LM2432TE/NOPB National Semiconductor, LM2432TE/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM2432TE/NOPB
• Manufacturer: National Semiconductor

Specifications of LM2432TE/NOPB

• Lead Free Status / RoHS Status: Compliant

Application Hints
calibrate the video levels. The operating point of the CRT
can be fine-tuned using the adjustment range of the proces-
sor’s RGB cut-off and/or gain controls via the I
normal video resumes, a protection element should shunt
high video current away from the IK input of the RGB pro-
cessor. Since the processor’s IK pin is not clamped during
normal video, V
(V
diode (D
sor input and LM2432 IK output in this case. Again, it may be
necessary for the RGB processor to have IK leakage com-
pensation for the leakage current attributed to the zener. See
a simplified current feedback interface circuit in Figure 15.
LM2432 IK Output and Protection Requirements
The LM2432 IK output sources a copy of the actual cathode
current to the interface circuit during the closed-loop IK
measurement interval and during normal video when the IK
feedback loop is opened. Because the cathode current dur-
ing normal video is much higher than the low current being
measured during the measurement interval, V
it’s maximum rating. To protect and prevent improper opera-
tion of the LM2432, V
specified in the Section Operating Ranges .
For voltage feedback topologies, it is recommended to use a
high-speed diode to clamp the IK voltage to V
supply during normal video. A small series resistor (R
Figure 14) can be placed at the IK pin to limit the current
through the diode when clamping. See the NSC Demonstra-
tion Board for an example. For current feedback topologies,
it is recommended to use a high-speed, low-leakage zener
diode to clamp V
shunt the high video current away from the RGB processor’s
IK input. The zener voltage should be higher than the clamp-
ing voltage of the processor’s IK pin and lower than the
maximum voltage rating of either the processor’s IK pin or
LM2432 V
In a Direct-View TV application with a single neck PCB, it is
possible for the three LM2432 IK outputs to share the one
feedback circuit and protection diode by connecting the IK
pins together on the neck PCB. This will reduce component
count. In a Rear-Projection TV application with three neck
PCBs, the IK pins can be connected on the central neck PCB
or the RGB processor mainboard through cabling. This way,
IK MAX
FIGURE 15. Simplified IK Interface for Current
). A properly chosen high speed, low-leakage zener
PROT
IK
, whichever is less.
) can be used to protect both the RGB proces-
IK
IK
of the LM2432 must not exceed +16V
Feedback Systems
to a properly chosen zener voltage and
IK
must be maintained within the range
(Continued)
IK
2
BB
C-bus. When
may exceed
or a lower
20137725
D
in
9
they can share the interface circuit to feed back the IK
voltage or current signal to the RGB processor. However,
each LM2432 IK output should have its own protection diode
on its PCB.
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance, an adequate ground plane, iso-
lation between channels, good supply bypassing and mini-
mizing unwanted feedback are necessary. Also, the length of
the signal traces from the preamplifier to the LM2432 and
from the LM2432 to the CRT cathode should be as short as
possible. The following references are recommended:
Ott, Henry W., “Noise Reduction Techniques in Electronic
Systems”, John Wiley & Sons, New York, 1976.
“Video Amplifier Design for Computer Monitors”, National
Semiconductor Application Note 1013.
Pease, Robert A., “Troubleshooting Analog Circuits”,
Butterworth-Heinemann, 1991.
Because of its high small signal bandwidth, the part may
oscillate in a TV if feedback occurs around the video channel
through the chassis wiring. To prevent this, leads to the video
amplifier input circuit should be shielded, and input circuit
wiring should be spaced as far as possible from output circuit
wiring.
TYPICAL APPLICATION
The high bandwidth, large swing capability, and simple ap-
plication make the LM2432 ideal for Rear-Projection and
Direct-View HDTV CRT applications. The IK output can be
made compatible with any RGB video processor with IK
feedback. If the IK output is not used in the application, it
should be connected to the same ground as pin 3 (GND).
See the section “Cathode Current Output for IK Feedback
Systems” for more information.
NSC DEMONSTRATION BOARD
Figure 16 and Figure 17 show the schematic and PCB layout
for the NSC demonstration neck board for a typical Rear-
Projection HDTV application with IK feedback. This single
channel neck board could be used for all three channels,
since each neck board receives video-related signals directly
from the RGB mainboard. The power supplies are daisy-
chained between each channel using inboard and outboard
connectors J6 and J7. This board provides a good example
of a layout that can be used as a guide for future layouts.
Samples of the NSC demonstration neck board are available
upon request to your local National Semiconductor Sales
Office.
Input Video Interface
On the RGB mainboard, the video output of the RGB pro-
cessor is buffered with a PNP transistor to drive the video
through flat cabling to the NSC neck board. The cabling from
the mainboard plugs into the neck board at connector J8 to
supply it with video, IK, GND, and other signals. Between the
video input (pin 3) of J8 and V
another buffer stage consisting of two NPN transistors. Both
NPN transistors drop the video levels from the preceding
PNP buffer by a total of two V
black level such that the LM2432 output (or cathode) black
level voltage is near the nominal cut-off voltage of the CRT.
The overall voltage shift from the processor output to the
LM2432 input is one V
shifting could have been accomplished using one NPN
buffer on the RGB mainboard to drive the processor’s video
output through cabling directly to the LM2432 input. How-
BE
drop. Note: The same video level
BE
. This shifts the nominal input
IN
(pin 7) of the LM2432 is
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