LM2423TE/NOPB National Semiconductor, LM2423TE/NOPB Datasheet - Page 8

• Manufacturer Part Number: LM2423TE/NOPB
• Description: IC CRT DTV DRIVER 15MHZ TO220-11
• Manufacturer: National Semiconductor

Specifications of LM2423TE/NOPB

• Display Type: CRT
• Current - Supply: 21mA
• Voltage - Supply: 100 V ~ 230 V
• Mounting Type: Through Hole
• Package / Case: TO-220-11 (Bent and Staggered Leads)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Operating Temperature: -
• Interface: -
• Configuration: -
• Digits Or Characters: -
• Other names: LM2423TE

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Application Hints
Figure 10 shows the maximum power dissipation of the
LM2423 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 LM2423 case temperature must be maintained below
110˚C. If the maximum expected ambient temperature is
60˚C and the maximum power dissipation is 17W (from
Figure 10, 15 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 LM2423. 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 LM2423 and
from the LM2423 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.
PP
inside a modified Sony TV using a
(Continued)
P-P
alternating one
8
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 LM2423 is shown in the sche-
matic for the NSC demonstration board in Figure 14. Used in
conjunction with a pre-amp with 1.2V black level output no
buffer transistors are required to obtain the correct black
level at the cathodes. If the pre-amp has a black level closer
to 2V, then an NPN transistor should be used as shown in
Figure 14 to drop the video black level voltage closer to 1.2V.
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 LM2423
from arc over. Diodes D1–D6 must also be used for proper
arc over protection. The NSC demonstration board can be
used to evaluate the LM2423 in a TV. If the NSC demonstra-
tion board is used for evaluating the LM2423, then U2, the
voltage regulator, would not be used and +12V would be
connected directly to V
NSC DEMONSTRATION BOARD
Figure 15 shows the routing and component placement on
the NSC LM2423 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:
• C5 — V
• C6 — V
• C7, C8 — V
The routing of the LM2423 outputs to the CRT is very critical
to achieving optimum performance. Figure 16 shows the
routing and component placement from pin 10 (V
LM2423 to the blue cathode. Note that the components are
placed so that they almost line up from the output pin of the
LM2423 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 D1, D3 and R15 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 D1 is con-
nected directly to a section of the ground plane that has a
short and direct path to the LM2423 ground pins. The cath-
ode of D3 is connected to V
capacitor C7 which is connected to the same area of the
ground trace as D1. The diode placement and routing is very
important for minimizing the voltage stress on the LM2423
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 LM2423 in a convenient manner. To benefit
from the enhanced LM2423 package with thin leads, the
device should be secured in small PCB holes to optimize the
metal-to-metal spacing between the leads.
and ground pins
ground
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 LM2423 and V
BB
by inserting JP1.
CC
very close to decoupling
OUT1
) of the
CC