NSC LM2412

Monolithic Triple 2.8 ns CRT Driver
General Description
The LM2412 is an integrated high voltage CRT driver circuit
designed for use in high resolution color monitor applications. 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 presented by other applications, limited only by the package’s
power dissipation. The LM2412 is a low power alternative of
the LM2402
The IC is packaged in an industry standard 11 lead TO-220
molded plastic power package. See thermal considerations
section for heat sinking requirements.
Lower power than LM2402 with the same bandwidth
Well matched with LM2202 video preamps
Output swing capability: 50 VPP for VCC = 80V
1V to 5V input range
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
n CRT driver for color monitors with display resolutions up
to 1600 x 1200 with 85 Hz refresh rate
n Pixel clock frequency up to 200 MHz
n Rise/fall times typically 2.8 ns with 8 pF load at 40 VPP
Schematic and Connection Diagrams
FIGURE 1. Simplified Schematic Diagram
(One Channel)
© 1999 National Semiconductor Corporation
Top View
Order Number LM2412T
See NS package Number
LM2412 Monolithic Triple 2.8 ns CRT Driver
December 1999
Absolute Maximum Ratings (Notes 1, 2)
ESD Tolerance
If Military/Aerospace specified devices are required,
please contact the National Semiconductor Sales Office/
Distributors for availability and specifications.
Human Body Model
2 kV
Machine Model
Operating Ranges (Note 3)
Supply Voltage, VCC
Bias Voltage, VBB
+60V to +85V
Input Voltage, VIN
0V to 6V
+8V to +15V
−65˚C to +150˚C
Storage Temperature Range, TSTG
Lead Temperature (Soldering, < 10 sec.)
+1V to +5V
VOUT (VCC = 80V, VBB = 12V)
+15V to +75V
Case Temperature
−20˚C to +100˚C
Do not operate the part without a heat sink.
Electrical Characteristics
(See Figure 2 for Test Circuit)
Unless otherwise noted: VCC = +80V, VBB = +12V, VIN = +3.3 VDC, CL = 8 pF, TC = 60˚C, no AC input.
Supply Current
Per Channel, No Output Load
Bias Current
DC Output Voltage
All Three Channels
VIN = 1.9V
DC Voltage Gain
Gain Matching
(Note 4)
Linearity Error
(Notes 4, 5)
Rise Time (Notes 6, 7)
10% to 90%, 40 VPP Output (1 MHz)
Fall Time (Notes 6, 7)
10% to 90%, 40 VPP Output (1 MHz)
40 VPP Output (1 MHz)
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 VIN = 1.6V to VIN = 5.0V.
Note 6: Input from signal generator: tr, tf < 1 ns.
Note 7: 100% tested in production. These limits are not used to calculate outgoing quality levels.
AC Test Circuit
FIGURE 2. Test Circuit (One Channel)
Figure 2 shows a typical test circuit for evaluation of the
LM2412. This circuit is designed to allow testing of the
LM2412 in a 50Ω environment without the use of an expensive FET probe. The combined resitors of 4950Ω at the output form a 200:1 voltage divider when connected to a 50Ω
load. The test board supplied by NSC also offers the option
to test theLM2412 with a FET probe. CL is the total capaciwww.national.com
tance at the LM2412 output, including the board capacitance.
Typical Performance Characteristics
FIGURE 6. Power Dissipation vs Frequency
FIGURE 4. Speed vs Temp.
FIGURE 7. Speed vs Offset
FIGURE 5. Rise/Fall Time
FIGURE 8. Bandwidth
shoot, ringing and oscillation. A 0.1 µF capacitor should be
connected from the supply pin, VCC, to ground, as close to
the supply and ground pins as is practical. Additionally, a
10 µF to 100 µF electrolytic capacitor should be connected
from the supply pin to ground. The electrolytic capacitor
should also be placed reasonably close to the LM2412’s
supply and ground pins. A 0.1 µF capacitor should be connected from the bias pin, VBB, to ground, as close as is practical to the part.
Theory of Operation
The LM2412 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 LM2412 operates 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 LM2412
is shown in Figure 1. A PNP emitter follower, Q5, provides input buffering. This minimizes the current loading of the video
pre-amp. R9 is used to turn on Q5 when there is no input.
With Q5 turned on, Q1 will be almost completely off, minimizing the current flow through Q1 and Q2. This will drive the
output stage near the VCC 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 LM2412. R1, R2 and R3 are all related when calculating the output voltage of the CRT driver.
Rb limits the current through the base of Q2. Q1 and Q2 are
in a cascode configuration. Q1 is a low voltage and very fast
transistor. Q2 is a higher voltage transistor. The cascode
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 LM2412. 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 current the output stage now becomes a class AB amplifier with a crossover distortion much lower than a class B amplifier.
During normal CRT operation, internal arcing may occasionally occur. Spark gaps, in the range of 200V, connected from
the CRT cathodes to CRT ground will limit the maximum voltage, but to a value that is much higher than allowable on the
LM2412. This fast, high voltage, high energy pulse can damage the LM2412 output stage. The application circuit shown
in Figure 9 is designed to help clamp the voltage at the output of the LM2412 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 VCC 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 LM2412 ground. This will significantly reduce the high frequency voltage transients that the LM2412
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 LM2412 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 LM2412 would be subjected to during an arc-over. 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. The values of L1 and R1 may need to be
adjusted for a particular application. The recommended minimum value for R1 is 75Ω, with L1 = .049 µH.
Figure 2 shows a typical test circuit for evaluation of the
LM2412. Due to the very wide bandwidth of the LM2412, it is
highly recommended that the stand alone board suplied by
NSC be used for the evaluation of the CRT driver’s performance. The 50Ω 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
Application Hints
National Semiconductor (NSC) is committed to providing application information that assists our customers in obtaining
the best performance possible from our products. The following information is provided in order to support this commitment. 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 component 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 purposes. When working with high bandwidth circuits, good layout practices are also critical to achieving maximum performance.
FIGURE 9. One Channel of the LM2412 with the
Recommended Arc Protection Circuit.
Referring to Figure 9, there are three components (R1, R2
and L1) that can be adjusted to optimize the transient response of the application circuit. Increasing the values of R1
and R2 will slow the circuit down while decreasing overshoot. Increasing the value of L1 will speed up the circuit as
well as increase overshoot. It is very important to use inductors with very high self-resonant frequencies, preferably
Since the LM2412 is a very high bandwidth amplifier, proper
power supply bypassing is critical for optimum performance.
Improper power supply bypassing can result in large overwww.national.com
above 300 MHz. Air core inductors from J.W. Miller Magnetics (part #75F518MPC) were used for optimizing the performance of the device in the NSC application board. The values shown in Figure 9 can be used as a good starting point
for the evaluation of the LM2412.
Divide the result from step 3 by 0.72. For 100 MHz, the
result is 18.1W.
Multiply the result in 4 by the new active time percentage.
Multiply 2.7W by the new inactive time.
Add together the results of steps 5 and 6. This is the expected power dissipation for the LM2412 in the designer’s application.
The LM2412 case temperature must be maintained below
100˚C. If the maximum expected ambient temperature is
70˚C and the maximum power dissipation is 13.8W (from
Figure 6. 100MHz) then a maximum heat sink thermal resistance can be calculated:
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 circuit (R1, R2 and L1 in Figure 9) should be chosen based on
the nominal load capacitance. Once this is done the performance of the design can be checked by varying the load
based on what the expected variation will be during production.
Effect of Offset
A typical application of the LM2412 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/LM2412 combination in a
Figure 7 shows the variation in rise and fall times when the
output offset of the device is varied from 35 to 55 VDC. The
rise and fall times show about the same overall variation.
The slightly slower fall time is fastest near the center point of
45V, making this the optimum operating point. At the low and
high output offset range, the characteristic of rise/fall time is
slower due to the saturation of Q3 and Q4. The recovery
time of the output transistors takes longer coming out of
saturation thus slows down the rise and fall times.
PC Board Layout Considerations
For optimum performance, an adequate ground plane, isolation between channels, good supply bypassing and minimizing unwanted feedback are necessary. Also, the length of the
signal traces from the preamplifier to the LM2412 and from
the LM2412 to the CRT cathode should be as short as possible. The red video trace from the buffer transistor to the
LM2412 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 following references are recommended for video board designers:
Ott, Henry W., “Noise Reduction Techniques in Electronic
Systems”, John Wiley & Sons, New York, 1976.
“Guide to CRT Video Design”, National Semiconductor Application Note 861.
“Video Amplifier Design for Computer Monitors”, National
Semiconductor Application Note 1013.
Pease, Robert A., “Troubleshooting Analog Circuits”,
Butterworth-Heinemann, 1991.
Figure 4 shows the performance of the LM2412 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
LM2412 become slightly longer as the case temperature increases from 40˚C to 125˚C. In addition to exceeding the
safe operating temperature, the rise and fall times will typically exceed 3 nsec. Please note that the LM2412 is never
to be operated over a case temperature of 100˚C. In addition to exceeding the safe operating temperature, the rise
and fall times will typically exceed 3 nsec.
Figure 6 shows the total power dissipation of the LM2412 vs.
Frequency when all three channels of the device are driving
an 8 pF load. Typically the active time is about 72% of the total time for one frame. Worst case power dissipation is when
a one on, one off pixel is displayed over the active time of the
video input. This is the condition used to measure the total
power disspation of the LM2412 at different input frequencies. Figure 6 gives all the information a monitor designer
normally needs for worst case power dissipation. However, if
the designer wants to calculate the power dissipation for an
active time different from 72%, this can be done using the information in Figure 14. The recommended input black level
voltage is 1.9V. From Figure 14, if a 1.9V input is used for
the black level, then power dissipation during the inactive
video time is 2.7W. This includes both the 80V and 12V supplies.
If the monitor designer chooses to calculate the power dissipation for the LM2412 using an active video time different
from 72%, then he needs to use the following steps when using a 1.9V input black level:
1. Multiply the black level power dissipation, 2.7W, by 0.28,
the result is 0.8W.
2. Choose the maximum frequency to be used. A typical
application would use 100 MHz, or a 200 MHz pixel
clock. The power dissipation is 13.8W.
3. Subtract the 0.8W from the power dissipation from Figure 6. For 100 MHz this would be 13.8 – 0.8 = 13.0W.
Because of its high small signal bandwidth, the part may oscillate in a monitor if feedback occurs around the video channel through the chassis wiring. To prevent this, leads to the
video amplifier input circuit should be shielded, and input circuit 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 LM2202/2412 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:
C47 - VCC bypass capacitor, located very close to pin 6
and ground pins. (Figure 12)
C49 - VBB bypass capacitor, located close to pin 10 and
ground. (Figure 12)
Application Hints
Application Hints
D25, R58 and D19 are placed to keep the size of the video
nodes to a minimum (R58 is located under D19). This minimizes parasitic capacitance in the video path and also enhances the effectiveness of the protection diodes. The traces
in the video nodes to these components are shown by the
white line. The anode of protection diode D25 is connected
directly to the ground plane giving a short and direct path to
the LM2412 ground pins. The cathode of D24 is connected
to VCC very close to decoupling capacitor C78 (Figure 13)
which is connected to the same section of the ground plane
as D25. The diode placement and routing is very important
for minimizing the voltage stress on the LM2412 during an
arc-over event. Lastly, notice that S3 is placed very close to
the blue cathode and is tied directly to CRT ground.
• C46 and C77 - VCC bypass capacitors, near LM2412 and
VCC clamp diodes. Very important for arc protection. (Figure 11)
The routing of the LM2412 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 LM2412 to the blue
cathode pin of the CRT connector. This is done to minimize
the length of the video path between these two components.
The direct video path is shown in by a dark gray line through
the components and the PCB traces. Note also that D24,
FIGURE 10. Demo Board Schematic
Application Hints
Application Hints
FIGURE 11. PCB Top Layer
Application Hints
FIGURE 12. PCB Bottom Layer
Application Hints
FIGURE 13. PCB CRT Driver, Blue Channel Output
LM2412 Monolithic Triple 2.8 ns CRT Driver
Physical Dimensions
inches (millimeters) unless otherwise noted
11 Lead Molded TO-220
NS Package Number TA11B
Order Number LM2412T
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