lm2432 National Semiconductor Corporation, lm2432 Datasheet - Page 7

• Manufacturer Part Number: lm2432
• Description: 220v Monolithic Single Channel 37 Mhz Hdtv Crt Driver
• Manufacturer: National Semiconductor Corporation

Application Hints
EFFECT OF LOAD CAPACITANCE
Figure 7 shows the effect of increased load capacitance on
the speed of the device. The rise and fall time increase by
7% and 7.5%, respectively, per additional pF above 10 pF.
EFFECT OF OFFSET
Figure 8 shows the variation in rise and fall times when the
DC offset of the 110V
120V and 150V
than 10% relative to the center data point (135V
time shows a variation of 14% relative to the center data
point.
THERMAL CONSIDERATIONS
Figure 9 shows the performance of the LM2432 as a function
of case temperature. The figure shows that the rise and fall
times of the LM2432 increase by approximately 17% and
20%, respectively, as the case temperature increases from
40˚C to 90˚C. This corresponds to a speed degradation of
only 3.5% and 4.0% for every 10˚C rise in case temperature.
POWER DISSIPATION AND HEATSINK CALCULATION
Worst-Case Power Dissipation
Figure 10 shows the maximum power dissipation of the
LM2432 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 a HDTV application. The other 20% of the
time the device is assumed to be sitting at the black level
(190V in this case). Under these worst-case condition, the
maximum power dissipated by the LM2432 is about 8.9W at
around 30 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 LM2432 was over
300 MHz. Using a RGB video processor or preamplifier with
less bandwidth may cause the LM2432 to dissipate less
power than shown in Figure 10 at the same conditions.
CC
of 220V and is driving a 10 pF load with 110V
DC
. The rise time shows a variation of less
PP
output swing is varied between
(Continued)
FIGURE 13. Recommended Application Circuit
DC
). The fall
PP
swing
7
A Practical Approach to Power Dissipation
The power curve (Figure 10) mentioned previously shows
the LM2432 power dissipation for square wave frequencies
ranging from 1 to 50 MHz at 110V
unrealistic 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 LM2432. Here is one approach:
A HDTV 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 LM2432 as a result
of this picture condition can be approximated by taking the
average of the power between 1 to 30 MHz in Figure 10. This
average is 7W. Because a square wave input was used to
generate this power curve, a sine wave would cause the
LM2432 to dissipate slightly less power, probably about
6.7W. This is one common way to determine a practical
figure for maximum power dissipation. It is the system de-
signer’s responsibility to establish the worst-case picture
condition for his particular application and measure dissipa-
tion under that condition to choose a proper heatsink.
Heatsink Calculation Example
Once the maximum dissipation is known, Figure 11 can be
used to determine the heatsink requirement for the LM2432.
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 LM2432 will dissipate about 6.7W.
The power derating curve shows that the maximum allowed
case temperature is 120˚C when 6.7W is dissipated. If the
maximum expected ambient temperature is 65˚C, then the
maximum thermal resistance from device case-to-sink (θ
can be calculated:
This example assumes a capacitive load of 10 pF and no
resistive load. The designer should note that if output swing,
V
increased, then the AC component of the total power dissi-
pation will also increase.
CC
supply voltage, input bandwidth, or load capacitance is
θ
CS
= (120˚C – 65˚C) / 6.7W = 8.2˚C/W
θ
CS
= (T
C MAX
20137710
– T
A MAX
PP
) / P
swing. In practice, it is
D MAX
www.national.com
CS
)