LM1276 National Semiconductor Corporation, LM1276 Datasheet - Page 11

• Manufacturer Part Number: LM1276
• Description: 150 Mhz I2c Compatible Rgb Preamplifier With Internal 512 Character Osd Rom, 512 Character Ram And 4 Dacs
• Manufacturer: National Semiconductor Corporation

Typical Performance
Characteristics
otherwise specified (Continued)
SYSTEM INTERFACE SIGNALS
The Horizontal Sync, Flyback, Vertical Sync, and the Clamp
input signals are important for proper functionality of the
LM1276. Both blanking inputs must be present for OSD
synchronization. In addition, the Horizontal blanking input
also assists in setting the proper cathode black level, along
with the Clamping pulse. The Vertical blanking input initiates
a blanking level at the LM1276 outputs, which is program-
mable from 3 to 127 lines (at least 10 is recommended). This
input is set up to only accept a vertical sync pulse, and the
leading edge is used to start the programmable vertical
blanking signal directly. The start position of the internal
Horizontal blanking pulse is programmable from 0 to 64
pixels ahead of the start position of the Horizontal flyback
input. Both horizontal and vertical blanking can be individu-
ally disabled, if desired.
Figure 3 and Figure 4 show the Horizontal Flyback input
when it is logic level and the Vertical input (which must
always be logic level). Figure 3 shows the smaller pin 28
voltage superimposed on the horizontal blanking pulse input
to the neck board with R
where the voltage at pin 28 is clamped to about 1V when the
pin is sinking current. Figure 4 shows the smaller pin 1
voltage superimposed on the vertical blanking input to the
neck board with R
spond to the application circuit of Figure 9.
Please note that the Horizontal Flyback signal to pin 28
MUST be continuously provided to the IC, even during en-
ergy save or sleep modes. In the application, this signal
should be always generated whether the VGA cable is dis-
connected, the monitor is in energy save mofe, or sleep
mode.
Figure 5 show the case where the horizontal input is from
deflection. Figure 5 shows the pin 28 voltage which is de-
rived from a horizontal flyback pulse of 35V peak to peak
with R
Figure 6 shows the pin 27 clamp input voltage superimposed
on the neck board clamp logic input pulse. R = 1k and should
be chosen to limit the pin 27 voltage to about 2.5V peak to
peak. This corresponds to the application circuit given in
Figure 9. The clamp input pin can also be internally con-
nected to the Horizontal Sync pin, thus eliminating the need
for a Clamp signal supplied to the neckboard. This can be
enabled with register 0x853E[4].
H
= 8.2K and C
V
= 4.7k. These component values corre-
1
jumpered.
H
V
CC
= 4.7k and C
= 5V, T
A
= 25˚C unless
1
= 0.1 µF. Note
11
H SYNC & V SYNC
V Sync at pin 1 and H Sync at pin 15 must be supplied with
logic level signals generated by the MCU. In an application
where a logic level clamp pulse is used, the same signal can
be used for the H Sync input. It is important that both V Sync
and H Sync are always receiving signals, even during VGA
cable disconnect, energy save mode, or sleep mode.
CATHODE RESPONSE
Figure 7 shows the response at the red cathode for the
application circuit in Figures 9, 10. The input video rise time
is 1.5 ns. The resulting leading edge has a 7.1 ns rise time
and 7.6% overshoot, while the trailing edge has a 7.1 ns rise
time and 6.9% overshoot using an LM2467 driver.
ABL GAIN REDUCTION
The ABL function reduces the contrast level of the LM1276
as the voltage on pin 26 is lowered from V
Figure 8 shows the amount of gain reduction as the voltage
is lowered from V
until V
increases. Many system designs will require about 3 dB to
5 dB of gain reduction in full beam limiting. Additional attenu-
ation is possible, and can be used in special circumstances.
However, in this case, video performance such as video
linearity and tracking between channels will tend to depart
from normal specifications.
The onset of ABL in the LM1276 is adjustable so that the
amount of beam limiting can be varied, especially for larger
Hi-Brite window displays where the contrast level is not
desired to be reduced as much as a normal video display.
The beam current limiting is 4-bit adjustable in steps of 80 µA
each all the way up to a delta of 1.2 mA. The value of the
ABL pull up resistor (R2) to the external +80V supply must
be selected carefully such that the ABL threshold current will
be at the desired maximum (i.e. 2 mA) when register 0x85C4
is at the lowest setting, 0x00.
There are 4 different ABL current registers corresponding to
4 different ABL settings. Each setting or register (0x85C4 -
0x85C7) can be assigned a different ABL current threshold.
ABL current register 0 can correspond to a minimal area of
the screen being highlighted, and ABL current register 4 can
correspond to the maximum area of the screen being high-
lighted. This area is calculated by the HiBrite software, and
the particular ABL register that is is to be activated is se-
lected by the software. The values of each register are
written by the MCU.
26
reaches the knee around 3.7V, where the slope
CC
(5.0V) to 2V. The gain reduction is small
CC
to around 2V.
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