a whole because an operational amplifier is used, and the operational amplifier always consumes power even when the circuit is not used. It is not possible to fully meet the demands of crystallization.

  Therefore, the present invention provides a versatile LED drive circuit that satisfies the demands for cost reduction and low power consumption and enables high-precision drive control regardless of the ambient temperature, and an imaging device using the same. This is the issue.

An LED drive circuit according to a first aspect of the present invention is an LED drive circuit that drives and controls an LED by receiving a pulse width modulation drive signal or a pulse frequency modulation drive signal from a drive control unit provided as a function of the microcomputer. A first switching element whose on / off state is controlled by the drive signal, a second switching element whose on / off state is controlled based on the on / off state of the first switching element, An LED having one end connected to a power supply via a second switching element, a first resistor having one end connected to the other end of the LED and the other end grounded, and differential together with the first switching element A third switching element as a component of the amplifier circuit; and a second resistor having one end connected to the connection end of the first and third switching elements and the other end grounded. With resistance.

  Therefore, when the voltage of the first resistor exceeds a predetermined value, the third switching element is turned on, the voltage of the second resistor is increased, and the first switching element is turned off. So negative feedback takes.

An LED drive circuit according to a second aspect of the present invention is an LED drive circuit that drives and controls an LED in response to a pulse width modulation drive signal or a pulse frequency modulation drive signal from a drive control unit provided as a function of the microcomputer. A base is connected to the drive control unit, a first transistor whose on / off state is controlled by the drive signal from the drive control unit, and a base is connected to a collector of the first transistor; A second transistor whose emitter is connected to a power supply and whose on / off state is controlled based on the on / off state of the first transistor; an LED whose anode is connected to the collector of the second transistor; A first resistor having one end connected to the cathode of the LED and the other end grounded, and a collector connected to the power source; One end is connected to the third transistor, which is a component of the differential amplifier circuit together with the first transistor, and the connection end of the emitter of the first transistor and the emitter of the second transistor, and the other end is grounded And a second resistor.

  Therefore, when the voltage of the first resistor exceeds the sum of the emitter potential of the first transistor and the base-emitter voltage of the third transistor, the third switching element is turned on, and the first The voltage of the resistor 2 increases, and negative feedback is applied so that the first switching element is turned off.

  An image pickup apparatus according to a third aspect of the present invention is an image pickup apparatus that emits an LED for at least one of autofocus auxiliary light and operation display during self-timer shooting. A control unit having a drive control unit that outputs a drive signal as a function, a first switching element whose on / off state is controlled by the drive signal, and on based on the on / off state of the first switching element A second switching element whose off-state is controlled, an LED having one end connected to a power source via the second switching element, a first end connected to the other end of the LED, and the other end grounded 1 resistor, a third switching element that constitutes a differential amplifier circuit together with the first switching element, and the first and third switches And an LED driving unit including a second resistor having one end connected to the connecting end of the ring element and the other end grounded.

  Therefore, when the voltage of the first resistor exceeds a predetermined value, the third switching element is turned on, the voltage of the second resistor is increased, and the first switching element is turned off. So negative feedback takes.

  According to the present invention, it is possible to obtain high accuracy in a wide temperature range, realize low cost, and satisfy the demand for low power consumption by suppressing power consumption when the circuit is not used. An LED driving circuit and an imaging apparatus using the LED driving circuit can be provided.

  DESCRIPTION OF EXEMPLARY EMBODIMENTS Hereinafter, the best mode for carrying out the invention (hereinafter, simply referred to as “embodiment”) will be described with reference to the drawings.

  FIG. 1 shows a configuration of an LED drive circuit according to a first embodiment of the present invention and will be described.

  As shown in FIG. 1, the output of a pulse width modulation (PWM) controller 4 is connected to the base of an NPN transistor Q1. The collector of the NPN transistor Q1 is connected to the base of the PNP transistor Q3, and the emitter of the NPN transistor Q1 is grounded via a connection node N1, a resistor 1, and a connection node N2. The power source Vcc is connected to the emitter of the PNP transistor Q3 via the connection node N3, and the collector of the PNP transistor Q3 is connected to the anode of the LED3. The cathode of the LED 3 is grounded via a connection node N4, a resistor 2, and a connection node N2. The power supply Vcc is further connected to the collector of the NPN transistor Q2 via the connection node N3. The base of the NPN transistor Q2 is connected to the connection node N4, and the emitter is grounded via the connection node N1, the resistor 1, and the connection node N2. The PWM control unit 4 is provided with a microcomputer or the like mounted on an imaging apparatus such as a digital camera as a function, but is not limited thereto. The PWM control unit 4 outputs a drive signal (drive pulse) based on a predetermined duty ratio. The NPN transistor Q1, the NPN transistor Q2, and the resistor 1 constitute a differential amplifier circuit.

  Hereinafter, the operation of such a configuration will be described in detail.

  When a drive signal is applied from the PWM controller 4 to the base of the NPN transistor Q1, the NPN transistor Q1 is turned on. As a result, a current flows to the base of the PNP transistor Q3, and the PNP transistor Q3 is turned on.

Then, a current (current I _LED ) flows through the _{LED 3, the LED 3} emits light, and a voltage proportional to the current I _LED is generated at the resistor 2 (resistance value R2). Although the voltage of the resistor 2 increases, when the voltage reaches the voltage obtained by adding the emitter potential of the NPN transistor Q1 and the base-emitter voltage of the PNP transistor Q2, a current flows to the base of the NPN transistor Q2, and the NPN Transistor Q2 is turned on. As a result, a current flows through the resistor 1 (resistance value R1) via the emitter of the NPN transistor Q2 and the node N1.

Due to this current, the voltage of the resistor 1 rises, and accordingly, the base-emitter voltage of the NPN transistor Q1 is limited, and negative feedback is applied to turn off the NPN transistor Q1. As a result, since the current flowing through the PNP transistor Q3 is limited, the current I _LED flowing through the _{LED 3} is stabilized at a certain value.

Here, the base voltage of the NPN transistor Q1 is V _B1 , the base voltage of the NPN transistor Q2 is V _B2 , the base-emitter voltage of the NPN transistor Q1 is V _BE1 , and the base-emitter voltage of the NPN transistor Q2 is V _BE2. The base voltage V _B2 of the NPN transistor Q2 is expressed by the following equation (1).

V _B2 = V _B1 −V _BE1 + V _BE2 (1)
Therefore, if the current flowing through the base of the NPN transistor Q2 is considered to be sufficiently smaller than the current flowing through the resistor 2 and ignored, the current I _LED flowing through the _{LED 3} is expressed by the following equation (2).

I _LED = V _B2 / R2 = (V _B1 −V _BE1 + V _BE2 ) / R2 (2)
As is apparent from the equation (2) showing the current I _LED flowing through the _{LED 3} , the base-emitter voltage V _BE1 of the NPN transistor Q1 and the base-emitter voltage V _{BE2 of the} NPN transistor Q2 are canceled. Accordingly, under the condition that the NPN transistors Q1 and Q2 are used in the same environment, the base-emitter voltages V _BE1 and V _BE2 change according to temperature in the same manner. The fluctuation due to the temperature change of the current I _LED is suppressed.

In addition, if the resistance value R1 of the resistors 1 and 2 is determined so that the collector currents I _C1 and I _{C2 of the} two NPN transistors Q1 and Q2 are the same, the base − between the two transistors Q1 and Q2 Differences between the collector currents I _C1 and I _C2 of the emitter-to-emitter voltages V _BE1 and V _BE2 can be suppressed, and the current I _LED flowing through the _{LED 3} can be controlled with higher accuracy. This will be described in detail below.

That is, the collector currents I _C1 and I _C2 of the NPN transistors Q1 and Q2 when no negative feedback is applied are expressed by the following equation (3).

I _C1 = (V _B1 −V _BE1 ) / R1, I _C2 = 0 (3)
When the collector current of the PNP transistor Q3 is I _C3 and the current amplification factor hfe is hfe3, the collector currents I _C1 and I _C2 of the NPN transistors Q1 and Q2 when negative feedback is applied are given by the following equation (4): Indicated by

I _C1 = I _C3 / hfe3, I _C2 = (V _B1 −V _BE1 ) / R1−I _C3 / hfe3 (4)
Accordingly, in order for the collector currents I _C1 and I _{C2 to} be the same in a state where negative feedback is applied, the resistance value R1 of the resistor 1 may be set so as to satisfy the following equation (5).

I _C1 = I _C2
I _C3 / hfe3 = (V _B1 −V _BE1 ) / R1−I _C3 / hfe3
(V _B1 −V _BE1 ) / R1 = 2 · (I _C3 / hfe3)
∴R1 = hfe3 (V _B1 −V _BE1 ) / 2I _C3 (5)
In general, it is known that the base-emitter voltage of a transistor has a property of fluctuating depending on its collector current. Therefore, theoretically, if the NPN transistors Q1 and Q2 are products of the same specification, the difference between the base-emitter voltages V _BE1 and V _BE2 is reduced to 0 if the collector currents I _C1 and I _C2 are the same. can do.

By the way, the voltage of the resistor 2 that can be fed back is limited to the range of (the voltage at which the NPN transistor Q2 is turned on) or more and (the power supply voltage Vcc−the collector-emitter saturation voltage of the PNP transistor Q3−the forward voltage drop of the LED3) Is done. Here, the voltage at which the NPN transistor Q2 is turned on is a voltage exceeding the sum of the emitter potential V _E1 of the NPN transistor Q1 and the base-emitter voltage V _BE2 of the NPN transistor Q2.

Accordingly, in order to control the light quantity of the _{LED 3} by changing the DC voltage level of the base of the NPN transistor Q1 and controlling the DC current I _LED of the _{LED 3} , in order to realize high-precision light quantity control over a wide range. Needs further technical innovation.

  In order to solve this problem together, in the present embodiment, as described above, the drive signal is a PWM drive signal having a constant peak voltage level. Then, by controlling the on-duty of the PWM drive signal, the time integral value of the luminous flux of the LED 3 by pulse drive is controlled, thereby realizing LED drive control with high accuracy from 100% of full light emission to several percent. Although described later, a pulse frequency modulation (PFM; Pulse Frequency Modulation, hereinafter abbreviated as PFM) drive signal may be employed instead of the PWM drive signal.

When not in use, all the transistors are turned off by setting the base voltage V _B1 of the NPN transistor Q1 to 0V. Therefore, even if the power supply voltage Vcc is applied, no current flows in the LED drive circuit except for the leakage current of the NPN transistor Q1, and therefore, the power consumption can be suppressed to almost 0W.

  Next, with reference to FIG. 2 thru | or FIG. 5, the 1st thru | or 4th modification of the LED drive circuit based on the 1st Embodiment of this invention is demonstrated in detail.

  In the following description, the same components as those in the first embodiment (FIG. 1) of the present invention will be denoted by the same reference numerals, description thereof will be omitted as appropriate, and improvements will be mainly described.

  FIG. 2 shows and describes the configuration of the LED drive circuit according to the first improved example.

  In the first improved example, the PWM control unit 4 in FIG. 1 is replaced with a PFM control unit 5. PFM control is one of the voltage control methods for a switching power supply, similar to PWM control. For example, PFM control realizes an operation of changing the off time (or on time) with the transistor on time always fixed. Control. According to the first improved example, the LED drive control with high accuracy is realized by controlling the time integral value of the luminous flux of the LED 3 by pulse drive by the duty control of the PFM signal.

  FIG. 3 shows and describes the configuration of an LED drive circuit according to a second improved example.

  In this second improved example, a resistor 6 (resistance value R3) is added between the base and emitter of the PNP transistor Q3. That is, the resistor 6 is connected between the connection node N5 between the power supply Vcc and the emitter of the PNP transistor Q3 and the connection node N6 which is the connection end of the base of the PNP transistor Q3 and the collector of the NPN transistor Q1. ing. According to this second improved example, the leakage current of the NPN transistor Q1 is negligible compared to the current flowing through the resistor 6 upon receiving the power supply Vcc. Therefore, the PNP transistor Q3 is turned on by the leakage current of the NPN transistor Q1. It is possible to prevent malfunctions that occur.

  FIG. 4 shows and describes the configuration of an LED drive circuit according to a third improved example.

  In this third improved example, a phase compensation capacitor 7 (capacitance C1) and a resistor 8 (resistance value R4) are newly added to prevent oscillation. That is, a resistor 8 is added between the PWM controller 4 and the base of the NPN transistor Q1, and a connection node N7 which is a connection end between the resistor 8 and the base of the NPN transistor Q1 and a connection end on the collector side of the NPN transistor Q1. A capacitor 7 is added to the connection node N8. According to the third improved example, by adding the capacitor 7 and the resistor 8 as a phase compensation circuit, it is possible to reduce the voltage gain in the high frequency region and prevent oscillation.

  FIG. 5 shows and describes the configuration of an LED drive circuit according to a fourth improved example.

  In this fourth improved example, the voltage dividing resistors 8 (resistance value R4) and 9 (resistance value R5) are used to adjust the peak voltage level of the drive signal (for example, PWM drive signal) applied to the base of the NPN transistor Q1. It has been added. That is, a resistor 8 is added between the output of the PWM control unit 4 and the base of the NPN transistor Q1, and one end of the resistor 9 is connected to a connection node N9 that is a connection end between the resistor 8 and the base of the NPN transistor Q1. The other end is grounded via the connection node N2. According to the fourth improved example, since the voltage obtained by dividing the drive signal by the resistors 8 and 9 is input to the base of the NPN transistor Q1, the peak voltage level is adjusted to an appropriate value.

  As described above, the LED drive circuit according to the first embodiment of the present invention has a drive signal (for example, a PWM control unit 4 or a PFM control unit 5) provided as a function of the microcomputer. , A PWM drive signal, or a PFM drive signal), and a first switching element (for example, an NPN transistor Q1) whose on / off state is controlled by the drive signal; A second switching element (for example, PNP transistor Q3) whose on / off state is controlled based on the on / off state of the first switching element, and a power source (for example, power source Vcc) via the second switching element An LED having one end connected to the LED (for example, LED3), a first end having the other end connected to the other end of the LED, and the other end grounded. A resistor (for example, resistor 2), a third switching element (for example, an NPN transistor Q2) that is a component of a differential amplifier circuit together with the first switching element, and a connection end of the first and third switching elements A second resistor (for example, resistor 1) having one end connected and the other end grounded. Therefore, in the LED driving circuit, when the voltage of the first resistor exceeds a predetermined value, the third switching element is turned on, the voltage of the second resistor is increased, and the first switching Negative feedback is applied so that the element is turned off.

More specifically, the LED drive circuit according to the first embodiment of the present invention includes a drive signal (for example, PWM) from a drive control unit (for example, the PWM control unit 4 or the PFM control unit 5) provided as a function of the microcomputer. A drive signal or a PFM drive signal), and a base is connected to the drive control unit, and an on / off state is controlled by the drive signal from the drive control unit. A first transistor (for example, NPN transistor Q1), a base connected to the collector of the first transistor, an emitter connected to a power source (for example, power source Vcc), and an on / off state of the first transistor A second transistor (eg, PNP transistor Q3) whose on / off state is controlled based on the second transistor, and the second transistor An LED having an anode connected to the collector (for example, LED 3), a first resistor having one end connected to the cathode of the LED and the other end grounded (for example, resistor 2), and a collector connected to the power source And a third transistor (for example, an NPN transistor Q2), which is a component of a differential amplifier circuit together with the first transistor, and one end at a connection end of the emitter of the first transistor and the emitter of the second transistor. A second resistor (for example, resistor 1) connected and grounded at the other end. Accordingly, when the voltage of the first resistor exceeds the sum of the emitter potential (eg, V _E1 ) of the first transistor and the base-emitter voltage (eg, V _BE2 ) of the third transistor, the third resistance is increased. The switching element is turned on, the voltage of the second resistor increases, and negative feedback is applied so that the first switching element is turned off.

Then, the base of the first transistor (e.g., NPN transistor Q1) - emitter voltage (e.g., V _BE1) based third transistor (e.g., NPN transistors Q2) - emitter voltage (e.g., V _BE2) and By canceling each other, it is possible to suppress fluctuations due to temperature changes in the current flowing through the LEDs (for example, LEDs 3).

Further, the first and second transistors are set so that collector currents (for example, collector currents IC1 and IC2) of the first transistor (for example, NPN transistor Q1) and the third transistor (for example, NPN transistor Q2) have the same value. And the collector current of the base-emitter voltage (for example, VBE1, VBE2) between the first and third transistors is determined, for example, the resistance value R2 of the resistor 2 and the resistance value R1 of the resistor 1. And the fluctuation of the current (for example, ILED) flowing through the LED can be suppressed. The drive signal is a PWM drive signal or a PFM drive signal.

  Therefore, according to the first embodiment, the amount of light can be controlled with high accuracy from a few percent to 100% of full light emission regardless of the ambient temperature by a drive signal by PWM (or PFM). An inexpensive LED driving circuit capable of reducing the power consumption during use to approximately 0 W is provided.

  Next, FIG. 6 shows and describes the configuration of an imaging apparatus according to the second embodiment of the present invention.

  For example, many imaging devices such as digital cameras are equipped with an autofocus (hereinafter abbreviated as AF) mechanism, but AF auxiliary light is used so that AF can be performed even when the amount of light is insufficient. Some are equipped with a function that automatically emits light. For such AF auxiliary light, LED lighting is generally used. However, the imaging apparatus according to the second embodiment uses the LED driving circuit according to the first embodiment described above for driving control of the LED that emits the AF auxiliary light. In this case, the LED often serves as a self-timer operation display in addition to the AF auxiliary light.

  As shown in FIG. 6, the optical system 12 includes a photographic lens, a diaphragm, and the like, and a CCD (Charge Coupled Device) 13 is disposed on the optical path of the incident light. The CCD 13 is electrically connected to the camera unit 14. The camera unit 14 is connected to the signal processing unit 15. The control unit 11 that controls the entire imaging apparatus is connected to the camera unit 14, the signal processing unit 15, the aperture / focus driving unit 17 that drives the optical system 12, the strobe light emission control unit 21 that controls the light emitting unit 20, and the card memory 23. Are connected to the media drive 22, the operation unit 24, and the display unit 16.

  What is characteristic is that the output of the PWM control unit 11a provided as a function of the control unit 11 is connected to the LED drive unit 18, and the drive control by the PWM drive signal of the LED 19 is possible. . The LED drive unit 18 and the LED 19 correspond to the LED drive circuit according to the first embodiment. It goes without saying that the first to fourth improved examples of the first embodiment can be adopted as the configuration.

  The control unit 11 is configured by a microcomputer including, for example, a CPU (Central Processing Unit), a ROM, a RAM, and the like, and executes various operation controls on the imaging apparatus according to a program stored in the internal ROM, for example. Is.

  In such a configuration, the LED drive unit 18 receives the PWM drive signal output from the PWM control unit 11a of the control unit 11, and performs drive control of the LED 19 as described above in the first embodiment. . The operation is as described above in the first embodiment.

  The optical system 12 forms an image on the subsequent CCD 13 using the incident light as imaging light. The optical system 12 includes a focus adjustment mechanism for focus adjustment, a diaphragm variable mechanism that varies the diaphragm according to the diaphragm value, and the like. Such a mechanism is driven by a drive signal output from the aperture / focus driver 17.

  The aperture / focus drive unit 17 outputs a predetermined drive signal in accordance with the control of the control unit 11 so as to obtain an appropriate focus state, aperture state, and the like.

  The CCD 13 photoelectrically converts the imaging light incident from the optical system 12 and imaged on the light receiving surface to generate an imaging signal and output it to the camera unit 14. At the time of shooting, for example, a shutter speed instruction determined according to the exposure setting result is notified from the control unit 11 to the camera unit 14. The camera unit 14 outputs a scanning timing signal corresponding to the notified shutter speed to the CCD 13. The CCD 13 performs scanning according to the scanning timing signal, executes photoelectric conversion processing, and outputs a video signal.

  In the camera unit 14, the analog video signal input from the CCD 13 is subjected to waveform shaping by performing gain adjustment and sample hold processing, for example, and then converted into digital image signal data by performing A / D conversion. . The signal processing unit 15 executes various types of image signal processing such as display luminance data generation processing on the digital image data input from the camera unit 14. The signal processing unit 15 can also perform signal processing for a so-called on-screen display so that a character image or the like can be displayed superimposed on the captured image under the control of the control unit 11. As the display unit 16, an LCD (Liquid Crystal Display) or the like is adopted, but the display unit 16 is not limited to this.

  The signal processing unit 15 performs, for example, image data compression processing by a predetermined method on the digital image signal from the camera unit 14 to generate compressed image data. The compressed image data is transferred to the media drive 22 and written to the card memory 23 under the control of the control unit 11. As the card memory 23, for example, a flash memory having a predetermined capacity is provided as a storage medium. When reproducing the compressed image data stored in the card memory 23, the media drive 22 selects and reads out the required image data in accordance with the instruction and control of the control unit 11 and transfers it to the signal processing unit 15. The signal processing unit 15 performs predetermined image signal processing so that the image of the image data is reproduced and displayed on the display unit 16.

  The operation unit 24 collectively shows various operators provided in the imaging apparatus. For example, a shutter button operated at the time of taking a picture, an operator for selecting a shooting mode, and an operator for up / down the parameter. However, it is not limited to this.

  The strobe light emission control unit 21 is a circuit for controlling the light emission of the light emitting unit 20. In this case, the light emission control of the light emitting unit 20 is performed by the control unit 11 outputting a control signal for instructing on / off of the light emission to the strobe light emission control unit 21. The light emitting unit 20 includes a light emitting element (for example, a xenon tube) driven to emit light by a strobe light emission control unit 21.

  As described above, according to the second embodiment of the present invention, in an imaging apparatus that emits LEDs for at least one of autofocus auxiliary light and operation display during self-timer shooting, PWM (or A control unit (for example, control unit 11) having as a function a drive control unit (for example, PWM control unit 11a) that outputs a drive signal of PFM), an LED drive unit (for example, LED drive unit 18 and LED 19; for example, the first embodiment) There is provided an imaging apparatus to which the LED driving circuit as described above is applied.

  Therefore, according to the second embodiment, the LED drive circuit according to the first embodiment is used by using the PWM drive signal output from the PWM (or PFM) control unit provided as a function of the control unit such as the microcomputer. An image pickup apparatus such as a digital camera is provided that drives and controls an LED that serves both as an autofocus auxiliary light and a self-timer operation display.

  The first and second embodiments of the present invention have been described above. However, the present invention is not limited to this, and it is needless to say that various improvements and modifications can be made without departing from the spirit of the present invention. . For example, in the first embodiment described above, an example in which one LED is driven and controlled has been shown. However, the present invention is not limited to this. The present invention is also applicable to a case in which a plurality of LED groups connected in series / parallel are driven and controlled. Is possible. In the second embodiment, an example of application to an imaging apparatus such as a digital camera has been described. However, the application destination is not limited to this, and any mobile phone or PDA can be used as long as an LED is used as a light source. It is applicable to various devices such as (Personal Digital Assistants).

The block diagram of the LED drive circuit which concerns on the 1st Embodiment of this invention. The block diagram of the 1st modification of the LED drive circuit which concerns on the 1st Embodiment of this invention. The block diagram of the 2nd modification of the LED drive circuit which concerns on the 1st Embodiment of this invention. The block diagram of the 3rd modification of the LED drive circuit which concerns on the 1st Embodiment of this invention. The block diagram of the 4th modification of the LED drive circuit which concerns on the 1st Embodiment of this invention. The block diagram which shows the structure of the imaging device which concerns on the 2nd Embodiment of this invention. The block diagram of the low noise constant current circuit which concerns on a prior art.

Explanation of symbols

  1, 2, 6, 8, 9 ... resistor, 3 ... LED, 4 ... PWM control unit, 5 ... PFM control unit, 7 ... capacitor, Q1, Q2 ... NPN transistor, Q3 ... PNP transistor

Claims (5)

A light emitting diode driving circuit for controlling driving of a light emitting diode in response to a pulse width modulation driving signal or a pulse frequency modulation driving signal from a drive control unit provided as a function of the microcomputer;
A first switching element whose on / off state is controlled by the drive signal;
A second switching element whose on / off state is controlled based on the on / off state of the first switching element;
A light emitting diode having one end connected to a power source via the second switching element;
A first resistor having one end connected to the other end of the light emitting diode and the other end grounded;
A third switching element that is a component of a differential amplifier circuit together with the first switching element;
A second resistor having one end connected to the connecting end of the first and third switching elements and the other end grounded;
When the voltage of the first resistor exceeds a predetermined value, the third switching element is turned on, the voltage of the second resistor is increased, and the first switching element is turned off. A light-emitting diode drive circuit, characterized in that negative feedback is applied. A light emitting diode driving circuit for controlling driving of a light emitting diode in response to a pulse width modulation driving signal or a pulse frequency modulation driving signal from a drive control unit provided as a function of the microcomputer;
A first transistor having a base connected to the drive controller, the on / off state of which is controlled by the drive signal from the drive controller;
A second transistor having a base connected to the collector of the first transistor, an emitter connected to a power source, and an on / off state controlled based on an on / off state of the first transistor;
A light emitting diode having an anode connected to a collector of the second transistor;
A first resistor having one end connected to the cathode of the LED and the other end grounded;
A third transistor that is connected to the power supply and is a component of a differential amplifier circuit together with the first transistor;
A second resistor having one end connected to the connection end of the emitter of the first transistor and the emitter of the second transistor and the other end grounded;
When the voltage of the first resistor exceeds the sum of the emitter potential of the first transistor and the base-emitter voltage of the third transistor, the third switching element is turned on, and the second switching element is turned on. A light-emitting diode driving circuit, wherein a negative feedback is applied so that a voltage of a resistor increases and the first switching element is turned off. The variation due to a temperature change of the current flowing through the light emitting diode is suppressed by mutually canceling the base-emitter voltage of the first transistor and the base-emitter voltage of the third transistor. Item 3. A light-emitting diode driving circuit according to Item 2 . The resistance values of the first and second resistors are determined so that the collector currents of the first transistor and the third transistor have the same value, and the base-emitter voltage between the first and third transistors is determined. The light emitting diode drive circuit according to claim 2 , wherein a difference in current due to the collector current is suppressed and fluctuations in current flowing in the light emitting diode are suppressed. In an imaging device that emits a light emitting diode for at least one of autofocus auxiliary light and operation display during self-timer shooting,
A control unit having as a function a drive control unit that outputs a pulse width modulation drive signal or a pulse frequency modulation drive signal;
A first switching element whose on / off state is controlled by the drive signal; a second switching element whose on / off state is controlled based on the on / off state of the first switching element; A light-emitting diode having one end connected to a power source via two switching elements, a first resistor having one end connected to the other end of the light-emitting diode and the other end grounded, and a differential together with the first switching element A light emitting diode driving unit comprising: a third switching element which is a component of an amplifier circuit; and a second resistor having one end connected to the connection end of the first and third switching elements and the other end grounded And
When the voltage of the first resistor exceeds a predetermined value, the third switching element is turned on, the voltage of the second resistor is increased, and the first switching element is turned off. An imaging apparatus characterized by being configured to receive negative feedback.

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