LM2403 National Semiconductor Corporation, LM2403 Datasheet - Page 5

• Manufacturer Part Number: LM2403
• Description: Monolithic Triple 4.5 CRT Driver
• Manufacturer: National Semiconductor Corporation

Application Hints
will also help optimize rise and fall times as well as minimize
EMI. For proper arc protection, it is important to not omit any
of the arc protection components shown in Figure 10 .
OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 10 , 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 Mag-
netics (part # 78FR12M) were used for optimizing the perfor-
mance of the device in the NSC application board. The val-
ues shown in Figure 10 can be used as a good starting point
for the evaluation of the LM2403. The NSC demo board also
has a position open to add a resistor in parallel with L1. This
resistor can be used to help control overshoot. Using vari-
able resistors for R1 and the parallel resistor is a great way
to help dial in the values needed for optimum performance in
a given application.
Pull-up Resistors
Optimizing the performance of the LM2403 does require the
use of pull-up resistors at the outputs of the CRT driver.
These resistors are shown as R100, R101, and R102 in the
schematic. If you have a demo board form National please
note that these resistors have been added on the back of the
board since there is no PCB location for the pull-up resistors.
Because of the improved performance with these resistors,
all demo boards have been shipped with the added pull-up
resistors. The LM2403 does have some crossover distortion,
normal for any AB amplifier such as the LM2403. Adding the
pull-up resistors does add more bias to Q3 ( Figure 1 ) thus
minimizing the crossover distortion. The LM2403 is normally
used in high end monitors, so it is highly recommended that
the 12k pull-up resistors be used in any design using the
LM2403. Selecting a 12k resistor provides the needed
pull-up current and limits the worst case power dissipation to
1/4W (white level at 25V).
In some applications pull-down resistors may be preferred.
Using 12k resistors gives acceptable performance, but this
will require the use of 1/2W resistors. Normally the power
save mode establishes whether pull-up or pull-down resis-
tors are preferred. If the setup of the power save mode in the
monitor gives a low output at the LM2403, then the pull-down
resistors would be preferred, if the 80V supply is still turned
on.
FIGURE 10. One Channel of the LM2403 with the Recommended Arc Protection Circuit
(Continued)
5
Effect of Load Capacitance
The output rise and fall times as well as overshoot will vary
as the load capacitance varies. The values of the output cir-
cuit (R1, R2 and L1 in Figure 10 ) should be chosen based on
the nominal load capacitance. Once this is done the perfor-
mance of the design can be checked by varying the load
based on what the expected variation will be.
For example, suppose you needed to drive a 10 pF (
load with a 40V
of R1, R2 and L1 that give the desired response with a 10 pF
load. Then you would test the design when driving an 8 pF
load and a 12 pF load. The table below summarizes the re-
sults from doing this exercise in a test board in the NSC lab.
The output signal swing was 40V
The example above clearly demonstrates the importance of
having a good estimate of the range of the load capacitance.
Effect of Offset
Figure 7 shows the variation in rise and fall times when the
output offset of the device is varied from 30 V
The rise time shows about twice as much variation as the fall
time, however the maximum variation relative to the center
data point (40 V
Operation with V
The closed loop topography of the LM2403 allows operation
down to 10V above ground. If the user can limit the white
level between 10V and 20V, then operation with V
is possible. Operating the LM2403 with V
quire the same current even though the supply voltage has
dropped by 12.5%. This results in a power savings of 12.5%
(as high a 1.5W), allowing a reduction in the size of the heat-
sink. Figure 8 shows the output waveform of the LM2403 op-
erating at a white level of 15V, and a peak-to-peak output
swing of 40V. Below is a summary of the LM2403 rise and
fall times with various output offset levels with V
Rise Time
Overshoot
Fall Time
Overshoot
Parameter
p-p
DC
waveform. First, you would pick the values
CC
) is less than 10%.
8 pF
4.1
1%
4.4
1%
= 70V
DS100082-10
p-p
10 pF
4.2
5%
4.6
2%
from 65V to 25V.
CC
= 70V will re-
DC
www.national.com
CC
12 pF
to 50 V
10%
5%
CC
4.3
4.7
= 70V.
±
= 70V
20%)
DC
.