LM1205 National Semiconductor, LM1205 Datasheet - Page 10

• Manufacturer Part Number: LM1205
• Description: 130 MHz/85 MHz RGB Video Amplifier System with Blanking
• Manufacturer: National Semiconductor

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Functional Description
The 0V to 4V drive control signal comes in on pin 18 Each
channel has its own drive section therefore the crosstalk
compensation needed for the contrast control voltages is
not required for the drive control thus no external pins for
the drive control The drive attenuator gives an attenuation
range from 0 dB to
for the drive attenuator is desirable and intentionally de-
signed because the drive is used only to balance the overall
gain of each color channel giving the correct color tempera-
ture on the CRT
The output of the drive attenuator stage goes to A2 the
amplifier in the DC restoration section The video signal
goes to the non-inverting input of A2 The inverting side of
A2 goes to the output of gm1 the clamp comparator and
the clamp capacitor at pin 8
During the back porch period of the video signal a negative
going clamp pulse from pin 14 is applied to the clamp com-
parator turning on the comparator This period is where the
black level of the video signal at the output of the LM1205
LM1207 is compared to the desired black level which is set
at pin 19 Figure 7 shows the timing of the clamp pulse
relative to the video signal The clamp capacitor is charged
or discharged by gm1 generating the correction voltage
needed at the inverting input of A2 to set the video output to
the correct DC level Removing the clamp pulse turns off
gm1 with the correction voltage being maintained by the
clamp capacitor during active video Both the clamp pulse
and the blank pulse at pin 13 are TTL voltage levels
There are actually two output sections
sections have been designed to be identical except
has more current drive capability The output transistor
shown is part of
the user knows the configuration of the output stage
does not go to the outside world it is used for feeding back
the video signal for DC restoration Its output goes directly
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b
A4 but has been shown separately so
b
6 dB A small gain adjustment range
(Continued)
b
A3 and
b
A4 Both
b
b
A4
A3
10
to the inverting input of the clamp comparator via the volt-
age divider formed by the 500
be close to the same output as
track due to the similar design of the two output stages
However the current at the output of
the current at the output of
load resistance of 390
the green video output pin to ground Another input to
is the blank pulse When a negative going blank pulse is
applied to pin 13 the output of the LM1205 LM1207 is driv-
en to less than 0 1V above ground Using the timing shown
in Figure 7 for the blank pulse the output of the LM1205
LM1207 will be less than 0 1V during the inactive portion of
the video signal This is a ‘‘blacker than black’’ condition
blanking the CRT at the cathodes By using the blank func-
tion of the LM1205 LM1207 no grid blanking is necessary
Note that the DC restoration is done by feeding back the
video signal from
using the two output stages blanking can be done at the
CRT cathodes and at the same time activate the DC resto-
ration loop
V
pins are all internally connected For proper operation of the
LM1205 LM1207 it is necessary to connect all the V
pins to the input power to the PCB and bypass each pin with
a 0 1 F capacitor V
23 for the three output stages This is a separate power
input from V
the two different power inputs There must be a connection
on the PCB between V
be bypassed by a parallel connection of a 10 F and 0 1 F
capacitors The ground connections for the LM1205
LM1207 are at pins 7 21 and 24 All three ground pins are
internally connected and these pins must also be connect-
ed externally to a good ground plane for proper operation of
the LM1205 LM1207
CC1
goes to pins 3 11 and 25 (see Figure 1 ) These three
CC1
there are no internal connections between
b
A3 but blanking is done at
CC2
CC1
needs to be connected from pin 20
is the input power at pins 22 and
b
and V
A3 To balance both outputs a
and 4k resistors
b
CC2
A3 and will temperature
b
Pins 22 and 23 must
A4 will be ten times
b
b
A4 By
A4 will
b
CC1
A4