lm2421 National Semiconductor Corporation, lm2421 Datasheet - Page 6

• Manufacturer Part Number: lm2421
• Description: Monolithic Triple Channel Hdtv Driver
• Manufacturer: National Semiconductor Corporation

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Theory of Operation
The LM2421 is a high voltage monolithic three channel CRT
driver suitable for HDTV applications. The LM2421 operates
with 145V and 8V power supplies. The part is housed in the
industry standard 11-lead TO-220 molded plastic power
package.
The circuit diagram of the LM2421 is shown in Figure 1 . The
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 −42. Emitter followers Q3 and Q4
isolate 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
biasing 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
LM2421. This circuit is designed to allow testing of the
LM2421 in a 50
sive FET probe. In this test circuit, the two 4.99k
form a 400:1 wideband, low capacitance probe when con-
nected to a 50
50
is ac coupled to the base of Q5.
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 fol-
lowing information is provided in order to support this com-
mitment. 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
purposes. When working with high bandwidth circuits, good
layout practices are also critical to achieving maximum per-
formance.
IMPORTANT INFORMATION
The LM2421 performance is targeted for the HDTV market.
The application circuits shown in this document to optimize
performance and to protect against damage from CRT ar-
cover are designed specifically for the LM2421. If another
member of the LM242X family is used, please refer to its
datasheet.
POWER SUPPLY BYPASS
Since the LM2421 is a wide bandwidth amplifier, proper
power supply bypassing is critical for optimum performance.
Improper power supply bypassing can result in large over-
shoot, ringing or oscillation. 0.1 µF capacitors should be
connected from the supply pins, V
close to the LM2421 as is practical. Additionally, a 47 µF or
larger electrolytic capacitor should be connected from both
supply pins to ground reasonably close to the LM2421.
ARC PROTECTION
During normal CRT operation, internal arcing may occasion-
ally occur. Spark gaps, in the range of 300V, connected from
the CRT cathodes to CRT ground will limit the maximum
oscilloscope input). The input signal from the generator
coaxial cable and a 50
environment without the use of an expen-
CC
and V
BB
load (such as a
, to ground, as
resistors
6
voltage, but to a value that is much higher than allowable on
the LM2421. This fast, high voltage, high energy pulse can
damage the LM2421 output stage. The application circuit
shown in Figure 10 is designed to help clamp the voltage at
the output of the LM2421 to a safe level. The clamp diodes,
D1 and D2, should have a fast transient response, high peak
current rating, low series impedance and low shunt capaci-
tance. FDH400 or equivalent diodes are recommended. D1
and D2 should have short, low impedance connections to
V
located very close to a separately decoupled bypass capaci-
tor (C3 in Figure 10 ). The ground connection of D2 and the
decoupling capacitor should be very close to the LM2421
ground. This will significantly reduce the high frequency
voltage transients that the LM2421 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 LM2421 as well as the voltage stress at the outputs
of the device. R2 should be a
R1 can be a
large value resistors for R1 and R2 would be desirable, but
this has the effect of increasing rise and fall times. Inductor
L1 is critical to reduce the initial high frequency voltage
levels that the LM2421 would be subjected to. The inductor
will not only help protect the device but it will also help
minimize 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 10 .
CC
and ground respectively. The cathode of D1 should be
1
⁄
4
W metal or carbon film type resistor. Having
1
⁄
2
W solid carbon type resistor.