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PDF LM1212 Data sheet ( Hoja de datos )
| Número de pieza | LM1212 | |
| Descripción | Video Amplifier System | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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www.DataSheet4U.com
August 1996
LM1212
230 MHz Video Amplifier System with OSD Blanking
General Description
The LM1212 is a very high frequency video amplifier system
intended for use in high resolution monochrome or RGB
color monitor applications with OSD In addition to the wide-
band video amplifier the LM1212 contains a gated differen-
tial input black level clamp comparator for brightness con-
trol a DC controlled attenuator for contrast control and a
DC controlled sub contrast attenuator for drive control The
DC control for the contrast attenuator is pinned out sepa-
rately to provide a more accurate control system for RGB
color monitor applications All DC controls offer a high input
impedance and operate over a 0V–4V range for easy inter-
face to bus controlled alignment systems During the OSD
window the output is blanked to k 0 4V The LM1212 oper-
ates from a nominal 12V supply but can be operated with
supply voltages down to 8V for applications that require re-
duced IC package power dissipation characteristics
Features
Y Wideband video amplifier
(fb3 dB e 230 MHz at VO e 4 VPP)
Y tr tf e 1 5 ns at VO e 4 VPP
Y Externally gated comparator for brightness control
Y 0V to 4V high input impedance DC contrast control
(l 40 dB range)
Y 0V to 4V high input impedance DC drive control
(g3 dB range)
Y Ouput blanked to k 0 4V for OSD window
Y Easy to parallel three LM1212s for optimum color
tracking in RGB systems
Y Output stage clamps to 0 65V and provides up to 9V
output voltage swing
Y Output stage directly drives most hybrid or discrete
CRT amplifier stages
Applications
Y High resolution CRT monitors with OSD
Y Video switches
Y Video AGC amplifier
Y Wideband amplifier with gain and DC offset control
Block and Connection Diagram
Order Number LM1212N
See NS Package Number N20A
TL H 12354 – 1
C1996 National Semiconductor Corporation TL H 12354
RRD-B30M86 Printed in U S A
http www national com
1 page  
Circuit Description
Figure 2 shows a block diagram of the LM1212 video ampli-
fier along with contrast and brightness (black level) control
Contrast control is a DC-operated attenuator which varies
the AC gain of the amplifier Signal attenuation (contrast) is
achieved by varying the base drive to a differential pair and
thereby unbalancing the current through the differential pair
As shown in Figure 2 a 5 3V bias voltage is internally con-
nected to the positive input of the attenuator and to Pin 1
Pin 3 provides a control voltage for the negative input (pin 2)
of the attenuator The voltage at pin 3 varies as the voltage
at the contrast control input (pin 8) varies thus providing
signal attenuation The gain is maximum (0 dB attenuation)
if the voltage at pin 8 is 4V and is minimum (maximum atten-
uation) if the voltage at pin 8 is 0V The 0V to 4V DC-operat-
ed drive control at pin 9 provides a 6 dB gain adjustment
range This feature is necessary for RGB applications where
independent gain adjustment of each channel is required
The brightness or black level clamping requires a ‘‘sample
and hold’’ circuit which holds the DC bias of the video ampli-
fier constant during the black level reference portion of the
video waveform Black level clamping often referred to as
DC restoration is accomplished by applying a back porch
clamp signal to the clamp gate input pin (pin 14) The clamp
comparator is enabled when the clamp signal goes low dur-
ing the black level reference period (see Figure 2 ) When
the clamp comparator is enabled the clamp capacitor con-
nected to pin 12 is either charged or discharged until the
voltage at the minus input of the comparator matches the
voltage set at the plus input of the comparator During the
video portion of the signal the clamp comparator is disabled
and the clamp capacitor holds the proper DC bias In a DC
coupled cathode drive application picture brightness func-
tion can be achieved by varying the voltage at the compara-
tor’s plus input Note that the back porch clamp pulse width
(tW in Figure 2 ) must be greater than 100 ns for proper
operation
OSD blanking is activated through an active high TTL signal
at pin 20 When pin 20 has a high logic level the video
output at pin 17 drops to k 100 mV During this time an
external signal can be applied to the output driving the vid-
eo level to the desired level for the OSD window This OSD
signal will back bias the emitter of the internal output tran-
sistor Q23 (see Figure 4 ) The maximum voltage to this
transistor under this condition is 5V This signal must also
supply enough current to the pull-down transistor Q24
which is about 100 mA The OSD blanking can only be done
during the active video period If activated during the clamp
pulse period the OSD blanking will interfere with the DC
restoration
VIDEO AMPLIFIER SECTION (Input Stage)
A simplified schematic of LM1212’s video amplifier input
stage is shown in Figure 3 The 5 4V zener diode Q1 Q6
and R2 bias the base of Q7 at 2 6V The AC coupled video
signal applied to pin 6 is referenced to the 2 6V bias voltage
FIGURE 2 Block Diagram of the LM1212 Video Amplifier
with Contrast and Brightness (Black Level) Control
5
TL H 12354 – 8
http www national com
5 Page 
Circuit Description (Continued)
FIGURE 7 Simplified Schematic of the LM1212 Clamp Gate Circuit
TL H 12354 – 13
11 http www national com
11 Page | ||
| Páginas | Total 18 Páginas | |
| PDF Descargar | [ Datasheet LM1212.PDF ] | |
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