LM2407
Monolithic Triple 7.5 nS CRT Driver
General Description
The LM2407 is an integrated high voltage CRT driver circuit
designed for use in color monitor applications. The IC con-
tains three high input impedance, wide band amplifiers
which directly drive the RGB cathodes of a CRT. Each chan-
nel has its gain internally set to -14 and can drive CRT ca-
pacitive loads as well as resistive loads present in other ap-
plications, limited only by the package's power dissipation.
The IC is packaged in an industry standard 11-lead TO-220
molded plastic power package. See thermal considerations
on page 6.
Features
n
Low power dissipation
n
Well matched with LM1279 video preamp
n
0V to 5V input range
n
Stable with 0 pF20 pF capacitive loads and inductive
peaking networks
n
Convenient TO-220 staggered lead package style
n
Standard LM240X Family Pinout which is designed for
easy PCB layout
Applications
n
1024 x 768 displays up to 85 Hz refresh
n
Pixel clock frequencies up to 100 MHz
n
Monitors using video blanking
Schematic and Connection Diagrams
DS100093-1
FIGURE 1. Simplified Schematic Diagram
(One Channel)
DS100093-2
Note: Tab is at GND
Top View
Order Number LM2407T
August 1999
LM2407
Monolithic
T
riple
7.5
nS
CRT
Driver
1999 National Semiconductor Corporation
DS100093
www.national.com
Absolute Maximum Ratings
(Notes 1, 3)
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Supply Voltage, (V
CC
)
+90V
Bias Voltage, (V
BB
)
+16V
Input Voltage, (V
IN
)
-0.5V to V
BIAS
+0.5V
Storage Temperature Range, (T
STG
)
-65C to +150C
Lead Temperature
(Soldering,
<
10 sec.)
300C
ESD Tolerance, Human Body Model
2 kV
Machine Model
300V
Operating Range
(Note 2)
V
CC
+60V to +85V
V
BB
+8V to +15V
V
IN
+0V to +5V
V
OUT
+15V to +75V
Case Temperature
-20C to +100C
Do not operate the part without a heat sink.
Electrical Characteristics
(See
Figure 2 for Test Circuit)
Unless otherwise noted: V
CC
= +80V, V
BB
= +12V, V
IN
= +2.7 V
DC
, C
L
= 8 pF, Output = 40 V
PP
at 1 MHz, T
C
= 50C.
Symbol
Parameter
Condition
LM2407
Units
Min
Typical
Max
I
CC
Supply Current
Per Channel, No Output Load
11.5
mA
I
BB
Bias Current
All Three Channels
11
mA
V
OUT
DC Output Voltage
No AC Input Signal, V
IN
= 1.2V
62
65
68
V
DC
A
V
DC Voltage Gain
No AC Input Signal
-13.3
-13.9
-14.5
A
V
Gain Matching
(Note 4), No AC Input Signal
1.0
dB
LE
Linearity Error
(Notes 4, 5), No AC Input Signal
8
%
t
R
Rise Time
10% to 90%
7.5
nS
t
F
Fall Time
90% to 10%
7.5
nS
OS
Overshoot
Rising Edge
8
%
Falling Edge
2
Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur.
Note 2: Operating ratings indicate conditions for which the device is functional, but do not guarantee specific performance limits. For guaranteed specifications and
test conditions, see the Electrical Characteristics. The guaranteed specifications apply only for the test conditions listed. Some performance characteristics may
change when the device is not operated under the listed test conditions.
Note 3: All voltages are measured with respect to GND, unless otherwise specified.
Note 4: Calculated value from Voltage Gain test on each channel.
Note 5: Linearity Error is the variation in dc gain from V
IN
= 1.0V to V
IN
= 4.5V.
Note 6: Input from signal generator: t
r
, t
f
<
1 nS.
AC Test Circuit
Figure 2 shows a typical test circuit for evaluation of the LM2407. This circuit is designed to allow testing of the LM2407 in a 50
environment without the use of an expensive FET probe. The 4950
resistor at the output forms a 100:1 voltage divider when
connected to a 50
load.
DS100093-3
Note: 8 pF load includes parasitic capacitance.
FIGURE 2. Test Circuit (One Channel)
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2
AC Test Circuit
(Continued)
DS100093-4
FIGURE 3. V
OUT
vs V
IN
DS100093-5
FIGURE 4. Speed vs Temp.
DS100093-6
FIGURE 5. LM2407 Pulse Response
DS100093-7
FIGURE 6. Power Dissipation vs Frequency
DS100093-8
FIGURE 7. Speed vs Offset
DS100093-9
FIGURE 8. Speed vs Load Capacitance
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Theory of Operation
The LM2407 is a high voltage monolithic three channel CRT
driver suitable for high resolution display applications. The
LM2407 operates using 80V and 12V power supplies. The
part is housed in the industry standard 11-lead TO-220
molded plastic power package.
The circuit diagram of the LM2407 is shown in
Figure 1. A
PNP emitter follower, Q5, provides input buffering. Q1 and
Q2 form a fixed gain cascode amplifier with resistors R1 and
R2 setting the gain at -14. Emitter followers Q3 and Q4 iso-
late the high output impedance of the cascode stage from
the capacitance of the CRT cathode which decreases the
sensitivity of the device to load capacitance. Q6 provides bi-
asing to the output emitter follower stage to reduce cross-
over distortion at low signal levels.
Figure 2 shows a typical test circuit for evaluation of the
LM2407. This circuit is designed to allow testing of the
LM2407 in a 50
environment without the use of an expen-
sive FET probe. In this test circuit, two low inductance resis-
tors in series totaling 4.95 k
form a 100:1 wideband, low
capacitance probe when connected to a 50
coaxial cable
and a 50
load (such as a 50
oscilloscope input). The in-
put signal from the generator is ac coupled to the base of
Q1.
Application Hints
INTRODUCTION
National Semiconductor (NSC) is committed to provide ap-
plication information that assists our customers in obtaining
the best performance possible from our products. The follow-
ing information is provided in order to support this commit-
ment. The reader should be aware that the optimization of
performance was done using a specific printed circuit board
designed at NSC. Variations in performance can be realized
due to physical changes in the printed circuit board and the
application. Therefore, the designer should know that com-
ponent value changes may be required in order to optimize
performance in a given application. The values shown in this
document can be used as a starting point for evaluation pur-
poses. When working with high bandwidth circuits, good lay-
out practices are also critical to achieving maximum perfor-
mance.
IMPORTANT INFORMATION
The LM2407 performance is targeted for the XGA resolution
market (1024 x 768, 85 Hz refresh). It is not designed to be
a direct replacement for the LM2405 or LM2406. The appli-
cation circuits required to optimize performance and to pro-
tect against damage from CRT arcover are different for each
part. The application section in this document provides infor-
mation for the LM2407. Please refer to the LM2405 and
LM2406 data sheets for specific application information on
each of those devices.
POWER SUPPLY BYPASS
Since the LM2407 is a high bandwidth amplifier, proper
power supply bypassing is critical for optimum performance.
Improper power supply bypassing can result in large over-
shoot, ringing and oscillation. A 0.01 F capacitor should be
connected from the supply pin, V
CC
, to ground, as close to
the supply and ground pins as is practical. Additionally, a 10
F to 100 F electrolytic capacitor should be connected from
the supply pin to ground. The electrolytic capacitor should
also be placed reasonably close to the LM2407's supply and
ground pins. A 0.1 F capacitor should be connected from
the bias pin, V
BB
, to ground, as close as is practical to the
part.
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. Spark gaps, in the range of 200V, connected from
the CRT cathodes to CRT ground will limit the maximum volt-
age, but to a value that is much higher than allowable on the
LM2407. This fast, high voltage, high energy pulse can dam-
age the LM2407 output stage. The application circuit shown
in
Figure 9 is designed to help clamp the voltage at the out-
put of the LM2407 to a safe level. The clamp diodes should
have a fast transient response, high peak current rating, low
series impedance and low shunt capacitance. FDH400 or
equivalent diodes are recommended. D1 and D2 should
have short, low impedance connections to V
CC
and ground
respectively. The cathode of D1 should be located very close
to a separately decoupled bypass capacitor (C3 in
Figure 9).
The ground connection of the diode and the decoupling ca-
pacitor should be very close to the LM2407 ground. This will
significantly reduce the high frequency voltage transients
that the LM2407 would be subjected to during an arcover
condition. Resistor R2 limits the arcover current that is seen
by the diodes while R1 limits the current into the LM2407 as
well as the voltage stress at the outputs of the device. R2
should be a 1/2W solid carbon type resistor. R1 can be a
1/4W metal or carbon film type resistor. Inductor L1 is critical
to reduce the initial high frequency voltage levels that the
LM2407 would be subjected to. Having large value resistors
for R1 and R2 would be desirable, but this has the effect of
increasing rise and fall times. The inductor will not only help
protect the device but it 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 9.
DS100093-10
FIGURE 9. One Channel of the LM2407 with the Recommended Arc Protection Circuit
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4
Application Hints
(Continued)
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 LM2407. 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. Once the optimum values are deter-
mined the variable resistors can be replaced with fixed val-
ues.
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. The pre-
vious section discussed how to optimize the transient re-
sponse in the application with the use of a series inductor.
Effect of Offset
Figure 7 shows the variation in rise and fall times when the
output offset of the device is varied from 40 V
DC
to 50 V
DC
.
The rise time shows a maximum variation relative to the cen-
ter data point (45 V
DC
) is about 20%. The fall time shows a
variation of about 5% relative to the center data point.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2407 in the test
circuit shown in
Figure 2 as a function of case temperature.
The figure shows that the rise time of the LM2407 decreases
by approximately 5% as the case temperature increases
from 50C to 100C. This corresponds to a speed degrada-
tion of 1% for every 10C rise in case temperature. There is
a negligible change in fall time versus temperature in the test
circuit.
Figure 6 shows the total power dissipation of the LM2407 vs.
Frequency when all three channels of the device are driving
an 8 pF load with a 40V
p-p
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 informa-
tion needed to determine the heat sink requirement for his
application. The designer should note that if the load capaci-
tance is increased the AC component of the total power dis-
sipation will also increase.
The LM2407 case temperature must be maintained below
100C. If the maximum expected ambient temperature is
50C and the maximum power dissipation is 6.2W, 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 LM2407 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 LM1279/2407 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 LM2407 and from
the LM2407 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" 2nd Edition, John Wiley & Sons, New York, 1988.
"Guide to CRT Video Design", National Semiconductor Appli-
cation Note 861.
"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 routing and component placement on the
NSC LM1279/2407 demonstration board. The schematic of
the board is shown in
Figure 10. This board provides a good
example of a layout that can be used as a guide for future
layouts. Note the location of the following components:
C55 -- V
CC
bypass capacitor, located very close to pin 6
and ground pins
C43, C44 -- V
BB
bypass capacitors, located close to pin
10 and ground
C53C55 -- V
CC
bypass capacitors, near LM2407 and
V
CC
clamp diodes. Very important for arc protection
The routing of the LM2407 outputs to the CRT is very critical
to achieving optimum performance.
Figure 12 shows the
routing and component placement from pin 1 of the LM2407
to the blue cathode. Note that the components are placed so
that they almost line up from the output pin of the LM2407 to
the blue cathode pin of the CRT connector. This is done to
minimize the length of the video path between these two
components. Note also that D14, D15, R29 and D13 are
placed to minimize the size of the video nodes that they are
attached to. This minimizes parasitic capacitance in the
video path and also enhances the effectiveness of the pro-
tection diodes. The anode of protection diode D14 is con-
nected directly to a section of the the ground plane that has
a short and direct path to the LM2407 ground pins. The cath-
ode of D15 is connected to V
CC
very close to decoupling ca-
pacitor C55 (see
Figure 12) which is connected to the same
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