Mitsubishi DPLUS 74SB -BKA Service Manual page 145

THEORY OF OPERATION
The LM2469 is high voltage monolithic three channel CRT driver
suitable for color monitor applications. The LM2469 operates
with 80V and 12V power supplies. The part is housed in the
industry standard 9-lead TO-220 molded plastic power package.
The circuit diagram of the LM2469 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 –20. Emitter followers Q3 and Q4 isolate the
high output impedance of the amplifier, decreasing the sensi-
tivity of the device to changes in load capacitance. Q6 pro-
vides biasing to the output emitter follower stage to reduce
crossover distortion at low signal levels.
Figure 3 shows a typical test circuit for evaluation of the LM2469.
This circuit is designed to allow testing of the LM2469 in a 50W
environment without the use of an expensive FET probe. In
this test circuit, two low inductance resistors in series totaling
4.95KW form a 200:1 wideband, low capacitance probe when
connected to a 50W load (such as 50W oscilloscope input).
The input signal from the generator is AC coupled to the base
of Q5.
APPLICATION HINTS
INTRODUCTION
National Semiconductor (NSC) is committed to provide appli-
cation 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 perfor-
mance was done using a specific printed circuit board designed
at NSC. Variations in performance can be realized due to physi-
cal changes in the printed circuit board and the application.
Therefore, the designer should know that component value
changes might 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 work-
ing with high bandwidth circuits, good layout practices are al-
ways critical to achieving maximum performance.
IMPORTANT INFORMATION
The LM2469 performance is targeted for the VGA (640 x 480)
to (1024 x 768, 70Hz refresh) resolution market. It is designed
to be a replacement for discrete CRT drivers. The application
circuits shown in this document to optimize performance and
to protect against damage from CRT arc-over are designed
specifically for the LM2469. If another member of the LM246X
family is used, pleased refer to its datasheet.
POWER SUPPLY BYPASS
Since the LM2469 is a wide bandwith amplifier, proper power
supply bypassing is critical for optimum performance. Improper
power supply bypassing can result in large overshoot, ringing or
oscillation. A 0.1uF capacitor should be connected from the sup-
ply pin, V to ground, as close to the supply and ground pins as
CC
is practical. Additionally, a 10uF to 100uF electrolytic capacitor
should be connected from the supply pin to ground. The electro-
lytic capacitor should also be placed reasonably close to the
LM2469's supply and ground pins. A 0.10uF capacitor should be
connected from the bias pin (V
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 LM2469.
This fast, high voltage, high energy pulse can damage the
LM2469 output stage. The application circuit shown in Figure
4 is designed to help clamp the voltage at the output of the
LM2469 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 capacitance. FDH400 or
equivalent diodes are recommended. Do not use 1N4148 di-
odes for the clamp diodes. D1 and D2 should have short, low
impedance connections to V
cathode of D1 should be located very close to a separately
decoupled bypass capacitor (C3 in Figure 4). The ground con-
nection of D2 and the decoupling capacitor should be very close
to the LM2469 ground. This will significantly reduce the high
frequency voltage transients that the LM2469 would be sub-
jected to during an arcover condition. Resistor R2 limits the
arcover current that is seen by the diodes while R1 limits the
current into the LM2469 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.
Having large value resistors for R1 and R2 would be desirable,
but has the effect of increasing rise and fall times. Inductor L1
is critical to reduce the initial high frequency voltage levels that
the LM2469 would be subjected to. The inductor will not only
help protect the device, but 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 4.
7-44
) to the ground, as close as is
bb
and ground respectively. The
CC