National Semiconductor LM1270 Technical data
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LM1270
Hi-Brite 200 MHz I
2
C Compatible RGB Image Enhancer
with Video Auto Sizing
LM1270 Hi-Brite 200 MHz I
May 2003
General Description
The LM1270 is a supplementary IC to the low cost chipset
(LCCS). It modifies the video stream prior to the LCCS
preamplifier. The key function of the LM1270 is image processing for enhancing pictures or motion video. Image enhancement is accomplished by special intermediate frequency boosting. 200 MHz bandwidth assures minimal
impact on the video change when adding the LM1270.
Other functions of the LM1270 are:
1. Measures the length of the active video and its front
porch time with respect to the horizontal flyback and
vertical sync for auto alignment information.
2. Calibration mode for determining the start of the image
coordinates.
3. Data receiver for receiving data encoded on video.
4. Window highlight with respect to the horizontal and vertical sync, used with the image enhancing feature.
5. Video source selection, allowing the choice of two analog inputs.
Block and Connection Diagram
Features
n Ideal companion IC to the LCCS to provide image
enhancing for viewing pictures or motion video
n Programmable clamp generator
n Programmable blank generator
n Video detection controlled by I
allowing easy and accurate Hi-Brite window calibration
n Programmable window Hi-Brite function
n Programmable image sharpness control (TV emphasis),
adjustable for different line rates
n Measures video position with respect to the horizontal
flyback and vertical sync pulses, providing necessary
information for auto-sizing
n Emphasis done either inside or outside the window
2
C compatible bus,
Applications
n Any monitor designed with the LCCS chip set
n Standard 0.7V video output assures compatibility with
any video pre-amp
2
C Compatible RGB Image Enhancer with Video Auto Sizing
20063501
FIGURE 1. Order Number LM1270N
See NS Package Number N24D
© 2003 National Semiconductor Corporation DS200635 www.national.com
Absolute Maximum Ratings (Notes 1,
3)
LM1270
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage, Pins 9 and 18 6.0V
Peak Video Output Source Current
Thermal Resistance to Ambient (θ
Thermal Resistance to Case (θ
Junction Temperature (T
ESD Susceptibility (Note 4) 2 kV
ESD Machine Model (Note 5) 200V
Storage Temperature −65˚C to +150˚C
Lead Temperature (Soldering, 10 sec.) 265˚C
(Any One Amp) Pins 19, 20 or 21 28 mA
Voltage at Any Input Pin
(V
)V
IN
Power Dissipation (P
)
D
+0.5>V
CC
IN
(Above 25˚C Derate Based
and TJ) 2.4W
on θ
JA
>
−0.5V
Operating Ratings (Note 2)
Temperature Range 0˚C to +70˚C
Supply Voltage (V
Video Inputs 0.0V
CC
Active Video Signal Electrical Characteristics
Unless otherwise noted: TA= 25˚C, VCC= +5V, VIN= 0.7V, CL= 5 pF, Video Signal Output = 0.7V
Symbol Parameter Conditions
I
CC
I
CC-PS
LE Linearity Error Triangular Signal Input Source
V
O BLK TYP
V
O BLK A-B
V
O WHITE-MAX
t
r
OS
R
t
f
OS
F
f (−3 dB) Video Amplifier Bandwidth
V
10 kHz Video Amplifier 10 kHz Isolation (Note 14) −70 dB
SEP
V
10 MHz Video Amplifier 10 MHz Isolation (Note 14) −50 dB
SEP
A
V MAX
A
V 1/2
A
V MIN
A
V MATCH
A
V TRACK
VID
THRESHOLD
V
CLAMP MAX
V
CLAMP MIN
I
CLAMP LOW
Maximum Supply Current Both Supplies, RL=∞(Note 8) 165 235 mA
Maximum Supply Current, Power
Both Supplies, RL=∞(Note 8)
Save Mode
(Note 9)
Typical Video Black Level Output Test Setting 4, No AC Input Signal 1.2 1.4 1.6 VDC
Typical Video Black Level
No AC Input Signal
Difference A-B
White Level Video Output Voltage Video in = 0.7V 0.9 1.0 1.1 V
Rise Time 10% to 90%, AC Input Signal
(Note 10)
Overshoot (Rising Edge) AC Input Signal (Note 10) 6 %
Fall Time 90% to 10%, AC Input Signal
(Note 10)
Overshoot (Falling Edge) AC Input Signal (Note 10) 8 %
V
=1V
O
P-P
(Note 12)
Maximum Voltage Gain Reg. 09 & 0A = 7Fh, AC Input
Signal
Contrast@50% Level Reg. 09 & 0A = 40h, AC Input
Signal
Maximum Contrast Attenuation Reg. 09 & 0A = 00h, AC Input
Signal
Absolute Gain Match@A
V MAX
Reg. 09 & 0A = 7Fh, AC Input
Signal
Gain Change between Amplifiers Tracking When Changing A
(Note 11)
A
V 1/2
V MAX
Video Threshold Normal Operation 80 mV
Clamp Gate Low Input Voltage Default Mode, Clamp Comparators
Off
Clamp Gate High Input Voltage Default Mode, Clamp Comparators
On
Clamp Gate Input Current Default Mode, V24= 0V −1.6 −1.4 −1.1 mA
) 51˚C/W
JA
) 32˚C/W
JC
) 150˚C
J
& PLL VCC) 4.75V<V
.
P-P
Min
(Note 7)
Typ
(Note 6)
<
V
IN
Max
(Note 7)
CC
<
15 25 35 mA
5%
−0.05 0 0.05 VDC
1.9 ns
2.0 ns
200 MHz
1.3 1.4 1.5 V/V
−5 dB
−10 dB
±
0.5 dB
to
±
0.5 dB
2.2 V
2.8 V
<
1.0V
5.25V
P-P
Units
www.national.com 2
Active Video Signal Electrical Characteristics (Continued)
Unless otherwise noted: TA= 25˚C, VCC= +5V, VIN= 0.7V, CL= 5 pF, Video Signal Output = 0.7V
Symbol Parameter Conditions
I
CLAMP HIGH
t
PW CLAMP
t
CLAMP-VIDEO
V
CLAMP HIGH OUT
I
CLAMP HIGH OUT
V
CLAMP LOW OUT
I
CLAMP LOW OUT
R
IN-VIDEO
V
REFREXT
V
SPOT
Clamp Gate Input Current Default Mode, V24= 4V 0.5 0.7 0.9 mA
Back Porch Clamp Pulse Width Default Mode (Note 15) 200 ns
End of Clamp Pulse to Start of
Active Video
Default Mode, Limit Is Guaranteed
by Design
Clamp Gate High Output Voltage Internal Clamp Is Output at Pin 24 3.2 V
Clamp Gate High Output Current Internal Clamp Is Output at Pin 24 10 µA
Clamp Gate Low Output Voltage Internal Clamp Is Output at Pin 24 0.4 V
Clamp Gate Low Output Current Internal Clamp Is Output at Pin 24 −10 µA
Input Resistance 20 MΩ
V
REFREXT
Output Voltage 10 kΩ, 1% Resistor; Pin 23 to GND 1.26 1.44 1.64 V
Spot Killer Voltage VCCAdjusted to Activate 3.40 4.0 4.25 V
(Note 7)
Min
50 ns
.
P-P
Typ
(Note 6)
Max
(Note 7)
External Interface Signals Electrical Characteristics
Unless otherwise noted: TA= 25˚C, VCC= +5V, VIN= 0.7V, CL= 5 pF, Video Output = 0.7 V
Symbol Parameter Conditions
(I2C) I2C compatible Low Input Voltage SDA or SCL Inputs −0.5 1.5 V
V
L
V
(I2C) I2C compatible High Input Voltage SDA or SCL Inputs
H
(I2C) I2C compatible Low Input Current SDA or SCL, Input Voltage = 0.4V
I
L
I
(I2C) I2C compatible High Input Current SDA or SCL, Input Voltage = 4.5V
H
V
(I2C) I2C compatible Low Output Voltage IO= 3 mA 0.5 V
OL
I
IN THRESHOLD
I
IN-OPERATING
IINH-Flyback Detection Threshold −35 µA
Minimum —Insure Normal
Operation
Maximum —Should Not Exceed in
Lowest Operating Horizontal
Frequency in Given Application
(Note 15)
Normal Operation
I
IN FLYBACK
Peak Current during Flyback
Period, Recommended Design
Range
Operating Range for All Horizontal
Scan Frequencies, Maximum
Current Should Not Exceed 4 mA
(Note 15)
Note 1: Limits of Absolute Maximum Ratings indicate limits below which damage to the device must not occur.
Note 2: Limits of operating ratings indicate required boundaries of conditions for which the device is functional, but may not meet specific performance limits.
Note 3: All voltages are measured with respect to GND, unless otherwise specified.
Note 4: Human body model, 100 pF capacitor discharged through a 1.5 kΩ resistor.
Note 5: Machine Model ESD test is covered by specification EIAJ IC-121-1981. A 200 pF capacitor is charged to the specified voltage, then discharged directly into
the IC with no external series resistor (resistance of discharge path must be under 50Ω).
Note 6: Typical specifications are specified at +25˚C and represent the most likely parametric norm.
Note 7: Tested limits are guaranteed to National’s AOQL (Average Outgoing Quality Level).
Note 8: The supply current specified is the quiescent current for V
the supply current is used by the pre-amp.
Note 9: Linearity Error is the variation in step height of a 16 step staircase input signal waveform with a 0.7 V
with each step approximately 100 ns in width.
Note 10: Input from signal generator: t
generator response have been removed from the output rise and fall times.
Note 11: ∆A
gain change between any two amplifiers with the contrast set to A
gains might be 0.1 dB, −0.1 dB, and −0.2 dB and change to −5.2 dB, −4.9 dB and −4.7 dB respectively for contrast set to A
of −5.0 dB with a tracking change of
Note 12: Adjust input frequency from 10 MHz (A
Note 13: Measure output levels of the other two undriven amplifiers relative to the driven amplifier to determine channel separation. Terminate the undriven amplifier
inputs to simulate generator loading. Repeat test at f
Note 14: A minimum pulse width of 200 ns is guaranteed for a horizontal line of 15 kHz. This limit is guaranteed by design. If a lower line rate is used then a longer
clamp pulse may be required.
track is a measure of the ability of any two amplifiers to track each other and quantifies the matching of the three gain stages. It is the difference in
V
<
1 ns. Scope and generator response used for testing: tr= 1.1 ns, tf= 0.9 ns. Using the RSS technique the scope and
r,tf
±
0.2 dB.
max reference level) to the −3 dB corner frequency (f
V
= 10 MHz for V
IN
with RL=∞. Load resistors are not required and are not used in the test circuit, therefore all
CC
level at the input, subdivided into 16 equal steps,
P-P
and measured relative to the AVmax condition. For example, at AVmax the three amplifiers’
V 1/2
).
−3 dB
SEP 10 MHz
.
.
P-P
Min
(Note 7)
3.0
Typ
(Note 6)
Max
(Note 7)
VCC+
0.5
±
10 µA
±
10 µA
−50 µA
−550
2.0 3.0 4.0 mA
. This yields a typical gain change
V 1/2
LM1270
Units
Units
V
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External Interface Signals Electrical Characteristics (Continued)
Note 15: Limits met by matching the external resistor going to pin 24 to the H Flyback voltage.
LM1270
Typical Performance Characteristics
VCC= 5V, TA= 25˚C unless otherwise specified.
Emphasis, Center Frequency at Maximum Center Frequency, Emphasis at Maximum
20063502
The above two plots show the processing done by the
LM1270 on the video input signal. There are two variables
for the video processing, emphasis and center frequency.
Emphasis is controlled by bits 0-2 in register 0B. This gives
8 different levels of emphasis. In the above plot the center
frequency is set at its maximum level and the 8 different
levels of emphasis is measured. The output video with no
emphasis is adjusted to a 0.7 V
emphasis the video is increased to a 0.9 V
level. Using maximum
P-P
level at the
P-P
rising edge of the video. If the falling edge was measured it
would show a similar waveform, but going in the negative
direction.
Center frequency is shown in the second plot. Control of the
center frequency is done with bits 0-3 in register 0C. This
gives 16 adjustments for this feature. Every other adjustment
is shown in the above plot, showing all 16 adjustments would
have made the plot too hard to read. The curves closely
20063503
approximate the peaking of an RC network, therefore the
term center frequency means the RC time constant that is
approximated by each curve in the above plot. A true RC
peaking network would give very large overshoots. The
LM1270 has special circuitry to clip the very large overshoots, yet has the complete benefit of the RC peaking. This
special circuitry allows for much more overshoot than one
could do with RC peaking and still not saturate the video
channel.
Note that the video channel with the emphasis also has its
own independent contrast control. This allows the user to
adjust his monitor for a brighter picture within the Hi-Brite
window and optimize the emphasis for the resolution he is
using with the monitor. Now the user of the monitor can give
his pictures or video a special “sparkle” when using the
capabilities of the LM1270.
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Pin Descriptions
LM1270
Pin
No.
1
2
3
Pin Name Schematic Description
Blue 1 Video In
Red 1 Video In
Green 1 Video In
4 Analog Input
Ground
5
Blue 2 Video In
6
Red 2 Video In
7
Green 2 Video In
Channel 1 video inputs. These inputs must be AC
coupled with a 0.1 µF cap. DC restoration is done at
these inputs. A series resistor of about 33Ω and
external ESD protection diodes should also be used.
Channel 1 video passes through the emphasis
section used for the Hi-Brite window. When using
Hi-Brite the channel 1 input pin must be shorted to
the corresponding channel 2 input pin.
Ground Pin for the input analog circuits of the
LM1270
Channel 2 video inputs. These video inputs must be
AC coupled with a 0.1 µF cap. DC restoration is
done at these inputs. A series resistor of about 33Ω
and external ESD protection diodes should also be
used. When using Hi-Brite the channel 2 input pin
must be shorted to the corresponding channel 1
input pin.
810Digital Ground
PLL V
CC
9 PLL Filter
11 V Sync
The ground pin should be connected to the rest of
the circuit ground by a short but independent PCB
trace to prevent contamination by extraneous
signals. The V
rest of the V
pin should be isolated from the
CC
line by a ferrite bead and bypassed
CC
to pin 8 with an electrolytic capacitor and a high
frequency ceramic.
Recommended topology and values are shown to
the left. It is recommended that both filter branches
be bypassed to the independent ground as close to
pin 8 as possible. Great care should be taken to
prevent external signals from coupling into this filter
2
from video, I
C compatible bus, etc.
Logic level vertical sync signal received from the
video card in the PC or sync stripper circuit.
www.national.com5
Pin Descriptions (Continued)
LM1270
Pin
No.
12 H Sync Logic level horizontal sync signal received from the
Pin Name Schematic Description
video card in the PC or sync stripper circuit.
13 SCL
14 SDA
15 Blank/Interrupt
Out
The I2C compatible Clock line. A pull-up resistor of
about 2 kΩ should be connected between this pin
and V
. A 100Ω resistor should be connected in
CC
series with the clock line for additional ESD
protection.
The I2C compatible data line. A pull-up resistor of
about 2 kΩ should be connected between this pin
and V
. A 100Ω resistor should be connected in
CC
series with the data line for additional ESD
protection.
This output pin can be selected for Vertical Blank,
blank all the video, or no blanking. Provides the
blanking signal normally used at G1 of the CRT,
blanking the video during vertical retrace. If blanking
is not required, this pin may be used as an interrupt
signal for the microcontroller.
16 H Flyback
www.national.com 6
Proper operation requires current reversal. R
H
should be large enough to limit the peak current at
pin 16 to about +2 mA during blanking, and
−300 µA during scan. C
is usually needed for logic
17
level inputs and should be large enough to make
the time constant, R
horizontal period. R
significantly larger than the
HC17
Fil
and C
are typically 300Ω
Fil
and 330 pF when the flyback waveform has ringing
and needs filtering. These two parts are not used on
the current neck board. C
may be needed to filter
57
extraneous noise and can be up to 100 pF.
Pin Descriptions (Continued)
LM1270
Pin
No.
Pin Name Schematic Description
1718Ground
V
CC
19
Green Video Out
20
Red Video Out
21
Blue Video Out
22 V
REF
Ground pin for the output analog portion of the
LM1270 circuitry, and power supply pin for all the
analog of the LM1270. Note the recommended
charge storage and high frequency capacitors which
should be as close to pins 17 and 18 as possible.
Video outputs of the LM1270. Typically the output is
set to 0.7 V
to drive any standard video pre-amp.
P-P
When Hi-Brite is activated the user can set the
video output as high as 1.0 V
P-P
.
Cap Provides filtering for the internal voltage which sets
the internal bias current in conjunction with R
EXT
.A
minimum of 0.1 µF is recommended for proper
filtering. This capacitor should be placed as close to
pin 22 and the analog ground return as possible.
This pin can also be set to use an external V
REF
.
23 V
REFREXT
24 Clamp
Sets the internal current sources through a
10 kΩ 1% external resistor. Resistor value and
accuracy is critical for optimum operation of the
LM1270. This resistor should be placed as close to
pin 23 and the analog ground return as possible.
This pin can either accept an external clamp pulse
or send an internally generated clamp pulse to the
video circuits. As an input this pin accepts either
TTL or CMOS logic levels. The internal switching
threshold is approximately one-half of V
external series resistor, R
, of about 1 kΩ is
CLP
CC
.An
recommended to avoid overdriving the input
devices, or for current limiting the drivers if being
used as an output. C
filtering noise on the clamp input. R
may be necessary for
CLP
LOAD
should be
used if this pin is an output with a value of about
10 kΩ.
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Functional Description
LM1270
VIDEO SECTION
The LM1270 gives the monitor designer the ability to add a
highlighted window to the CRT monitor. Special software
provided manufacturer enables the user to select the area
he wants to have highlighted. National Semiconductor refers
to this feature as Hi-Brite. Referring to Figure 1, the Block
Diagram of the LM1270, note that there are two channels for
each video input. Of the three video channels only the blocks
of the blue channel are shown since all three channels have
the same blocks. A buffer stage is located at the video inputs
and this is followed by a contrast control. Referring to channel one (B1 input), the contrast control is followed by an
emphasis block. The combination of the emphasis control
and the contrast control gives the desired highlighted video
inside the Hi-Brite window. The output of the emphasis stage
and the output of the contrast control in channel two go to a
high-speed video switch, which selects the video channel
going to the output stage.
Having an independent contrast control in each video channel allows the user to adjust the video gain, normally having
the higher gain in channel one, giving a “brighter” picture
within the Hi-Brite window. The Emphasis stage is used to
give more “sparkle” to the highlighted video. Video going into
the emphasis stage has peaking added to the video. Both
the amplitude and the duration of the peaking are adjustable
through the I
different video resolutions. Maximum peaking is 20%. Although most uses of the Hi-Brite would be for the video
inside the selected window, the reverse may be selected
where all the video outside the window is processed through
the emphasis stage. Please remember that for all video
inputs the corresponding channel one input and the channel
two input must be shorted together. Standard AC coupling
such as used with the LM123X or LM124X pre-amps is to be
used with the LM1270.
2
C COMPATIBLE INTERFACE
I
Pins 13 and 14 receive signals from an I
The interface section is used to decode the I
signal and update the registers. Note that all functions of the
LM1270 are controlled through an I
bus. Details on the internal registers are covered in the I
compatible Interface Registers Section.
PLL
A phase locked loop (PLL) is required for the LM1270 to sync
all the timing circuits to the incoming horizontal sync. Proper
operation of both the Hi-Brite window and the video detection requires a proper lock of the PLL reference pulse to the
horizontal sync. Careful layout of the loop filter at pin 10 is
necessary for keeping noise out of the PLL section. The
register to the PLL is used to set the number of pixels per line
for the internal counters that set the Hi-Brite window, generate the internal clamp pulse, and set the counters for the
video detection. The maximum number of pixels per line is
2,047, or 7Fh. For good resolution of the window, 640 is the
recommended minimum pixels per line. The VCO has a
running frequency range of 110 MHz to 160 MHz when using
prescaler 7. For the other pre-scaler settings the frequency
range is 120 MHz to 220 MHz. The output of the VCO goes
to a pre-scaler with a 3 bit register, dividing the VCO output
by up to 8. Minimum division is 1, just a straight pass through
the pre-scaler. Calculating the VCO frequency:
F
VCO
2
C compatible bus, optimizing the emphasis for
2
C compatible bus.
2
CoranI2C compatible
= VCO frequency
2
C compatible
F
= Horizontal line frequency
H
N = Number of pixels per line
M = PLL pre-scaler
Using the above terms gives the following formula for the
PLL:
=N*M*F
F
VCO
H
It is easiest to run the PLL in AUTO mode, setting bit 7 of
address 0E to a “1”. Here the user programs in the desired
pixels per line and the part will select the preferred pre-scaler
number. If one is using manual mode below is a table
showing the relationship between the pre-scaler and the
VCO frequency. The chart shows the pixel frequency, F
F
P=FVCO
/M
.
P
The necessary pixels per line is calculated by the following
formula:
N=F
P/FH
PRESC2PRESC2PRESC2PRES
DIV RATIO
FP(max)
MHz
Fp(min)
MHz
1 1 1 1 160.0 110.0
1 1 0 2 110.0 60.0
1 0 1 3 73.3 40.0
1 0 0 4 55.0 30.0
0 1 1 5 44.0 24.0
0 1 0 6 36.7 20.0
0 0 1 7 31.4 17.1
0 0 0 8 27.5 10.0
TIMING AND WINDOW GENERATION SECTION
All timing is referenced to the vertical and horizontal sync
inputs that come in on pins 11 and 12 respectively. The two
registers near the inputs sets the start of the window for both
the horizontal and vertical direction. The window generator
and the following logic are used to generate a pulse to
control the video switch used to switch between the video
with emphasis and the standard video. The switching between the two video channels is the generation of the window seen by the user. There is the option to switch off the
Hi-Brite window. An additional switch located just before the
2
C
video switch is used to turn off the control signal.
Video detection is another important function inside the
LM1270. Note that both video channels, the PLL, and both
vertical sync and horizontal flyback signals all go to this
block. From these inputs the video detection is used to
detect the length of the horizontal and vertical front porch as
well as the length of the active video for both the horizontal
and vertical directions. This information allows the monitor
designer to offer auto alignment in his monitor. Further processing of the video information in the video and window
timing data section generates an internal clamp pulse. This
is the DC restoration pulse for the LM1270. Pin 24 is the
output for the clamp pulse, allowing the monitor designer to
use it for the clamp input of the pre-amp. There is also the
option to program the LM1270 to accept an external clamp
pulse instead of using its own internally generated pulse.
The video detection section also has a data receiver for
receiving data encoded on the video. When this feature is
used, the MCU in the monitor is used to control the LM1270,
keeping all communication with the LM1270 within the monitor. This results in a faster response time by eliminating the
need for the computer to control the LM1270. Control by the
computer requires communication through the monitor MCU.
www.national.com 8
Functional Description (Continued)
Using the data encoded on the video eliminates one stage of
communicating with the LM1270, allowing for a faster response time for changes in the Hi-Brite window.
V
REF
Proper operation of the LM1270 does require a very accurate reference voltage. This voltage is generated in the V
block. To insure an accurate voltage over temperature, an
external resistor is used to set the current in the V
The external resistor is connected to pin 23. This resistor
should be 1% and have a temperature coefficient under
100 ppm/˚C. ALL VIDEO SIGNALS MUST BE KEPT AWAY
FROM PIN 23. This pin has a very high input impedance and
will pick up any high frequency signals routed near it. The
board layout shown in Figure 12 is a good example of trace
routing near pin 23. The output of the V
stage goes to a
REF
number of blocks in the video section and also to pin 22. This
pin allows capacitor filtering on the V
output and offers an
REF
accurate external reference. A buffer must be used with this
reference, the maximum current loading should be only
100 µA. Pin 22 is bi-directional, allowing the LM1270 to be
referenced to a pre-amp V
. This configuration makes it
REF
possible to DC couple to the pre-amp, but for accurate DC
level, it is recommended that AC coupling be used between
the LM1270 and the pre-amp. It is also recommended that
the internal V
be used for best performance. Note: Any
REF
noise injected into pin 22 will appear on the video. The
voltage reference must be kept very clean for best performance of the LM1270.
REF
REF
stage.
LM1270
H FLYBACK
H Flyback is an analog signal input from the monitor horizontal scan. This flyback signal goes to the internal horizontal flyback pulse generation circuit. An optional capacitor
and/or resistor to ground may be needed if noise interferes
with the H Flyback signal or if there is ringing on the H
Flyback signal. Note: there is no blanking pedestal on the
output video. If horizontal blanking is to be added to the
video signal it must be done at the pre-amp.
is used to limit the input current into the IC to a
R
LIMIT
maximum value of +1 mA during flyback and −100 µA during
normal forward scan. For example if an H flyback with a peak
of 100V is used, R
= 100 kΩ. The internal input imped-
LIMIT
ance of pin 16 is low to limit the maximum voltage swing at
the input to be within the supply rail and ground. Figure 2
shows the H flyback waveforms and the location of R
LIMIT
.A
56 pF capacitor has been added to the H Flyback pin for
filtering noise on the H Flyback signal.
20063519
FIGURE 2. H Flyback Input Pulse
VERTICAL BLANKING
A similar vertical pulse generation circuit to the LM1238 is
included internal to the LM1270. The leading edge of the
vertical sync is used to start the programmable vertical
blanking signal directly. The Vertical Blank Duration Control
Register sets the width of the pulse. When vertical blanking
is enabled its polarity is programmable.
CLAMPING
Pin 24, the clamp pin, can be set for either an input or and
output. If used as an input, then this pin functions just like the
clamp input pins to the video pre-amps. One should use a
series 1 kΩ resistor to pin 24 and a small capacitor may also
be needed between pin 24 and ground to filter high frequency noise.
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