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National Semiconductor

LM2402T Datasheet

Monolithic Triple 3 ns CRT Driver

LM2402T Datasheet Preview

August 1999
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.
n Rise/fall times typically 3.0/2.8 ns with 8 pF load at
40 VPP
n Well matched with LM2202 video preamps
n Output swing capability: 50 VPP for VCC = 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
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
FIGURE 1. Simplified Schematic Diagram
(One Channel)
Top View
Order Number LM2402T
© 1999 National Semiconductor Corporation DS101016
Page 1

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, VCC
Bias Voltage, VBB
Input Voltage, VIN
−0.5V to VBIAS + 0.5V
Storage Temperature Range, TSTG −65˚C to +150˚C
Lead Temperature (Soldering, <10 sec.)
ESD Tolerance
Human Body Model
Machine Model
2 kV
Recommended Operating
Conditions (Note 3)
VOUT (VCC = 80V, VBB = 12V)
Case Temperature
+60V to +85V
+8V to +15V
+1V to +5V
+17V to +72V
−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.
Supply Current
Bias Current
DC Output Voltage
DC Voltage Gain
Gain Matching
Linear Error
Per Channel, No Output Load
All Three Channels
VIN = 1.9V
(Note 4)
(Notes 4, 5)
22 27 32
40 50 60
62 65 68
−12 −14 −16
tr Rise Time
tf Fall Time
OS Overshoot
10% to 90%, 40 VPP Output (1 MHz)
10% to 90%, 40 VPP Output (1 MHz)
40 VPP Output (1 MHz)
3.0 ns
2.8 ns
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 = 1.5V to VIN = 5V.
Note 6: Input from signal generator: tr, tf < 1 ns.
AC Test Circuit
FIGURE 2. Test Circuit (One Channel)
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.
Page 2

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
Page 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 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 LM2402. 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 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
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-
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 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 LM2402’s
supply and ground pins. A 0.1 µF capacitor should be con-
nected from the bias pin, VBB, to ground, as close as is prac-
tical to the part.
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 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 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 12W solid
carbon type resistor. R1 can be a 14W 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 µH.
FIGURE 9. One Channel of the LM2402 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 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-
Page 4
Part Number LM2402T
Manufactur National Semiconductor
Description Monolithic Triple 3 ns CRT Driver
Total Page 11 Pages
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