LM1209 Datasheet, PDF (19/24 Pages) National Semiconductor (TI) – 130 MHz/100 MHz RGB Video Amplifier System

Circuit Description (Continued)
age of Q29 There are also four diode drops from the base
of Q30 to the output pin 20 Therefore during blanking pin
20 will be less than 100 mV above ground enabling the
designer to blank at the cathode of the CRT R23 is added
to quickly turn off Q25 by discharging its base when the
blanking signal is removed
Figure 14 also shows the power and ground pins to the
LM1208 LM1209 All the V_CC1 pins (pins 3 11 25) are all
internally connected together A 0 1 mF bypass capacitor
must be located close to each pin and connected to ground
Further bypassing is done by a 100 mF capacitor This ca-
pacitor needs to be located on the board close to the
LM1208 LM1209 Pins 22 and 23 are the V_CC2 pins These
pins may need a ferrite bead in series with the input power
A 10 mF and a 0 1 mF bypass capacitors must be located
close to pins 22 and 23 Correct bypassing of pins 22 and
23 is very important If the bypassing is not adequate then
the outputs of the LM1208 LM1209 will have ringing or
even worse they may oscillate The ground side of the by-
pass capacitors at pins 22 and 23 must be returned to a
ground plane with no interruptions from other traces be-
tween these capacitors and the ground pins 21 and 24 of
the LM1208 LM1209
Applications of the LM1208
LM1209
Figure 15 is the schematic of the demonstration board de-
signed at National Figure 16 is the actual layout of the dem-
onstration board Note that the schematic shown in
Figure 15 is almost identical to the schematic shown in Fig-
ure 4 The only difference between the two schematics is
that in Figure 15 each channel has individual adjustments
for both drive and cutoff making this circuit a good design
for monitor applications Each CRT will have a slightly differ-
ent cutoff voltage for each color making it necessary to
provide separate adjustments in order to accurately set the
cutoff for each color The gain of each color of the CRT is also
slightly different if the color temperature of the display is to
be accurately set then each channel of the LM1208
LM1209 must have individual gain adjustments Thus each
channel has its own drive control Once the drive control is
set the gain between the three color channels will closely
track as the contrast is adjusted All the jumpers needed to
design a single sided PC board are shown in the schematic
The resistors and jumpers with no reference designation are
the connections between the PC board and the connectors
mounted on the PC board CN1 thru CN8 are BNC connec-
tors
A 30X resistor is in series with each of the video inputs A
voltage surge may occur at these inputs when either the
inputs are first connected to another system or when the
system is powered up before the monitor is turned on If this
voltage surge exceeds the supply voltage (at ground poten-
tial if the monitor is not powered up) of the LM1208
LM1209 or goes below ground current will flow through the
parasitic devices of the LM1208 LM1209 This current is
limited by the 30X resistors preventing a potential cata-
strophic failure A 100X resistor is added to the Blank Gate
and Clamp Gate inputs These two resistors also limit
TL H 11884 19
FIGURE 14 Simplified Schematic of LM1208 LM1209 Video Amplifier Output Stage with Blank Circuit
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