JP4641677B2 - Digital camera shutter drive circuit - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a shutter drive circuit of a digital camera that drives a shutter of a digital camera by controlling a current to a motor coil of a motor that drives the shutter, and more particularly to improvement of drive characteristics.
[0002]
[Prior art]
FIG. 4 is a diagram showing a conventional example of a shutter drive circuit of a digital camera. The other end of the motor coil 10 of the motor whose one end is connected to the power supply VB is connected to the ground via the NPN transistor 12 and the current detection resistor 14. The output of the current control amplifier 16 is connected to the base of the NPN transistor 12, and the connection point between the emitter of the transistor 12 and the current detection resistor 14 is connected to the negative input terminal of the current control amplifier 16. The positive input terminal of the current control amplifier 16 is connected to the connection point of a pair of resistors 18 and 20 connected in series with one end connected to the ground. The other ends (upper side) of the resistors 18 and 20 are connected to the power source via the constant current circuit 22 and to the ground via the Zener diode 24.
[0003]
Therefore, the constant current from the constant current circuit 22 flows to the resistors 18 and 20, and the upper end voltage of the resistors 18 and 20 is regulated by the Zener diode, so that the positive input end of the current control amplifier 16 is set to a constant set voltage. Is set.
[0004]
The negative input terminal of the current control amplifier 16 becomes the same potential as the positive input terminal due to an imaginary short. Therefore, the transistor 12 is driven so that the upper voltage of the current detection resistor 14 becomes the set voltage, and the current flowing through the motor coil 10 is controlled to a constant current.
[0005]
In the digital camera, exposure (charge accumulation) of the CCD image sensor is started in response to pressing of the shutter, and when a time corresponding to the shutter speed elapses, an electric current is passed through the motor coil 10 before the CCD image sensor. Although the shutter blades that block outside light are moved, a constant current flows through the motor coil 10 as described above, so that the movement of the shutter blades can be kept constant and the shutter can be released at the target shutter speed.
[0006]
[Problems to be solved by the invention]
Here, the shutter speed has been increased with the advancement of functions of digital cameras, and the speed of more than 1/2000 seconds has also increased. In such a case, it is necessary to improve the accuracy of current supply to the motor coil 10.
[0007]
That is, in the above conventional circuit, the current flowing through the current detection resistor 14 is controlled to be constant and the current flowing through the motor coil 10 is controlled to be constant as described above. In this circuit, the power supply voltage VB varies. Then, the problem that the rising of the current flowing through the motor coil 10 fluctuated occurred.
[0008]
This is because the rising slope of the coil current determined by the relationship with the inductance of the motor coil 10 increases as the voltage applied to both ends of the motor coil 10 increases.
[0009]
That is, as shown in FIG. 5, the current (coil current) of the motor coil 10 has different rising slopes when the power supply voltage VB is low and when the power supply voltage VB is high. The time until the current reaches 200 mA is different, which causes a problem that the shutter speed cannot be controlled with high accuracy.
[0010]
SUMMARY An advantage of some aspects of the invention is that it provides a shutter drive circuit for a digital camera that can accurately control the current of a motor coil regardless of fluctuations in power supply voltage.
[0011]
[Means for Solving the Problems]
The present invention relates to a shutter drive circuit for a digital camera that drives a shutter of a digital camera by controlling a current to a motor coil of a motor that drives a shutter, and has one end connected to the motor coil and the other end a current detection resistor. A control transistor connected to the ground, a current control amplifier having an output connected to the control signal input terminal of the control transistor, and an intermediate point of the control transistor and the current detection resistor connected to the negative input terminal, A constant voltage source connected to the positive input terminal of the current control amplifier for supplying a constant voltage thereto, a capacitor connected to the positive input terminal of the current control amplifier and connected to the ground at the other end, and charging of this capacitor A discharge transistor for extracting a voltage, and turning off the discharge transistor to charge the capacitor, and the current with a predetermined characteristic. Up the voltage of the positive input terminal of the control amplifier, and drives the shutter by applying a current corresponding to the voltage of the positive input to the motor coil.
[0012]
Thus, according to the present invention, the voltage at the time of starting up the positive input terminal of the current control amplifier is determined by the time constant of the capacitor and the resistor. Therefore, the rise of the amount of current to the motor coil can be set regardless of the inductance of the motor coil, and fluctuations in the motor coil current due to fluctuations in the power supply voltage can be prevented.
[0013]
In addition, it is preferable that a resistor is disposed between the capacitor and the constant voltage circuit. The charging time of the capacitor can be changed by adjusting the resistance value of the resistor.
[0014]
The motor is preferably a voice coil motor or a solenoid motor.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a diagram illustrating a configuration example of a shutter drive circuit of a digital camera according to an embodiment.
[0016]
As in the conventional example of FIG. 4 described above, the other end of the motor coil 10 whose one end is connected to the power source VB is connected to the ground via the NPN transistor 12 and the current detection resistor 14. Here, the motor for driving the shutter is intended to open and close by moving the shutter blades, and a voice coil motor or a solenoid motor is suitable as the motor. However, other types of motors can be employed.
[0017]
The output of the current control amplifier 16 is connected to the base of the NPN transistor 12, and the connection point between the emitter of the transistor 12 and the current detection resistor 14 is connected to the negative input terminal of the current control amplifier 16. The positive input terminal of the current control amplifier 16 is connected to a connection point of resistors 18 and 20 having one end connected to the ground. The other ends of the resistors 18 and 20 are connected to the power source through the constant current circuit 22 and are connected to the ground through the Zener diode 24.
[0018]
Therefore, the constant current from the constant current circuit 22 flows to the resistors 18 and 20, and the upper end voltage of the resistors 18 and 20 is regulated by the Zener diode, so that the voltage at the positive input end of the current control amplifier 16 becomes the set voltage. Is set.
[0019]
Further, since the negative input terminal of the current control amplifier 16 has the same potential as the positive input terminal, the transistor 12 is driven so that the upper end voltage of the current detection resistor 14 becomes the set voltage, and the current flowing through the motor coil 10 is constant. Controlled by current.
[0020]
For example, if the resistance value of the current detection resistor 14 is about 1Ω and a current of about 200 mA flows through the motor coil 10, the set voltage at the positive input terminal of the current control amplifier 16 is about 0.2V.
[0021]
The positive input terminal of the current control amplifier 16 is connected to a capacitor 26 having the other end connected to the ground and to a switch transistor 28 having the other end connected.
[0022]
Therefore, when the switch transistor 28 is off, the current from the constant current source 22 is used to charge not only the resistor 20 but also the capacitor 26. Therefore, the voltage at the rising edge of the positive input terminal of the current control amplifier 16 is determined by the time constant of the CR integration circuit composed of the resistors 18 and 20 and the capacitor 26.
[0023]
The circuit for driving the motor is accommodated in the semiconductor integrated circuit as a whole. However, it is preferable that the resistors 18 and 20 and the capacitor 26 are externally attached to the integrated circuit to be arbitrarily adjustable. Although both the resistors 18 and 20 may be adjusted, it is also preferable to change the resistance value of only the resistor 18 because the resistor 18 is dominant in the charging time constant.
[0024]
Then, by making the rise of the voltage at the positive input terminal of the current control amplifier 16 slower than the rise of the coil current when the power supply voltage VB is lowered, the rise characteristic of the upper voltage of the current detection resistor 14, that is, the coil current of the motor coil 10. Is determined only by the time constant of the CR integration circuit and does not depend on the fluctuation of the power supply voltage VB.
[0025]
That is, as shown in FIG. 2, when there is no capacitor 26, the coil current rises rapidly when the power supply voltage VB is 3V according to the rise characteristic of the coil current. On the other hand, when the power supply voltage VB is 2V, the rise is delayed.
[0026]
On the other hand, when the capacitor 26 is provided, there is no influence of the rise of the current control amplifier, and the coil current is always determined according to the time constant of the CR integration circuit. That is, the imaginary short between the positive input terminal and the negative input terminal of the current control amplifier 16 is always maintained. The solid line in FIG. 2 shows the case where the power supply voltage VB is 2 V, no capacitor, and the case where the capacitor 26 is provided. When the capacitor 26 is provided, this solid line coil current rising characteristic is always obtained. In this example, it is assumed that the minimum voltage at which the circuit can operate is 2V.
[0027]
In this way, the influence of the fluctuation of the power supply voltage VB on the time until the coil current of the motor coil 10 reaches the set current can be eliminated, and the constant time can always be maintained. This rise starts from the time when the switch transistor 28 is turned off.
[0028]
Therefore, in this embodiment, as shown in FIG. 3, when the shutter is released, the CCD capture signal is turned on to start exposure. Further, the driving start time of the motor coil 10 is calculated from the shutter speed at that time. Then, after a predetermined time has elapsed, the switch close signal is turned on to turn off the switch transistor 28. As a result, a voltage that rises in accordance with the time constant of the CR integration circuit is supplied to the positive input terminal of the current control amplifier 16, the upper voltage of the current detection resistor 14 is controlled to that voltage, and the coil current is set to the set current for a certain time. To. Therefore, the shutter blade driven by the motor coil 10 always closes the shutter at a constant speed, and a highly accurate shutter operation is guaranteed.
[0029]
That is, the time from when the CCD capture signal is turned on to when the switch close signal is turned on is a predetermined time calculated by the shutter speed. On the other hand, the shutter is actually turned off when the shutter blades are closed. Therefore, there is a certain delay time from when the switch close signal is turned on until the shutter blades are actually closed. In the present embodiment, the delay time can be made constant regardless of power supply voltage fluctuation by simply making the rise of the coil current of the motor coil 10 constant.
[0030]
As described above, the image signal is captured by closing the shutter blade. Thereafter, the shutter blades are returned to the open state to prepare for the next image capture.
[0031]
If the motor is a solenoid motor, use the spring force to open and close the shutter blades (usually in the “open” direction), and energize the motor when the shutter is closed. Close the shutter blade. For this reason, by stopping energization, the shutter is opened by the spring force. Therefore, energization is single energization that energizes only when the shutter blades are closed.
[0032]
On the other hand, when the motor is a voice coil motor, the shutter blades are opened and closed by energization in opposite directions. That is, when the motor coil is energized in one direction, the shutter blade is moved in one direction and the shutter is closed, and when the other is energized, the shutter blade is moved in the opposite direction and the shutter is opened.
[0033]
In the above-described example, the voltage generated by the constant current circuit 22 and the Zener diode 24 is divided by the resistors 18 and 20 to create the positive input terminal voltage of the amplifier 16, but the constant current circuit 22 and the Zener diode 24 are used. The portion formed by the above may have any configuration as long as it is a circuit that outputs a constant voltage. For example, it is also preferable to replace the Zener diode 24 with a band gap constant voltage circuit. Furthermore, a constant voltage source outside the IC may be connected to the upper end of the resistor 18.
[0034]
【The invention's effect】
As described above, according to the present invention, the voltage at the time of starting up the positive input terminal of the current control amplifier is determined by the charging voltage of the capacitor. Therefore, it is possible to set the rise of the current amount to the motor coil regardless of the rise of the current in the inductance of the motor coil, and it is possible to prevent the fluctuation of the motor coil current due to the fluctuation of the power supply voltage.
[Brief description of the drawings]
FIG. 1 is a diagram illustrating a configuration of a circuit according to an embodiment.
FIG. 2 is a diagram showing a change state of a coil current of a motor coil 10;
FIG. 3 is a timing chart showing the operation of each part.
FIG. 4 is a diagram showing a configuration of a conventional circuit.
FIG. 5 is a diagram showing a change state of a coil current of a motor coil 10;
[Explanation of symbols]
10 motor coil, 12 transistor, 14 current detection resistor, 16 current control amplifier, 18, 20 resistor, 22 constant current circuit, 24 Zener diode, 26 capacitor, 28 switching transistor.

Claims (2)

A shutter driving circuit of a digital camera that controls a current to a motor coil of a motor that drives a shutter to drive a shutter of the digital camera,
A control transistor having one end connected to the motor coil and the other end connected to the ground via a current detection resistor;
An output connected to the control signal input terminal of the control transistor, and a current control amplifier having a negative input terminal connected to an intermediate point of the control transistor and the current detection resistor;
A constant voltage source connected to the positive input terminal of the current control amplifier via a first resistor, and supplying a constant voltage to the positive input terminal via the first resistor ;
A capacitor connected to the positive input terminal of the current control amplifier and having the other end connected to the ground;
A second resistor connected in parallel to the capacitor;
A discharge transistor that pulls out the charging voltage of this capacitor;
Have
The discharge transistor is turned off to charge the capacitor, the voltage at the positive input terminal of the current control amplifier is raised with a predetermined characteristic, and a current corresponding to the voltage at the positive input terminal is supplied to the motor coil to drive the shutter. Digital camera shutter drive circuit. The circuit of claim 1, wherein
The motor is a shutter drive circuit of a digital camera which is a voice coil motor or a solenoid motor .

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