lm2452 National Semiconductor Corporation, lm2452 Datasheet - Page 7

• Manufacturer Part Number: lm2452
• Description: 220v Monolithic Triple Channel 17 Mhz Dc Coupled Crt Dtv Driver
• Manufacturer: National Semiconductor Corporation

Application Hints
in voltage and the fall time decreases only about 3 ns with
the same offset adjustment. Offset voltage variation has a
minimal affect on the rise and fall times of the driver if the
saturation area is avoided.
THERMAL CONSIDERATIONS
Figure 9 shows the performance of the LM2452 in the test
circuit shown in Figure 3 as a function of case temperature.
The figure shows that the rise and fall times of the LM2452
increases by under 4 ns as the case temperature increases
from 30˚C to 110˚C. Please note that this part should not be
operated with a case temperature over 100˚C. The response
above 100˚C is shown only for reference.
Figure 10 shows the maximum power dissipation of the
LM2452 vs. Frequency when all three channels of the device
are driving into a 10 pF load with a 130V
pixel on, one pixel off. Note that the frequency given in
Figure 10 is half of the pixel frequency. The graph assumes
an 80% active time (device operating at the specified fre-
quency), which is typical in a TV application. The other 20%
of the time the device is assumed to be sitting at the black
level (190V in this case). A TV picture will not have frequency
content over the whole picture exceeding 15 MHz. It is
important to establish the worst case condition under normal
viewing to give a realistic worst-case power dissipation for
the LM2452. One test is a 1 to 30 MHz sine wave sweep
over the active line. This would give a slightly lower power
than taking the average of the power between 1 and 30 MHz.
This average is 15.2 W. A sine wave will dissipate slightly
less power, probably about an even 15W of power dissipa-
tion. All of this information is critical for the designer to
establish 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 will
also increase.
The LM2452 case temperature must be maintained below
100˚C given the maximum power dissipation estimate of
15W. If the maximum expected ambient temperature is 60˚C
and the maximum power dissipation is 15W then a maximum
heat sink thermal resistance can be calculated:
This example assumes a capacitive load of 10 pF and no
resistive load. If the maximum ambient temperature is 50˚C,
then the heat sink thermal resistance can increase to
3.3˚C/W. The designer should note that if the load capaci-
tance 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 LM2452. Using a variable
(Continued)
P-P
alternating one
7
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
using the LM1237 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 LM2452 and
from the LM2452 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 LM2452 is shown in Figure 14.
Used in conjunction with a pre-amp with a 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 to drop
the video black level voltage closer to 1.2V. When using only
one NPN transistor as an emitter follower, a jumper needs to
be added in each channel. In the red channel a jumper
needs to be added between C7 and R25. With just one
transistor neither of these components would be installed.
In addition to the video inputs are the DAC inputs. These
inputs are used to vary the LM2452 output black level by a
DAC. in the past when a driver was used with a CMOS AVP
there was not enough range on the video output to vary the
black level. A clamp circuit had to be used in conjunction with
the AVP and the driver. The DAC inputs of the LM2452 are
driven in the same way the clamp circuit had been driven,
eliminating the need for a clamp circuit. Figure 4 shows the
variation in the black level as the DAC input voltage is
changed. This is shown for both V
The neck board in Figure 14 has two transistors in each
channel enabling this board to work with pre-amps with a
black level output as high as 2.5V. Each transistor stage has
a gain of −1. This setup still gives the two diode drop at the
driver input; however, now additional peaking can be done
on the video signal before reaching the driver inputs. Some
popular AVPs do have a black level of 2.5V. For lower black
levels either one or both transistors would not be used.
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 LM2452
from arc over. Diodes D1,D8, D9, and D13–D15 must also
be used for proper arc over protection. The NSC demonstra-
tion board can be used to evaluate the LM2452 in a TV.
PP
inside a modified production TV set
IN
= 1.2V and V
www.national.com
IN
= 2.1V.