lm2425 National Semiconductor Corporation, lm2425 Datasheet - Page 7

• Manufacturer Part Number: lm2425
• Description: 220v Monolithic Triple Channel 10 Mhz Crt Dtv Driver
• Manufacturer: National Semiconductor Corporation

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tion of 1.9% for every 10˚C rise in case temperature. The fall
time has a smaller increase of about 12% over this tempera-
ture range.
Figure 10 shows the maximum power dissipation of the
LM2425 vs. Frequency when all three channels of the device
are driving into a 10 pF load with a 130V
pixel on, one pixel off. The graph assumes a 72% active time
(device operating at the specified frequency), which is typical
in a TV application. The other 28% of the time the device is
assumed to be sitting at the black level (190V in this case).
This graph gives the designer the information needed to
determine the heat sink requirement for his application. The
designer should note that if the load capacitance is in-
creased the AC component of the total power dissipation
would also increase.
The LM2425 case temperature must be maintained below
110˚C. If the maximum expected ambient temperature is
60˚C and the maximum power dissipation is 12W (from
Figure 10, 10 MHz) then a maximum heat sink thermal
resistance can be calculated:
This example assumes a capacitive load of 10 pF and no
resistive load. The designer should note that if the load
capacitance is increased the AC component of the total
power dissipation will also increase.
OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 13, there are three components (R1, R2
and L1) that can be adjusted to optimize the transient re-
sponse of the application circuit. Increasing the values of R1
and R2 will slow the circuit down while decreasing over-
shoot. Increasing the value of L1 will speed up the circuit as
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 13 can be used as a good starting
point for the evaluation of the LM2425. 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. Due to arc over considerations it is
recommended that the values shown in Figure 13 not be
changed by a large amount.
Figure 12 shows the typical cathode pulse response with an
output swing of 130V
Sony pre-amp.
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 signal inputs to the LM2425 and
from the LM2425 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.
PP
inside a modified Sony TV using a
(Continued)
P-P
alternating one
7
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 LM2425 is shown in the sche-
matic for the NSC demonstration board in Figure 14. Used in
conjunction with an LM1246 preamplifier, a complete video
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 LM2425 combination with
the LM2485 and the LM1246 in a TV.
It is important that the TV designer use component values for
the driver output stage close to the values shown in Figure
14. These values have been selected to protect the LM2425
from arc over. Diodes D1, D2, D4, and D7–D9 must also be
used for proper arc over protection. The NSC demonstration
board can be used to evaluate the LM2425 in a TV. If the
NSC demonstration board is used for evaluating the
LM2425, then U3, the +5V voltage regulator may be used,
eliminating the need to route +5V to the neck board for the
LM1246.
NSC DEMONSTRATION BOARD
Figure 15 shows the routing and component placement on
the NSC LM242X/LM124X/LM2485 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 components:
The routing of the LM2425 outputs to the CRT is very critical
to achieving optimum performance. Figure 16 shows the
routing and component placement from pin 10 (V
LM2425 to the blue cathode. Note that the components are
placed so that they almost line up from the output pin of the
LM2425 to the blue cathode pin of the CRT connector. This
is done to minimize the length of the video path between
these two components. Note also that D8, D9, R24, and D6
are placed to minimize the size of the video nodes that they
are attached to. This minimizes parasitic capacitance in the
video path and also enhances the effectiveness of the pro-
tection diodes. The anode of protection diode D8 is con-
nected directly to a section of the ground plane that has a
short and direct path to the LM2425 ground pins. The cath-
ode of D9 is connected to V
capacitor C7 which is connected to the same area of the
ground trace as D8. The diode placement and routing is very
important for minimizing the voltage stress on the LM2425
during an arc over event.
This demonstration board uses large PCB holes to accom-
modate socket pins, which function to allow for multiple
insertions of the LM2425 in a convenient manner. To benefit
from the enhanced LM2425 package with thin leads, the
device should be secured in small PCB holes to optimize the
metal-to-metal spacing between the leads.
• C19 — V
• C20 — V
• C46, C48 — V
and ground pins
ground
V
CC
clamp diodes. Very important for arc protection.
CC
BB
bypass capacitor, located close to pin 11 and
bypass capacitor, located very close to pin 2
CC
bypass capacitors, near LM2425 and
CC
very close to decoupling
OUT1
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