JP2001218118A - Ccd driver - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a CCD driver that can warrant its stable operation even when application of a high voltage is temporarily required to drive a CCD. SOLUTION: A low voltage generating charge pump 210 and a control section 230 configure a power supply circuit that provides a prescribed boosted voltage VSS to a vertical driver 240 being a drive circuit and an electronic shutter 250. The control section 230 of this power supply circuit applies interleave control to a boosted voltage clock CLK to apply constant voltage control to the voltage VSS. In the case of vertical transfer operation of a CCD image sensor 100 requiring a large current, the thinning of the boosted voltage clock CLK (constant voltage control) is inhibited before the operation so as to temporarily over-boost (pre-charge) the voltage VSS.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a CCD for driving a CCD (Charge Coupled Device) which is a solid-state image pickup device.
It relates to a driving device.

[0002]

2. Description of the Related Art As is well known, CCD image sensors are widely used as image pickup devices for various cameras and the like. And, as this CCD image sensor, (a) the light use efficiency is enhanced. (B) It is easy to increase the density (higher resolution) of pixels. For example, there is known a frame transfer type in which an imaging unit that performs photoelectric conversion and a storage unit that temporarily stores the photoelectrically converted charges are separately arranged in an element. .

The frame transfer type CCD is provided with a horizontal transfer section for outputting charges stored in the storage section in addition to the image pickup section and the storage section. An operation (vertical transfer) of collectively transferring the charges photoelectrically converted by the imaging unit to the storage unit at a predetermined timing (2) transferring the charges to the storage unit and transferring the stored charges at high speed line by line through the horizontal transfer unit (Transfer to the output unit) (horizontal transfer).

In order to realize such an operation of the frame transfer type CCD, the driving circuit is also provided with a vertical driver for applying the pulse for vertical transfer to each gate provided in the image pickup section and the storage section. , And a horizontal driver that applies a pulse for the horizontal transfer to each gate provided in the storage unit and the horizontal transfer unit.

The timing of generating each pulse output from the vertical driver and the horizontal driver is as follows.
It is determined according to an output from a timing generation circuit that generates a timing clock corresponding to the application based on the system clock.

The peak value of each pulse output from the vertical driver and the horizontal driver, that is, a pulse voltage, is required through a system voltage or a power supply circuit that raises and lowers the system voltage in a predetermined manner. The voltage level is secured.

[0007]

As described above, in the frame transfer type CCD, the image pickup signal based on the pulse applied from the vertical driver, that is, the charge photoelectrically converted by the image pickup unit is stored in the storage unit. The vertical transfer is performed, and based on the pulse applied from the horizontal driver, the same transfer and the horizontal transfer of the accumulated charges through the horizontal transfer unit are performed.

Normally, however, a high voltage is required for the vertical transfer to the storage section, and therefore, even if the required voltage is secured through the power supply circuit,
At the time of the vertical transfer, a large voltage drop temporarily occurs. In addition, when such a voltage drop occurs, it takes time to return to the desired voltage.

In any case, when such a voltage drop occurs during the vertical transfer, the effect on the operation itself of the vertical transfer cannot be ignored.
Not only the frame transfer type CCD but also an interline type CCD in which an imaging stage for performing photoelectric conversion and a transfer stage for transferring the photoelectrically converted charge to a horizontal transfer unit are respectively connected in parallel. A high voltage is still required for the charge transfer, and such a situation as a device for driving the charge is almost common.

The present invention has been made in view of the above circumstances, and has as its object to assure stable operation of a CCD even when a high voltage is required to be applied temporarily when driving the CCD. An object of the present invention is to provide a CCD driving device.

[0011]

The means for achieving the above object and the effects thereof will be described below. The invention according to claim 1 is a CCD driving device comprising: a driving circuit for applying a charge transfer pulse to a CCD; and a power supply circuit for applying a voltage required for generating the pulse to the driving circuit. Prior to the charge transfer operation of the CCD,
The gist of the invention is to provide an over-boosting means for temporarily over-boosting the voltage applied from the power supply circuit to the driving circuit.

As described above, a high voltage is required for the charge transfer operation of the CCD. However, according to such a configuration as the CCD driving device, before the voltage drop occurs due to the charge transfer operation, In addition, since the voltage applied from the power supply circuit to the drive circuit is temporarily excessively boosted, the voltage drop is minimized. In addition, since the voltage drop is minimized, the time required to return to a desired voltage is greatly reduced, and stable operation of the CCD is guaranteed. Here, boosting means that if the voltage applied to the drive circuit is a positive voltage, the voltage is raised to the positive side, and if the voltage applied to the drive circuit is a negative voltage, the voltage is reduced to the negative side.

According to a second aspect of the present invention, in the first aspect of the present invention, the power supply circuit includes a constant voltage control means for making a voltage applied to the drive circuit constant, and the over-boost means comprises The gist of the present invention is to make the constant voltage control means inactive during the over-boost operation.

According to the above configuration, even when the voltage applied from the power supply circuit to the drive circuit is temporarily over-boosted, it is possible to preferably balance the constant voltage with the over-boost. .

According to a third aspect of the present invention, in the second aspect, the power supply circuit includes a charge pump type booster circuit, and the constant voltage control means applies a clock applied to the charge pump type booster circuit. The voltage applied to the drive circuit is reduced to a constant voltage by thinning out, and the over-boosting means inhibits the thinning-out of the clock by the constant voltage control means to reduce the voltage applied to the drive circuit. The gist is that the pressure is temporarily increased.

According to the above configuration, the power supply circuit can be made compact, and the above-mentioned constant voltage and over-boost can be achieved through relatively simple control such as thinning out the clock applied to the charge pump type booster circuit. Both are possible.

According to a fourth aspect of the present invention, in the first aspect of the present invention, the CCD is an imaging unit that performs photoelectric conversion and a storage unit that temporarily stores the photoelectrically converted charge. Are frame transfer type CCDs separately disposed in the device, and the driving circuit is
The gist of the present invention is that the vertical driver performs batch transfer of charges from the imaging unit to the storage unit.

According to the above configuration, it is possible to preferably suppress the voltage drop caused by the above-described vertical transfer of the frame transfer type CCD, and as a result, the stable operation of the frame transfer type CCD driving device can be achieved. You can be assured.

According to a fifth aspect of the present invention, in the first aspect of the present invention, the drive circuit and the power supply circuit are integrated as a one-chip semiconductor integrated circuit device. Make a summary.

According to the above configuration, it is possible to reduce the cost of the CCD driving device and improve the yield. The same configuration is applied to the invention described in claim 3 and the invention described in claim 4, thereby making it easier to realize the configuration. In particular, the same configuration is applied to the invention described in claim 4. In a frame transfer type CCD driving circuit, a vertical driver that requires a higher breakdown voltage structure is required as a semiconductor integrated circuit device, and a semiconductor integrated circuit device is originally required to have a higher breakdown voltage structure. This is because the power supply circuit and the power supply circuit are integrated as one chip, which has a special significance from the viewpoint of the structure of the semiconductor integrated circuit device or the manufacturing method thereof.

According to a sixth aspect of the present invention, in the fifth aspect of the present invention, the gist is that the power supply circuit is excited using a period during which the image pickup signal output from the CCD is stopped. .

As described above, the drive circuit and the power supply circuit
When integrated as a semiconductor integrated circuit device of a chip, a signal (step-up clock) used to excite a power supply circuit is CC.
There is a concern that this may cause noise, such as jumping into an image pickup signal output from D. In this regard, according to the above configuration, since the power supply circuit is excited using the period during which the CCD does not output the imaging signal, even when these circuits are integrated as a one-chip semiconductor integrated circuit device. , The occurrence of such noise is preferably avoided.

[0023]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an embodiment in which a CCD driving device according to the present invention is embodied as the above-described frame transfer type CCD driving device will be described with reference to FIGS.
It will be described according to.

FIG. 1 is a block diagram showing the configuration of the driving device of the present embodiment and the CCD image sensor to be driven. The CCD image sensor 100 shown in FIG. 1 includes an imaging unit 110 that performs photoelectric conversion, and a storage unit 12 that temporarily stores the photoelectrically converted charges.
0, and a horizontal transfer unit 130 for outputting the charge stored in the storage unit 120 to an output unit (not shown).

Here, the imaging unit 110 and the storage unit 12
The function and operation of the horizontal transfer unit 130 will be further described. First, in the imaging unit 110, photoelectric conversion corresponding to the emitted light image is performed. Since the amount of charge generated by the photoelectric conversion is proportional to the light intensity and the storage time, an imaging signal is formed corresponding to the pattern of the charge generated for each pixel by the photoelectric conversion.

The charge photoelectrically converted for each pixel is transferred to the storage section 120 at high speed (frame shift) for each frame during the above-described vertical transfer. At the end of the frame shift, since all the charges photoelectrically converted by the imaging unit 110 have been transferred to the storage unit 120, the same charge pattern as the charge pattern generated in the imaging unit 110 is stored in the storage unit 120. A pattern is formed.
Then, the charge pattern formed in this storage portion is
The signal is output to a signal processing system (not shown) through the horizontal transfer operation described above.

At the time of this horizontal transfer, the charge pattern temporarily stored in the storage section 120 is transferred to the horizontal transfer section 130.
Thus, the signal is transferred to an output unit (not shown) at high speed line by line, and the signal transferred to the output unit is output to the signal processing system as an imaging signal of the CCD image sensor 100. Then, using this horizontal transfer period, the photoelectric conversion of the next light image by the imaging unit 110 is performed.

On the other hand, in the CCD image sensor 100, an electronic shutter operation of discharging unnecessary charges generated during a non-photoelectric conversion period of the imaging unit 110 to a drain (not shown) is also performed.

Next, a description will be given of a driving device of the present embodiment for realizing such an operation of the CCD image sensor 100. As shown in FIG. 1, the driving device according to the present embodiment includes a power supply / driving circuit IC 200, a horizontal driver 300, and a timing generator 400. The power supply / drive circuit IC 200 includes a low-voltage generation charge pump 210, a high-voltage generation charge pump 220, and a control unit 230 that constitute a power supply circuit.
And a vertical driver 240 and an electronic shutter 250 constituting a drive circuit are integrated as a one-chip IC (semiconductor integrated circuit device).

Here, the vertical driver 240 is connected to the image pickup unit 110 and the storage unit 12 of the CCD image sensor 100.
0 is a circuit for performing a vertical transfer of the charge by outputting a drive pulse (frame shift pulse) FSP to a gate (not shown) provided in each of the imaging units 110. This is a circuit for realizing the above-mentioned electronic shutter operation by outputting a drive pulse ESP for discharging the above-mentioned electric charges as needed. Each of the drive pulses FSP, E output from the vertical driver 240 and the electronic shutter 250
The output timing of the SP is determined according to the timing signals TV and TS output from the timing generator 400, respectively. The drive pulses FSP and ES output from the vertical driver 240 and the electronic shutter 250
The peak value of P, that is, the pulse voltage is the output voltage V of the low-voltage generation charge pump 210 constituting the power supply circuit.
SS (for example, −6 V) and the system power supply voltage VDD
(For example, 3.3 V).

This low voltage generating charge pump 210
Performs a boosting operation to the negative voltage side based on a boosting clock CLK1 applied from the control unit 230 which also forms a power supply circuit, and is configured as, for example, a circuit illustrated in FIG. Incidentally, in the charge pump circuit illustrated in FIG. 2, the output voltage VSS is, for example, “−” in the capacitor 211 by the boost clock CLK1 applied from the control unit 230.
3.3 V ”, and the capacitor 212 has“ −
6.6V ", and the capacitor 213 has"-
9.9V "in theory. The boosted clock CLK1 is obtained by thinning out the clock CLK output from the timing generator 400 through the controller 230. The controller 230 monitors the output voltage VSS of the low-voltage generation charge pump 210. While
The constant voltage control of the output voltage VSS is executed by controlling the thinning mode of the clock CLK so that the output voltage VSS is maintained at a predetermined voltage, for example, “−6 V”. Further, in the charge pump circuit illustrated in FIG.
3 and the like are external to the power supply / drive circuit IC 200.

The charge pump 220 for generating high voltage
Basically, the low-voltage generation charge pump 210
In the same manner, the output voltage VB of the charge pump circuit is controlled by the control unit 230 at a constant voltage. Note that this output voltage VB (for example, +8 V) is
Is used as a bias voltage for.

On the other hand, the horizontal driver 300 is connected to the storage section 120 and the horizontal transfer section 1 of the CCD image sensor 100.
30 is a circuit for performing the horizontal transfer of the electric charge by outputting a drive pulse HTP to a gate (not shown) provided in each of the gates 30. The output timing of the drive pulse HTP output from the horizontal driver 300 is determined according to the timing signal TH output from the timing generator 400. In addition, since the above-described vertical transfer operation of the CCD image sensor 100 requires a high voltage,
The generation of the driving pulse (frame shift pulse) FSP also required the boosting by the power supply circuit.
A very high voltage is not required for the horizontal transfer operation of the CD image sensor 100. Therefore, the peak value of the driving pulse HTP output from the horizontal driver 300, that is, the pulse voltage is equal to the system power supply voltage VDD.
(For example, 3.3 V).

In the present embodiment, as described above, the low-voltage generation charge pump 210 and the high-voltage generation charge pump 220 and the control unit 230 constituting the power supply circuit, and the vertical driver 240 constituting the drive circuit
By integrating the electronic shutter 250 and the electronic shutter 250 as a one-chip power supply / drive circuit IC 200, the drive circuit and the power supply circuit can be manufactured collectively.
The cost and the yield of the D drive device can be reduced. Further, when these drive circuits and power supply circuits are formed as separate semiconductor integrated circuit devices, it is inevitable to restrict the mounting of these devices on a substrate or the like. If such a problem is avoided, the size of the CCD driving device can be further reduced.

However, when the drive circuit and the power supply circuit are integrated as described above, a signal used for exciting the power supply circuit,
That is, there is a new concern that the boosted clocks CLK1 and CLK2 may cause noise, such as jumping into an image pickup signal output from the CCD image sensor 100.

Therefore, in the present embodiment, the above CC
The timing of the timing generator 400 is set so that the boost clock CLK is applied from the timing generator 400 to the controller 230 using the period in which the output of the imaging signal from the D image sensor 100 is stopped. It has been done.

The CCD image sensor 10
When driving 0, a high voltage is required due to the vertical transfer operation being performed under a high load, so even if the required voltage is secured through the power supply circuit, At the time of the vertical transfer, there is a concern that a large voltage drop may temporarily occur. In addition, when such a voltage drop occurs, it takes time to return to the desired voltage.
In any case, when such a voltage drop occurs, the effect on the operation pertaining to the vertical transfer itself cannot be ignored.

Therefore, in the present embodiment, in addition to the timing setting of the boosted clock CLK in the above-described mode, C
Prior to the vertical transfer operation of the CD image sensor 100, a power supply circuit, in particular, a low-voltage generation charge pump 210
, The voltage VSS applied to the vertical driver 240 and the electronic shutter 250 is temporarily increased excessively. In the present embodiment, this over-boost is performed by sending a precharge command signal PCG from the timing generation section 400 to the control section 230 constituting the power supply circuit immediately before the vertical transfer operation. Based on the addition of the precharge command signal PCG, this is realized by an operation of stopping the above-described constant voltage control on the low-voltage generation charge pump 210 for a predetermined period, that is, stopping the thinning out of the boost clock CLK. .

FIG. 3 shows the operation of the timing setting and the over-boosting (precharge) of the CCD driving apparatus according to the present embodiment. Referring to FIG. The timing setting operation and the over-boost operation of the drive device will be described in further detail.

First, as shown in FIG. 3A, an image signal is horizontally transferred as a signal (one line data) for each horizontal line in one captured image. In this data transfer, as is well known, a horizontal blanking period is provided each time one line of data is transferred, and line data for one captured image (interlace type is assumed. Is provided with a vertical blanking period every time one field of data is transferred. These horizontal blanking period and vertical blanking period
(Cathode Ray Tube) is provided to correspond to the scanning characteristics of the display device.

As described above, the CCD driving apparatus according to the present embodiment basically utilizes the output suspension period, that is, the horizontal blanking period and the vertical blanking period, to transfer such an image pickup signal. In the mode shown in FIG. 3B, the timing generator 400
30 is applied with the boost clock CLK. FIG. 3B shows a step-up clock CLK applied from the control unit 230 to the low-voltage generation charge pump 210.
1 shows the application mode, and in fact, the control unit 230 performs the thinning of the clock for performing the above-described constant voltage control.

As described above, the CCD image sensor 100
The excitation of the power supply circuit is performed using a period during which no image signal is output from the device, so that generation of noise due to a signal (boost clock) used for the excitation jumping into the image signal is preferably avoided. Become like

On the other hand, in the CCD driving device of the present embodiment, prior to the vertical transfer operation of the CCD image sensor 100, for example, the timing generator 400 controls the controller 230 in the manner shown in FIG. , A precharge command signal PCG is sent.

When the precharge command signal PCG is transmitted in this manner, the control unit 230 receiving the precharge command signal temporarily stops the constant voltage control, that is, the clock thinning, as described above. As a result, in the low-voltage generation charge pump 210, the output voltage V
The SS is over-boosted (precharged).
This aspect is shown as a solid line PG in FIG.

After that, when the vertical transfer operation based on the output of the frame shift pulse FSP is started in the mode shown in FIG. 3D, the large current consumption causes the output voltage VSS to change to the level shown in FIG. A voltage drop occurs as shown by the solid line VD in FIG. This voltage drop VD is continued over the execution period of the vertical transfer operation. However, in this embodiment, since the output voltage VSS is preliminarily boosted (precharged), its influence is also reduced. Even if the constant voltage control by the control unit 230 is restarted after that, the return time to the target control voltage (for example, -6 V) is greatly reduced.

The transition of the output voltage VSS indicated by the two-dot chain line in FIG. 3E shows the transition when the voltage VSS is not overcharged (precharged). In this case, the output voltage VSS is greatly attenuated by the voltage drop VD, and it takes a long time to return to the target control voltage thereafter. In this case, the effect on the vertical transfer operation itself cannot be ignored as described above.

As described above, according to the CCD driving apparatus of the present embodiment, many excellent effects as listed below can be obtained. (1) Before a voltage drop occurs due to the vertical transfer operation of the CCD image sensor 100, the voltage VSS output from the power supply circuit (the low voltage generation circuit 210) to the drive circuit (the vertical driver 240, the electronic shutter 250) temporarily Since the voltage is excessively boosted, the voltage drop is minimized. In addition, since the voltage drop is suppressed to the minimum, the time required to return to the target control voltage is greatly reduced, and the stable operation of the CCD image sensor 100 is ensured.

(2) The power supply circuit is provided with a control section 230 for converting the output voltage VSS to a constant voltage, and the constant voltage is made inactive at the time of over-boosting. Even in the case where the output voltage VSS is temporarily over-boosted, it is possible to preferably achieve both constant voltage and over-boosting of the same voltage VSS.

(3) The charge pump type booster circuit is employed for the power supply circuit, and the control unit 230 performs the above constant voltage by thinning out the clock applied to the charge pump type booster circuit. The configuration in which the thinning of the clock is prohibited during boosting makes it possible to form a compact power supply circuit,
Through the relatively simple control, it is possible to achieve both the constant voltage and the over-boost described above.

(4) By applying the driving device to a frame transfer type CCD in which a voltage drop due to a vertical transfer operation is particularly concerned, a stable operation of the frame transfer type CCD can be guaranteed.

(5) Vertical driver 2 constituting drive circuit
40 and the electronic shutter 250, and the low-voltage generation charge pump 210, the high-voltage generation charge pump 220, and the control unit 230, which constitute the power supply circuit, are integrated as a one-chip semiconductor integrated circuit (IC). CCD
It is possible to reduce the cost as a driving device and improve the yield.

(6) Also, a frame transfer type CC
Among the drive circuits of D, the vertical driver 240 and the electronic shutter 250 that require a higher withstand voltage structure as the semiconductor integrated circuit device, and the power supply circuit that originally requires the higher withstand voltage structure as the semiconductor integrated circuit device By being integrated as one chip, it is easy to realize the structure or the manufacturing method.

(7) Since the power supply circuit is excited using the period during which the output of the image pickup signal from the CCD image sensor 100 is stopped, each of the above circuits is used as a one-chip semiconductor integrated circuit device. Even in the case of integration, it is possible to avoid generation of noise due to the integration.

(8) As shown in FIG. 3, there is usually a large blank period, especially before and after the vertical transfer is performed, during the period in which the image signal output from the CCD image sensor 100 is stopped. Therefore, as described above,
Even when the power supply circuit is excited using the period in which the output of the imaging signal from the CCD image sensor 100 is stopped, an efficient over-boost (precharge) using the blank period to the maximum is performed. .

The above embodiment may be modified and implemented as follows. In the above embodiment, the precharge command signal PC
It is assumed that G is transmitted in the manner shown in FIG. 3C, but the fall time is not limited to the same manner. In short, the precharge command signal PCG only needs to keep its rising period at least until the output start timing of the frame shift pulse FSP.

In the above embodiment, the vertical driver 240 and the electronic shutter 25 constituting the power supply circuit and the drive circuit
Although the case where 0 and 1 are configured as a one-chip IC has been illustrated, the present invention can be similarly applied to a case where each circuit is configured as a separate chip.

In this case, depending on the arrangement of these circuits, the mounting mode, etc., the excitation timing of the power supply circuit,
There is no need to limit the excitation period. In the above embodiment, the case where the above-described driving device is applied to the frame transfer type CCD has been described. However, the CCD driving device according to the present invention can be similarly applied to the above-mentioned interline type CCD.

In the above embodiment, the charge pump type booster circuit is used as the power supply circuit, but other booster circuits can be used as appropriate. Further, in that case, as a method of the constant voltage control, a method according to the booster circuit employed can be employed. What is essential is that the boosting circuit only needs to be a circuit having a boosting ability higher than the voltage controlled by the constant voltage in anticipation of the over boosting.

Further, the power supply circuit does not necessarily need to be controlled at a constant voltage, but may be any one provided with a means for temporarily increasing the output voltage. In the above embodiment, for example, “+3.3 V” is used as the system power supply voltage VDD, and “−6 V” is used as the output voltage VSS of the power supply circuit. However, these voltages can be set arbitrarily. is there. Further, in the above-described embodiment, the case where the imaging signal is transferred in synchronization with the CRT scanning characteristic has been described. However, the transfer mode can be arbitrarily changed.

[Brief description of the drawings]

FIG. 1 is a block diagram showing one embodiment of a CCD driving device according to the present invention.

FIG. 2 is a circuit diagram showing a specific example of the low-voltage generation charge pump of the embodiment.

FIG. 3 is a timing chart showing an operation mode of the embodiment.

[Explanation of symbols]

100: CCD image sensor, 110: imaging unit, 12
0: accumulation unit, 130: horizontal transfer unit, 200: power supply / drive circuit IC, 210 low-voltage generation charge pump, 220
... Charge pump for generating high voltage, 230 ... Control unit, 24
0: vertical driver, 250: electronic shutter, 300: horizontal driver, 400: timing generator.

Claims (6)

[Claims] 1. A CCD driving apparatus comprising: a driving circuit for applying a charge transfer pulse to a CCD; and a power supply circuit for applying a voltage required for generating the pulse to the driving circuit. An overdrive means for temporarily overboosting the voltage applied from the power supply circuit to the drive circuit prior to the transfer operation. 2. The power supply circuit includes constant voltage control means for converting a voltage applied to the drive circuit into a constant voltage, and the over-boost means makes the constant-voltage control means inactive during the over-boost operation. The CCD driving device according to claim 1. 3. The power supply circuit includes a charge pump type booster circuit, and the constant voltage control means reduces a voltage applied to the drive circuit by thinning out a clock applied to the charge pump type booster circuit. 3. The method according to claim 2, wherein the voltage is increased, and the over-boost means temporarily over-boosts the voltage applied to the drive circuit by inhibiting the thinning of the clock by the constant voltage control means. CCD drive. 4. The CCD driving device according to claim 1, wherein said CCD has an image pickup unit for performing photoelectric conversion and a storage unit for temporarily storing the photoelectrically converted electric charges. A CCD which is a frame transfer type CCD which is separately disposed, wherein the driving circuit includes a vertical driver which transfers charges from the imaging unit to the storage unit of the CCD collectively. 5. The CCD driving device according to claim 1, wherein said driving circuit and said power supply circuit are integrated as a one-chip semiconductor integrated circuit device.
D drive. 6. The CCD driving apparatus according to claim 5, wherein the power supply circuit is excited using a period during which the output of the imaging signal from the CCD is stopped.