LM2437 National Semiconductor Corporation, LM2437 Datasheet - Page 5

• Manufacturer Part Number: LM2437
• Description: LM2437 - Monolithic Triple 7.5 NS CRT Driver, Package: to 220, Pin Nb=9
• Manufacturer: National Semiconductor Corporation

Application Hints
OPTIMIZING TRANSIENT RESPONSE
Referring to Figure 9 , 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 # 78FR56M) were used for optimizing the perfor-
mance of the device in the NSC application board. The val-
ues shown in Figure 9 can be used as a good starting point
for the evaluation of the LM2437. 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 will simplify
finding the values needed for optimum performance in a
given application. Once the optimum values are determined
the variable resistors can be replaced with fixed values.
EFFECT OF LOAD CAPACITANCE
Figure 8 shows the effect of increased load capacitance on
the speed of the device. This demonstrates the importance
of knowing the load capacitance in the application.
EFFECT OF OFFSET
Figure 7 shows the variation in rise and fall times when the
output offset of the device is varied from 40 to 50 V
rise time shows a maximum variation relative to the center
data point (45 V
variation of about 5% relative to the center data point.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2437 in the test
circuit shown in Figure 2 as a function of case temperature.
The figure shows that the rise time of the LM2437 increases
by approximately 5% as the case temperature increases
from 50˚C to 100˚C. This corresponds to a speed degrada-
tion of 1% for every 10˚C rise in case temperature.There is a
negligible change in fall time vs. temperature in the test cir-
cuit.
Figure 6 shows the maximum power dissipation of the
LM2437 vs. Frequency when all three channels of the device
are driving an 8 pF load with a 40 V
on, one pixel off signal. The graph assumes a 72% active
time (device operating at the specified frequency) which is
typical in a monitor application. The other 28% of the time
the device is assumed to be sitting at the black level (65V in
this case). This graph gives the designer the information
DC
FIGURE 9. One Channel of the LM2437 with the Recommended Arc Protection Circuit
) of about 20%. The fall time shows a
(Continued)
p-p
alternating one pixel
DC
. The
5
needed to determine the heat sink requirement for his appli-
cation. The designer should note that if the load capacitance
is increased the AC component of the total power dissipation
will also increase.
The LM2437 case temperature must be maintained below
100˚C. If the maximum expected ambient temperature is
70˚C and the maximum power dissipation is 6.2W (from Fig-
ure 6 , 50 MHz bandwidth) then a maximum heat sink thermal
resistance can be calculated:
This example assumes a capacitive load of 8 pF and no re-
sistive load.
TYPICAL APPLICATION
A typical application of the LM2437 is shown in Figure 10 .
Used in conjunction with an LM1279, a complete video chan-
nel from monitor input to CRT cathode can be achieved. Per-
formance is ideal for 1024 x 768 resolution displays with
pixel clock frequencies up to 100 MHz. Figure 10 is the sche-
matic for the NSC demonstration board that can be used to
evaluate the LM1279/2437 combination in a monitor.
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance, an adequate ground plane, isola-
tion between channels, good supply bypassing and minimiz-
ing unwanted feedback are necessary. Also, the length of the
signal traces from the preamplifier to the LM2437 and from
the LM2437 to the CRT cathode should be as short as pos-
sible. 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 os-
cillate in a monitor if feedback occurs around the video chan-
nel through the chassis wiring. To prevent this, leads to the
video amplifier input circuit should be shielded, and input cir-
cuit wiring should be spaced as far as possible from output
circuit wiring.
NSC DEMONSTRATION BOARD
Figure 11 shows the routing and component placement on
the NSC LM1279/2437 demonstration board. The schematic
of the board is shown in Figure 10 . This board provides a
DS100932-10
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