LM2459 National Semiconductor, LM2459 Datasheet - Page 8

• Manufacturer Part Number: LM2459
• Description: Monolithic Single Channel 15 MHz DTV CRT Driver
• Manufacturer: National Semiconductor

www.national.com
Application Hints
well as increase overshoot. It is very important to use induc-
tors with very high self-resonant frequencies, preferably
above 300 MHz. Ferrite core inductors from J.W. Miller
Magnetics (part # 78FR_ _k) were used for optimizing the
performance of the device in the NSC application board. The
values shown in Figure 14 can be used as a good starting
point for the evaluation of the LM2459. Using a variable
resistor for R1 will simplify finding the value needed for
optimum performance in a given application. Once the opti-
mum value is determined, the variable resistor can be re-
placed with a fixed value.
Figure 12 shows the typical cathode pulse response with an
output swing of 110V
The transient response can also be improved by adding a
capacitor from pin 6 to the ground plane used by the
LM2459. A small capacitor, such as a 22pF ceramic, will
notably improve the fall time and only increase the over-
shoots and settling times slightly. Note that increasing the
capacitance beyond 22pF will only improve the fall time
marginally, but will increase the settling times significantly.
This option allows for better matching between the rise and
fall time.
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 LM2459 and
from the LM2459 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
A typical application of the LM2459 is shown in the sche-
matic for the NSC demonstration board in Figure 14. Used in
conjunction with an LM126X preamplifier, a complete video
PP
using a LM1269 preamplifier.
(Continued)
8
channel from input to CRT cathode can be achieved. Perfor-
mance is ideal for DTV applications. The NSC demonstration
board can be used to evaluate the LM2459 in a TV.
NSC DEMONSTRATION BOARD
Figure 15 shows the routing and component placement on
the NSC LM2459 demonstration board. This board provides
a good example of a layout that can be used as a guide for
future layouts. Note the location of the following compo-
nents:
• C7203 — V
• C7205 — V
• C7207 — V
The routing of the LM2459 output to the CRT is very critical
to achieving optimum performance. Figure 16 shows the
routing and component placement from pin 2 (V
LM2459 to the cathode. Note that the components are
placed so that they almost line up from the output pin of the
LM2459 to the cathode pin of the CRT connector. This is
done to minimize the length of the video path between these
two components. Note also that D7204, D7205, R7251 and
D7214 are placed to minimize the size of the video nodes
that they are attached to. This minimizes parasitic capaci-
tance in the video path and also enhances the effectiveness
of the protection diodes. The anode of protection diode
D7204 is connected directly to a section of the the ground
plane that has a short and direct path to the LM2459 ground
pins. The cathode of D7205 is connected to V
to decoupling capacitor C7207 (see Figure 16) which is
connected to the same section of the ground plane as
D7204. The diode placement and routing is very important
for minimizing the voltage stress on the LM2459 during an
arcover event. Lastly, notice that SG7202 is placed very
close to the cathode and is tied directly to CRT ground.
This demonstration board uses medium-sized PCB holes to
accommodate socket pins, which function to allow for mul-
tiple insertions of the LM2459 in a convenient manner. To
benefit from the enhanced LM2459 package with thin leads,
the device should be secured in small PCB holes to optimize
the metal-to-metal spacing between the leads.
1 and ground pins
and ground
clamp diodes. Very important for arc protection.
CC
BB
CC
bypass capacitor, located very close to pin
bypass capacitor, located close to pin 4
bypass capacitor, near LM2459 and V
CC
OUT
very close
) of the
CC