LM2402
Monolithic Triple 3 ns CRT Driver
General Description
The LM2402 is an integrated high voltage CRT driver circuit
designed for use in high resolution color monitor applica-
tions. The IC contains three high input impedance, wide
band amplifiers which directly drive the RGB cathodes of a
CRT. Each channel has its gain internally set to -14 and can
drive CRT capacitive loads as well as resistive loads pre-
sented by other applications, 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 5.
Features
n
Rise/fall times typically 3.0/2.8 ns with 8 pF load at
40 V
PP
n
Well matched with LM2202 video preamps
n
Output swing capability: 50 V
PP
for V
CC
= 80V
n
1V to 5V input range
n
Stable with 0-20 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
CRT driver for color monitors with display resolutions up
to 1600 x 1200
n
Pixel clock frequency up to 200 MHz
Schematic and Connection Diagrams
DS101016-1
FIGURE 1. Simplified Schematic Diagram
(One Channel)
DS101016-2
Top View
Order Number LM2402T
August 1999
LM2402
Monolithic
T
riple
3
n
s
CRT
Driver
1999 National Semiconductor Corporation
DS101016
www.national.com
Absolute Maximum Ratings
(Notes 1, 2)
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
250V
Recommended Operating
Conditions
(Note 3)
V
CC
+60V to +85V
V
BB
+8V to +15V
V
IN
+1V to +5V
V
OUT
(V
CC
= 80V, V
BB
= 12V)
+17V to +72V
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
= +3.3 V
DC
, C
L
= 8 pF, T
C
= 60C.
Symbol
Parameter
Conditions
LM2402
Units
Min
Typ
Max
I
CC
Supply Current
Per Channel, No Output Load
22
27
32
mA
I
BB
Bias Current
All Three Channels
40
50
60
mA
V
OUT
DC Output Voltage
V
IN
= 1.9V
62
65
68
V
DC
A
V
DC Voltage Gain
-12
-14
-16
A
V
Gain Matching
(Note 4)
1.0
dB
LE
Linear Error
(Notes 4, 5)
3.5
%
t
r
Rise Time
10% to 90%, 40 V
PP
Output (1 MHz)
3.0
ns
t
f
Fall Time
10% to 90%, 40 V
PP
Output (1 MHz)
2.8
ns
OS
Overshoot
40 V
PP
Output (1 MHz)
5
%
Note 1: "Absolute Maximum Ratings" are those values beyond which the safety of the device cannot be guaranteed. They are not meant to imply that the devices
should be operated at these limits. The table of "Electrical Characteristics" specifies conditions of device operation.
Note 2: All voltages are measured with respect to GND, unless otherwise specified.
Note 3: 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 4: Calculated value from voltage gain test on each channel.
Note 5: Linearity error is the variation in DC gain from V
IN
= 1.5V to V
IN
= 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
LM2402. This circuit is designed for testing the LM2402 with
a FET probe. When calculating the total load capacitance,
the Tektronix P6201 FET probe with a 100:1 divider is speci-
fied to have 1.5 pF. The total board capacitance should be
6.5 pF.
DS101016-3
FIGURE 2. Test Circuit (One Channel)
www.national.com
2
Typical Performance Characteristics
DS101016-4
FIGURE 3. V
IN
vs V
OUT
DS101016-6
FIGURE 4. Speed vs Temp.
DS101016-8
FIGURE 5. Rise/Fall Time
DS101016-5
FIGURE 6. Power Dissipation vs Frequency
DS101016-7
FIGURE 7. Speed vs Offset
DS101016-9
FIGURE 8. Bandwidth
www.national.com
3
Theory of Operation
The LM2402 is a high voltage monolithic three channel CRT
driver suitable for very high resolution display applications,
up to 1600 x 1200 at 85 Hz refresh rate. The LM2402 oper-
ates using 80V and 12V power supplies. The part is housed
in the industry standard 11-lead TO-220 molded plastic
power package.
The simplified circuit diagram of one channel of the LM2402
is shown in
Figure 1. A PNP emitter follower, Q5, provides in-
put buffering. This minimizes the current loading of the video
pre-amp. R9 is used to turn off Q5 when there is no input.
This will drive the output stage to the V
CC
rail, minimizing the
power dissipation with no inputs. R6 is a pull-up resistor for
Q5 and also limits the current flow through Q5. R3 and R2
are used to set the current flow through Q1 and Q2. The ratio
of R1 to R2 is used to set the gain of the LM2402. R1, R2
and R3 are all related when calculating the output voltage of
the CRT driver. R
b
limits the current through the base of Q2.
Q1 and Q2 are in a cascade configuration. Q1 is a low volt-
age and very fast transistor. Q2 is a higher voltage transistor.
The cascade configuration gives the equivalent of a very fast
and high voltage transistor. The two output transistors, Q3
and Q4, form a class B amplifier output stage. R4 and R5 are
used to limit the current through the output stage and set the
output impedance of the LM2402. Q6, along with R7 and R8
set the bias current through Q3 and Q4 when there is no
change in the signal level. This bias current minimizes the
crossover distortion of the output stage. With this bias cur-
rent the output stage now becomes a class AB amplifier with
a crossover distortion much lower than a class B amplifier.
Figure 2 shows a typical test circuit for evaluation of the
LM2402. Due to the very wide bandwidth of the LM2402, it is
necessary to use a FET probe that is DC coupled to the out-
put for evaluation of the CRT driver's performance. The 50
resistor is used to duplicate the required series resistor in the
actual application. This resistor would be part of the arc-over
protection circuit. The input signal from the generator is AC
coupled to the input of the CRT driver.
Application Hints
INTRODUCTION
National Semiconductor (NSC) is committed to providing 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.
POWER SUPPLY BYPASS
Since the LM2402 is a very 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 0.1 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 LM2402's
supply and ground pins. A 0.1 F capacitor should be con-
nected from the bias pin, V
BB
, to ground, as close as is prac-
tical 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
LM2402. This fast, high voltage, high energy pulse can dam-
age the LM2402 output stage. The application circuit shown
in
Figure 9 is designed to help clamp the voltage at the out-
put of the LM2402 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. The ground
connection of the diode and the decoupling capacitor should
be very close to the LM2402 ground. This will significantly re-
duce the high frequency voltage transients that the LM2402
would be subjected to during an arc-over condition. Resistor
R2 limits the arc-over current that is seen by the diodes while
R1 limits the current into the LM2402 as well as the voltage
stress at the outputs of the device. R2 should be a
1
/
2
W solid
carbon type resistor. R1 can be a
1
/
4
W metal or carbon film
type resistor. Inductor L1 is critical to reduce the initial high
frequency voltage levels that the LM2402 would be sub-
jected to during an arc-over. Having large value resistors for
R1 and R2 would be desirable, but this has the effect of in-
creasing 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. The values of L1 and R1 may need to be
adjusted for a particular application. The recommended mini-
mum value for R1 is 43
, with L1 = .049 H.
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. Air core inductors from J.W. Miller Magnet-
ics (part #75F518MPC) were used for optimizing the perfor-
DS101016-10
FIGURE 9. One Channel of the LM2402 with the
Recommended Arc Protection Circuit.
www.national.com
4
Application Hints
(Continued)
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 LM2402.
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 9) 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 during produc-
tion.
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
DC
. The
rise and fall times show about the same overall variation.
The slightly faster fall time is fastest near the center point of
45V, making this the optimum operating point since there is
little increase in the rise time.
THERMAL CONSIDERATIONS
Figure 4 shows the performance of the LM2402 in the test
circuit shown in
Figure 2 as a function of case temperature.
Figure 4 shows that both the rise and fall times of the
LM2402 become slightly slower as the case temperature in-
creases from 40C to 125C. In addition to exceeding the
safe operating temperature, the rise and fall times will typi-
cally exceed 3 nsec. Please note that the LM2402 is never
to be operated over a case temperature of 100C.
Figure 6 shows the total power dissipation of the LM2402 vs.
Frequency when all three channels of the device are driving
both an 8 pF load and a 20 pF load. This graph gives the de-
signer the information needed to determine the heat sink re-
quirement for his application. The designer should note that
if the load capacitance is increased the AC component of the
total power dissipation will also increase as shown in
Figure
6. The designer should also remember that the actual video
signal has a period of around 70% to 75%. The remainder of
the time the video signal is inactive, or at the black level (be-
low the black level if blanked). During this time the LM2402
will be at the black level, or below, dissipating under 4W. Re-
ferring to
Figure 14 and using an input black level voltage of
1.9V, the power dissipation during the inactive video time is
3.8W, including both the 80V and 12V supplies.
The LM2402 case temperature must be maintained below
100C. Assume the worst case operating condition is a
100 MHz square wave during active video (a pixel clock of
200 MHz with one pixel on, one pixel off). From
Figure 6 one
can see that the power dissipation of the LM2402 is 28W if
the 100 MHz square wave is applied all the time. One must
also compensate for the inactive period of video. From
Fig-
ure 14 it has been calculated that the power dissipation dur-
ing the inactive video is 4W. Therefore there is an additional
24W of power dissipation due to the AC signal. Assume that
the AC signal is active 72% of the time. Now the AC power
dissipation is:
24W x 0.72 = 17W
The total power dissipation for 72% active video time is:
17W + 4W = 21W
If the maximum expected ambient temperature is 50C and
using the maximum power dissipation of 21W (video being
active only 72% of the frame), 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 LM2402 is shown in
Figure 10.
Used in conjunction with three LM2202s, a complete video
channel from monitor input to CRT cathode can be achieved.
Performance is excellent for resolutions up to 1600 x 1200
and pixel clock frequencies at 200 MHz.
Figure 10 is the
schematic for the NSC demonstration board that can be
used to evaluate the LM2202/2402 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 LM2402 and from
the LM2402 to the CRT cathode should be as short as pos-
sible. The red video trace from the buffer transistor to the
LM2402 input is about the absolute maximum length one
should consider on a PCB layout. If possible the traces
should actually be shorter than the red video trace. The fol-
lowing references are recommended for video board design-
ers:
Ott, Henry W., "Noise Reduction Techniques in Electronic
Systems", John Wiley & Sons, New York, 1976.
"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
Figures 11, 12 show routing and component placement on
the NSC LM2202/2402 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 fu-
ture layouts. Note the location of the following components:
C47 - V
CC
bypass capacitor, located very close to pin 6
and ground pins. (
Figure 12)
C49 - V
BB
bypass capacitor, located close to pin 10 and
ground. (
Figure 12)
C46 and C77 - V
CC
bypass capacitors, near LM2402 and
V
CC
clamp diodes. Very important for arc protection. (
Fig-
ure 11)
The routing of the LM2402 outputs to the CRT is very critical
to achieving optimum performance.
Figure 13 shows the
routing and component placement from pin 1 to the blue
cathode. Note that the components are placed so that they
almost line up from the output pin of the LM2402 to the blue
www.national.com
5
PDF 
ZIP