lm2412a National Semiconductor Corporation, lm2412a Datasheet - Page 4

• Manufacturer Part Number: lm2412a
• Description: Monolithic Triple 2.8 Ns Crt Driver
• Manufacturer: National Semiconductor Corporation

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Theory of Operation
The LM2412A is a high voltage monolithic three channel
CRT driver suitable for very high resolution display applica-
tions, up to 1600 x 1200 at 85 Hz refresh rate. The LM2412A
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
LM2412A 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 V
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 LM2412A. R1, R2
and R3 are all related when calculating the output voltage of
the CRT driver. R
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 tran-
sistors, 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 LM2412A. 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.
Figure 2 shows a typical test circuit for evaluation of the
LM2412A. Due to the very wide bandwidth of the LM2412A,
it is highly recommended that the stand alone board suplied
by NSC be used for the evaluation of the CRT driver’s
performance. The 50
quired 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
application information that assists our customers in obtain-
ing 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 optimiza-
tion 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.
b
limits the current through the base of Q2.
resistor is used to duplicate the re-
CC
rail, minimizing the
4
POWER SUPPLY BYPASS
Since the LM2412A is a very high bandwidth amplifier,
proper power supply bypassing is critical for optimum per-
formance. Improper power supply bypassing can result in
large overshoot, ringing and oscillation. A 0.1 µF capacitor
should be connected from the supply pin, V
close to the supply and ground pins as is practical. Addition-
ally, 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
LM2412A’s supply and ground pins. A 0.1 µF capacitor
should be connected from the bias pin, V
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
voltage, but to a value that is much higher than allowable on
the LM2412A. This fast, high voltage, high energy pulse can
damage the LM2412A output stage. The application circuit
shown in Figure 9 is designed to help clamp the voltage at
the output of the LM2412A 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
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 LM2412A ground. This will
significantly reduce the high frequency voltage transients
that the LM2412A 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 LM2412A as
well as the voltage stress at the outputs of the device. R2
should be a
metal or carbon film type resistor. Inductor L1 is critical to
reduce the initial high frequency voltage levels that the
LM2412A 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 75 , with L1 =
.049 µH.
FIGURE 9. One Channel of the LM2412A with the
Recommended Arc Protection Circuit.
1
⁄
2
W solid carbon type resistor. R1 can be a
BB
CC
, to ground, as
, to ground, as
DS200124-10
CC
1
and
⁄
4
W