Vertical blanking circuit and bias clamp boost supply

Details Vertical blanking circuit and bias clamp boost supply Vertical blanking circuit and bias clamp boost supply

2000-06-22

2002-04-09

Wong, Don (Department: 2821)

Electric lamp and discharge devices: systems

Cathode ray tube circuits

Cathode-ray deflections circuits

C348S379000

Reexamination Certificate

active

06369527

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to boosted power supplies of the type commonly found in a display device using a cathode ray tube (CRT), and in particular, to a circuit for generating such a boosted power supply potential using the vertical blanking signal pulses.
2. Description of the Related Art
A computer system essentially comprises a system unit housing a microprocessor, computer memory, and various other support logic, as well as various input/output (I/O) devices which are connected to the system unit and enable a user to intelligently interact with the system unit. Examples of various types of input devices include a keyboard, a mouse, a trackball, and a pen computer, as well as others. The primary output device in a computer system include a video display monitor (video monitor).
Video monitors, such as for use with digital computers, include a cathode ray tube (CRT), and driver circuitry including a video amplifier. The CRT includes three primary color cathode ray guns which are manipulated to converge on a screen that produces the color image. The three guns produce converged scanning rasters having red, green and blue fields which combine to produce white light. The typical scanning raster is a left to right horizontal and top to bottom vertical scan operated in accordance with the Video Electronics Standards Association (VESA) requirements.
A conventional monitor amplifier circuit
100
for displaying screen control states of a monitor is illustrated in FIG.
1
. In general, low level color video signals blue b, red r, and green g from a video source (not shown), such as a personal computer (PC) are provided to respective video preamplifiers
101
a
,
101
b
and
101
c
. These preamplifiers in turn provide the respective video signals blue b, red r, and green g, via buffer amplifiers BUFF
11
, BUFF
12
, BUFF
13
, to video output driver stages
103
a
,
103
b
,
103
c
which supply high level amplified color video signals B, R and G to respective cathode intensity control electrodes of a CRT (not shown). As can be seen, in
FIG. 1
, each video signal blue b, red r, and green g is applied to a respective amplifier circuit AMP
11
-AMP
13
, each of which includes four main components: a video preamplifier
101
a
-
101
c
, a bias/brightness circuit
105
a
-
105
c
, a video amplifier
103
a
-
103
c
, and a clamp amplifier
107
a
-
107
c
. Since the monitor amplifier circuits AMP
11
-AMP
13
are identical in structure and operation, only the circuit operation of amplifier circuit AMP
12
for the red video signal r will be discussed by referring to FIG.
2
.
As can be seen in
FIG. 2
, the four main components of monitor amplifier circuit AMP
12
are numbered
1
-
4
, number
1
being bias/brightness circuit
105
b
, number
2
being video preamplifier
101
b
, number
3
being clamp amplifier
107
b
, and number
4
being video amplifier
103
b.
Operation of this red video channel r is as follows. Terminal
10
constitutes the red video signal input r which originates from an external source, such as a PC. Capacitor CAP
12
couples the red video signal r to the noninverting input of video preamplifier
101
b.
At this point, the amplification of red video signal r is controlled by a single-throw switch SW
12
and a video clamp pulse VC. In any video signal, the clamp pulses are developed just following the synchronization pulses and make it possible to restore the voltage reference level of a video signal, in this case red video signal r. This clamp pulse VC is located in the “back porch” of the composite red video signal r and is employed to operate switch SW
12
. When clamp pulse VC is high, switch SW
12
is closed. Thus, each time the CRT scans a horizontal line, capacitor CAP
12
will be charged to black level reference voltage VREF, which is the potential reference level of the black region of an image. This level corresponding to the black color in an image makes it possible to restore the potential reference level of the red video signal r, this level having disappeared on account of the presence of the input capacitor CAP
12
.
On the other hand, when video clamp pulse VC is low, switch SW
12
opens and red video signal r is applied directly to video preamplifier
101
b
, which is shown in
FIG. 2
as a unity gain amplifier. Thus, red video signal r is passed through video preamplifier
101
b.
At this point, the amplification of red video signal r is controlled by double-throw switch SW
14
and signal
11
. Signal
11
represents a horizontal blanking pulse that is derived from the display scanning circuits in a manner well known in video display monitors. This signal
11
is employed to operate a double-throw switch SW
14
which switches the input IN
12
to output buffer BUFF
12
, between the output of video preamplifier
101
b
and circuit ground. When signal
11
is high, input IN
12
couples to video preamplifier
101
b
, the output of which is inversely amplified by video amplifier
103
b
to a voltage level suitable for driving a CRT and then applied to cathode electrodes of the CRT. On the other hand, when signal
11
is low, input IN
12
is at circuit ground and the CRT is blanked by driving the output of the video amplifier
103
b
to a high level.
During operation of this amplifier circuit AMP
12
, output coupling capacitor CAP
22
changes the DC level at the CRT cathode. Thus, a bias clamp circuit
105
b
is used to restore the DC level at the CRT cathode through a series diode D
11
. Bias clamp circuit
105
b
outputs a bias clamp DC voltage which, in a typical video monitor, is usually factory set. This bias clamp voltage reinstates the charge on output capacitor CAP
22
only during the blanking period. The voltage is preset, typically, in the range of 100-140 volts to compensate for differences in CRT cathode bias levels, required by each cathode in the CRT to set the black level. In addition, an adjustable voltage component of typically +/−10 volts may be added to this bias level to accomplish the ‘brightness’ feature, such that the black level can be manually adjusted by an external source. Thus, for example, increased image brightness results when the bias clamp voltage is reduced. This results in a less positive DC bias potential at the red cathode and a related increase in image brightness.
Although the conventional monitor amplifier system
100
amplifies and conditions video signals to drive the CRT, there are several disadvantages to the circuit configuration. Referring again to
FIG. 1
, it can be seen that this architecture involves a significant number of interconnections. Such a low level of integration has several disadvantages. First, the circuit architecture requires a large printed circuit board (PCB), yielding higher design costs due to shielding for the radio frequency (RF) interface. Second, the conventional circuit architecture has inferior high frequency performance due to long interconnection traces between the components and due to electromagnetic interference (EMI) stemming from long signal lines and large signal swings across the video interface between each preamplifier
101
a
-
101
c
and corresponding video amplifier
103
a
-
103
c
. Third, the high number of interconnections require higher pin count packages which are undesirably large and expensive. Finally, the complexity of the system
100
due to the low level of integration results in longer design time.
Referring to
FIG. 3
, a conventional video display circuit
200
a
shown in more detail includes, as three of its primary integrated circuits, a pre-amplifier
202
, an on-screen display (OSD) generator and pulse width modulation (PWM) circuit
204
, and a CRT driver
206
, interconnected substantially as shown. The pre-amplifier
202
clamps and amplifies the component blue
201
b
, green
201
g
and red
201
r
video signals, while providing gain and contrast control as well as the ability to introduce OSD characters. The OSD and PWM circuit
204
receives the horizontal
201
h
and vertical
201

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