Auto variable anti-blooming bias control circuit and method

This invention relates to auto variable anti-blooming bias circuits which can automatically vary the anti-blooming bias of the CCD image sensors according to the intensity of light incident thereon. The circuits comprise a DC voltage generation part for receiving signals fed-back from the output terminal of the CCD image sensor and generating DC voltage by averaging the applied signals, an input voltage generation part for receiving DC voltage transmitted from the DC voltage generation part and generating variable input voltage according to the received DC voltage, and an anti-blooming bias generation part for receiving the variable input voltage transmitted from the input voltage generation part and reference voltage and transmitting to the input, terminal of the CCD image sensor as an anti-blooming bias after comparing the two signals.

FIELD OF THE INVENTION 
This invention relates to CCD (Charge Coupled Device) image sensors, more 
particularly to auto variable anti-blooming bias circuits which can 
automatically vary the anti-blooming bias of the CCD image sensors and 
according to the intensity of light incident thereon. 
BACKGROUND OF THE INVENTION 
Shown in FIG. 1 is a section of a conventional CCD image sensor. 
A conventional CCD sensor includes a n-type substrate 61, a p-type well 62 
formed on the n-type substrate 61, a n+-type photo diode 63 formed on the 
p-type well 62 for generating signal charge, a n+-type VCCD (Vertical 
Charge Coupled Device) area 64 formed on the p-type well 62 spaced a 
certain distance from the photo diode 63 and transmitting the signal 
charge transmitted from the photo diode 63 to a HCCD (Horizontal Charge 
Coupled Device) area (not shown), a p++ layer 65 formed on the surface of 
the photo diode 63 for forming a electric potential barrier, a transfer 
gate 66 for transmitting the signal charge generated in the photo diode 63 
to the VCCD area 64, a HCCD area transmitting the signal charge from the 
VCCD area 64 to a power amplifier(not shown), a power amplifier(not shown) 
for transmitting an image signal Vout after receiving the signal charge 
transmitted from the HCCD area, a polygate 67 for transmitting the signal 
charge from the VCCD area 64 to the HCCD area(not shown), an insulation 
layer 68 formed on the substrate 61 between the polygate 67 and the 
transfer gate 66 for insulating the polygate 67 from the transfer gate 66, 
and a p++-type channel stop area 69 for insulating between cells by 
forming high electric potential barriers. 
In a CCD image sensor having a construction as shown in FIG. 1, the photo 
diode 63, on receiving light, generates a signal charge corresponding to 
the intensity of the light, which signal charge is transmitted to the VCCD 
area 64 in response to the signal applied to the transfer gate 66. This 
signal charge transmitted to the VCCD area 64 is transmitted to the HCCD 
area in response to the signal applied to the polygate 67, which is 
transmitted to the power amplifier in response to the signal applied to 
the HCCD area and, finally, output as a video signal. 
However, in the event that the photo diode 63 receives a light having too 
high intensity, the photo diode generates excessive signal charge. 
The image signal output of this excessive signal charge exhibits the 
blooming phenomena. 
As for the methods for eliminating such excessive signal charge, there are 
methods in which the excessive signal charge is made to escape in a 
direction opposite to the VCCD area 64 through an overflow drain region 
formed between the channel stop area 69 and the photo diode 63, or toward 
the substrate by applying anti-blooming bias to the substrate 61. 
In general, the blooming is controlled by applying bias to a substrate of 
integrated solid state image elements, which anti-blooming bias is of 
voltage of direct current, generally above 5 V and below 18 V. 
However, for controlling a photo accumulation period of time, electrical 
shutter pulses having a magnitude above 20 V can be applied thereto. 
Shown in FIG. 2 is a conventional manual variable anti-blooming DC bias 
circuit. 
The conventional manual variable anti-blooming DC bias circuit has 
resistances R11 and R12 and a variable resistance VR11, wherein the 
variable resistance VR11 is adjusted to apply desired anti-blooming bias 
voltage to the CCD image sensor 60; the 24 V is divided by the resistances 
R11 and R12 and the variable resistance VR11 adjusted by a user, which is 
applied to the substrate SUB of the CCD image sensor 10. 
Therefore, the user can prevent blooming due to the excessive light 
incident on the CCD image sensor by adjusting the variable resistance VR11 
image sensor so as to apply appropriate anti-blooming bias to the CCD 
image sensor. 
Referring to FIG. 2, as the 24 V power is fed through a circuitry 
connection, it can be more or less unstable, but with a circuit as shown 
in FIG. 3, a stable power can be fed to a CCD image sensor. 
Shown in FIG. 3 is an example of application of the manual variable 
anti-blooming bias circuit shown in FIG. 2. 
In the circuit shown in FIG. 3, when the stable power(15 V) is applied to 
the manual variable anti-blooming bias circuit, a constant voltage divided 
by resistances R13 and R14 is applied to a transistor Q11 at a base 
terminal thereon. 
And the 24 V transmitted from a vertical operation part 40 is adjusted by 
the variable resistance VR11 and applied therefrom to a transistor Q12 at 
a base terminal thereon. 
According to operation of the transistor Q12, a desired anti-blooming bias 
can be applied to an input terminal SUB of the CCD image sensor 60 through 
a transistor Q13. 
In this time, in case the 24 V power is unstable, a permanent direct 
current anti-blooming bias is fed to the CCD image sensor 60 at the input 
terminal SUB thereon due to a continuous induction of a current flowing in 
the transistor by a continuous current flowing in the transistor Q11. 
Accordingly, the CCD image sensor 60 transmits anti-blooming bias, an 
output voltage Vout based on which is transmitted to a signal processor 80 
through terminal OUT and, consequently, the signal processor 80, 
processing the signal transmitted from the CCD image sensor 60, transmits 
video signals. 
On the other hand, in case pulses above 15 V are transmitted from one 
output terminal Vsub of the vertical driving part 40, shutter pulses of 15 
V DC are applied to the input terminal SUB of the CCD image sensor 60 
under a condition that the 15 V voltage is set up by the diode 11, 
irrespective of the anti-blooming bias transmitted from a anti-blooming 
bias circuit 20. 
Shown in FIG. 4 is potential distributions based on anti-blooming bias VOFD 
in accordance with the CCD image sensor of FIG. 1. 
The higher the anti-blooming bias VOFD, the lower a electric potential 
barrier toward the substrate 61 making a signal saturation quantity ie., 
the quantity of signal charge which can be accumulated in the photo diode 
63 less. 
Shown in FIG. 5 is a graph showing relation between anti-blooming bias VOFD 
and smear noise in accordance with the CCD image sensor shown in FIG. 1, 
wherein it shows that, when anti-blooming bias VOFD becomes higher, ie., 
the signal saturation quantity becomes less, the smear noise increases. 
Shown in FIG. 6 are graphs showing relation between the intensity of light 
according to anti-blooming bias and the output voltage of the CCD image 
sensor of FIG. 1, wherein it shows that when anti-blooming bias VOFD 
becomes higher, the electric potential toward the substrate is formed 
lower, accumulating less signal saturation quantity, so that, when little 
quantity of light is incident, desired output voltage Vout can not be 
obtained. 
The lower the anti-blooming bias VOFD become, the higher the electric 
potential barrier toward the substrate is formed making the signal 
saturation quantity greater resulting to obtain the desired output voltage 
Vout even with little quantity of light. 
Referring to FIGS. 4 to 6, in case a user adjusts the variable resistance 
VR11 of FIG. 2 setting the anti-blooming bias VOFD at a higher value of 
VOFD1 in advance, even though an excessive signal charge would be 
generated in the photo diode 63 when excessive light is incident thereto, 
it is possible to prevent blooming because the sufficient excessive signal 
charge is made to escape to the substrate 61. 
However, it raises a problem of making the smear noise increase due to the 
high anti-blooming bias VOFD. on the other hand, when little quantity of 
light is incident thereon, even though desired quantity of signal charge 
be generated, due to low electric potential barrier formed toward the 
substrate by the high anti-blooming bias, accumulating little quantity of 
charge saturation, the signal charge is made to escape to the substrate 
creating a problem of obtaining no desired video signal. 
In the meantime, in case the variable resistance VR11 of FIG. 2 is set the 
anti-blooming bias VOFD at a lower value of VOFD3 in advance, the 
anti-blooming bias VOFD becomes lower making the smear noise decrease. 
However, because the anti-blooming bias is low, electric potential barrier 
is formed high preventing the excessive signal charge from escaping to the 
substrate 61, but making it transmitted to the VCCD area 64. Consequently, 
it raises a problem of developing blooming. 
SUMMARY OF THE INVENTION 
The object of this invention for solving the problems developed by setting 
anti-blooming bias in advance as the foregoing description in conventional 
way, is providing an auto variable anti-blooming bias circuit which can 
improve photo sensitivity in weak light and prevent blooming in intense 
light by varying anti-blooming bias automatically according to the 
intensity of light. 
These and other objects and features of this invention can be achieved by 
providing a CCD image sensor having an input terminal and an output 
terminal, including a DC voltage generation part for receiving signals 
fed-back from the output terminal of the CCD image sensor and generating 
DC voltage by averaging the signals transmitted from the CCD image sensor, 
an input voltage generation part for receiving DC voltage transmitted from 
the DC voltage generation part and generating variable input voltage 
according to the received DC voltage, and an anti-blooming bias generation 
part for receiving the variable input voltage transmitted from the input 
voltage generation part and reference voltage and transmitting to the 
input terminal of the CCD image sensor as an anti-blooming bias after 
comparing the two signals transmitted thereto.

DETAILED DESCRIPTION OF THE INVENTION 
An embodiment of this invention is to be explained in detail hereinafter, 
referring the attached drawings. 
FIG. 7 is a block diagram of an auto variable anti-blooming bias circuit in 
accordance with this invention. 
Referring to FIG. 7, an auto variable anti-blooming bias circuit 70 is a 
circuit for varying anti-blooming bias automatically by comparing signals 
Vout fed back from a CCD image sensor 60 to a reference voltage Vref, 
including a DC voltage generation part 71 for generating DC voltage by 
integrating intermediate output signals VIRIS of a signal processor 80 for 
processing output signals Vout from the CCD image sensor 60, an input 
voltage generation part 72 for generating and transmitting variable input 
voltage according to the DC voltage transmitted from the DC voltage 
generation part 71, and an anti-blooming bias generation part 73 for 
receiving the output of the input voltage generation part 72 and the 
reference voltage as applied signals, varying anti-blooming bias according 
to the variable input voltage transmitted from the input voltage 
generation part 72, and transmitting therefrom to a receiving terminal SUB 
of the CCD image sensor 60. 
FIG. 8 is a detailed drawing of the auto variable anti-blooming bias 
circuit of FIG. 7. 
Referring to FIG. 8, the DC voltage generation part 71 having two 
integrator (low pass filter) IT21 and IT22, obtains an average value of 
the signals VIRIS transmitted from the signal processor 80, and transmits 
voltage converted into DC. 
Each integrator IT21 and IT22 has a resistance and a condenser R21 and C21, 
and R22 and C22. 
The input voltage generation part 72 serves to apply the input voltage 
varying depending on the DC voltage transmitted from the DC voltage 
generation part 71 to the anti-blooming bias generation part 73, and 
includes a transistor having a base terminal the output signal of the DC 
voltage generation part 71 is applied thereto, and resistances R23 and R24 
each connected to a collector and an emitter of the transistor Q21 to 
divide the source voltage 24 to transmit variable input voltage Vin. 
The anti-blooming bias generation part 73 serves to generate and transmit 
the anti-blooming bias VOFD varying depending on the difference between 
the variable input voltage Vin and the reference voltage Vref, and 
includes differential amplification transistors Q22 and Q23 each having a 
base terminal the variable input voltage Vin from the input voltage 
generation part 72 and the reference voltage Vref are transmitted thereto, 
respectively, current driving transistors Q24 and Q25 connected to a 
collector of the transistors Q22 and Q23, respectively, and a transistor 
Q26 operated by the differentially amplified signals applied to the 
transistors Q22 and Q23 at each of base terminals thereon. 
The operation of an auto variable anti-blooming bias circuit in accordance 
with this invention as per the foregoing description is to be explained 
hereinafter. 
The output signal Vout of the CCD image sensor 60 is applied to the signal 
processor 80, and the intermediate output signal VIRIS of the signal 
processor 80 is applied to the DC voltage generation part 71 of the auto 
variable anti-blooming circuit 70. 
The DC voltage generation part 71 integrates the intermediate output 
signals VIRIS of the signal processor 80 through the two integrators IT21 
and IT22 each having the resistor and the condenser R21 and C21, and R22 
and C22 to obtain average value. 
The DC voltage obtained in the DC voltage generation part 71 is transmitted 
to the base terminal of the transistor Q21 of the input voltage generation 
part 72. 
The transistor Q21, operated by the output signal of the DC voltage 
generation part 71 applied on the base terminal, in case the intermediate 
output signal VIRIS of the signal processor 80 is great due to little 
quantity of light incident to the CCD image sensor 60, makes low 
anti-blooming bias applied on the CCD image sensor. 
Accordingly, the intermediate output signal VIRIS of the signal processor 
80 become high making the output signal of the DC voltage generation part 
71 high enough to operate the transistor Q21. 
The source voltage 24 V is divided by the resistances R23 and R24, which is 
applied to the base terminal of the transistor Q23 of the anti-blooming 
bias generation part 73 as a variable input signal Vin. 
The anti-blooming bias generation part 73, being applied with low variable 
input signal Vin having little difference with the reference voltage Vref, 
applies low anti-blooming bias VOFD to the CCD image sensor 60 at the 
input terminal SUB through the transistor Q26. 
In this time, the reference voltage is set by a user in advance using a 
variable resistance. 
Therefore, in case little quantity of light is to incident to the CCD image 
sensor 60, the output signal Vout of the CCD image sensor is fed-back 
through the signal processor 80, and the low anti-blooming bias VOFD 
obtained automatically according to the output signal of the CCD image 
sensor 60 is transmitted to the CCD image sensor 60. 
On the other hand, in case an excessive light is incident to the CCD image 
sensor 60 forming a low intermediate output signal VIRIS of the signal 
processor 80, high anti-blooming bias is made to be applied to the CCD 
image sensor 60. 
In this case, because the intermediate output signal VIRIS of the signal 
processor 80 is low, the output signal of the DC voltage generation part 
71 becomes low making operation of the transistor Q23 impossible. 
Consequently, the source voltage 24 V is applied to the transistor Q23 of 
the anti-blooming bias generation part 73 at the base terminal through the 
resistance as a variable input signal Vin. 
The anti-blooming bias generation part 73 is applied with high variable 
input signal Vin having great difference with the reference voltage Vref, 
according to which, a high anti-blooming bias VOFD is applied to the CCD 
image sensor 60 at the input terminal SUB through the transistor Q26. 
Therefore, in case an excessive light is incident to the CCD image sensor 
60, the output signal Vout of the CCD image sensor 60 is fed-back through 
the signal processor 80. 
In the meantime, also in case middle or slightly bright light is incident 
to, an appropriate anti-blooming bias VOFD can be set automatically in 
response to the light. 
FIG. 9 shows an example of application of the auto variable anti-blooming 
bias circuit of FIG. 7. 
When the auto variable auto-blooming bias circuit 70 is applied with the 
source voltage 15 V, a user can adjust the variable resistance VR21 to set 
the reference voltage Vref. 
In this time, when the output signal VIRIS of the signal processor 80 the 
output signal Vout of the CCD image sensor 60 is transmitted thereto as an 
input signal, is applied to the auto variable anti-blooming bias circuit 
70 of this invention, the auto variable anti-blooming bias circuit 70 of 
this invention transmits an appropriate anti-blooming bias VOFD to the CCD 
image sensor at the input terminal SUB in response to the output signal 
VIRIS of the signal processor 80. 
Accordingly, the CCD image sensor 60 can generate desired output signal 
Vout in response to an appropriate anti-blooming bias VOFD transmitted 
from the auto variable anti-blooming bias circuit 70. 
Meantime, in case pulses above 15 V is transmitted from one output terminal 
Vsub of the vertical driving part 40, shutter pulses of DC 15 V are 
applied to the CCD image sensor 60 at the input terminal SUB under the 
condition that 15 V voltage be set up by the diode D21, irrespective of 
the anti-blooming bias transmitted from the auto variable anti-blooming 
bias circuit 70 of this invention. 
In accordance with an auto variable anti-blooming bias circuit of this 
invention as explained in the foregoing description, it is possible to 
reduce smear than before improving the visibility of images by feeding 
back the signals transmitted from a CCD image sensor and adjusting the 
anti-blooming bias automatically according to the fed-back signals, making 
the anti-blooming bias decreased in weak light improving photo 
sensitivity, increased in intense light improving blooming, and setting an 
appropriate anti-blooming bias in middle or slightly bright light. 
Although the invention has been described in conjunction with specific 
embodiments, it is evident that many alternatives and variations will be 
apparent to those skilled in the art in light of the foregoing 
description. Accordingly, the invention is intended to embrace all of the 
alternatives and variations that fall within the spirit and scope of the 
appended claims.