lm2433 National Semiconductor Corporation, lm2433 Datasheet - Page 7

• Manufacturer Part Number: lm2433
• Description: 220v Monolithic Single Channel 16 Mhz Edtv Crt Driver
• Manufacturer: National Semiconductor Corporation

Application Hints
EFFECT OF OFFSET
Figure 7 shows the variation in rise and fall times when the
DC offset of the 110V
120V and 150V
mum variation of about 6% relative to the center data point
(135V
the 30V DC offset range.
THERMAL CONSIDERATIONS
Figure 8 shows the performance of the LM2433 as a function
of case temperature. The figure shows that the rise and fall
times of the LM2433 increase by approximately 4.5% and
6.5%, respectively, as the case temperature increases from
40˚C to 90˚C. This corresponds to a speed degradation of
about 0.9% and 1.3% for every 10˚C rise in case tempera-
ture, which is very stable performance over the temperature
range.
POWER DISSIPATION AND HEATSINK CALCULATION
Worst-Case Power Dissipation
Figure 9 shows the maximum power dissipation of the
LM2433 vs. square wave frequency when the device uses
V
alternating one pixel on, one pixel off signal. Note that the
frequency range shown in the power dissipation figure is
one-half the actual pixel frequency. The graph assumes 80%
active time (device operating at the specified frequency),
which is typical in an EDTV application. The other 20% of the
time the device is assumed to be sitting at the black level
(190V in this case). Under this worst-case condition, the
maximum power dissipated by the LM2433 is about 6.8W at
around 40 MHz. It is important to note that this power dissi-
pation is a result of a high frequency square wave input,
which is unrealistic in practical TV applications. The band-
width of the input source used to drive the LM2433 was over
300 MHz. Using a RGB video processor or preamplifier with
less bandwidth will cause the LM2433 to dissipate less
power than shown in Figure 9 at the same conditions.
CC
of 220V and is driving a 10 pF load with 110V
DC
), which is a relatively small variation in speed over
DC
. The rise time and fall time show a maxi-
PP
output swing is varied between
(Continued)
FIGURE 12. Recommended Application Circuit
PP
swing
7
A Practical Approach to Power Dissipation
The power curve (Figure 9) mentioned previously shows the
LM2433 power dissipation for square wave frequencies
ranging from 1 to 50 MHz at 110V
uncommon for a TV to display average frequency content
over the entire picture exceeding 20 MHz. Therefore, it is
important to establish the worst-case picture condition under
normal viewing to give a realistic maximum power dissipa-
tion for the LM2433. Here is one approach:
An EDTV signal generator pattern that yields a practical
worst-case picture condition is a “multi-burst” pattern that
consists of a 1-to-30 MHz sine wave sweep over each of the
active lines. The power dissipated by the LM2433 as a result
of this picture condition can be approximated by taking the
average of the power between 1 to 30 MHz in Figure 9. This
average is 5.1W. Because a square wave input was used to
generate this power curve, a sine wave would cause the
LM2433 to dissipate slightly less power, say 5.0W. This is
one common way to determine a practical figure for maxi-
mum power dissipation. It is the system designer’s respon-
sibility to establish the worst-case picture condition for his
particular application and measure dissipation under that
condition to choose a proper heatsink.
Heatsink Calculation Example
Once the maximum dissipation is known, Figure 10 can be
used to determine the heatsink requirement for the LM2433.
If the 1-to-30 MHz multi-burst test described previously is
assumed to be worst-case picture condition that yields maxi-
mum dissipation, then the LM2433 will dissipate about 5.0W.
The power derating curve shows that the maximum allowed
case temperature is 127.5˚C when 5.0W is dissipated. If the
maximum expected ambient temperature is 65˚C, then the
maximum thermal resistance from device case-to-air (θ
can be calculated:
θ
case-to-heatsink interface and θ
of the heatsink at the rated conditions.
This example assumes a capacitive load of 10 pF and no
resistive load. The designer should note that if the V
supply voltage, output swing, input bandwidth, or load ca-
pacitance is increased, then the power dissipation will also
increase.
CS
is the thermal resistance of the thermal compound at the
θ
θ
CA
CA
= (127.5˚C – 65˚C) / 5.0W = 12.5˚C/W.
= (T
C MAX
– T
20146210
A MAX
) / P
SA
D MAX
PP
is the thermal resistance
swing. In practice, it is
= θ
CS
+ θ
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SA
CA
CC
)