LM2419 National Semiconductor, LM2419 Datasheet - Page 3

• Manufacturer Part Number: LM2419
• Description: Triple 65 MHz CRT Driver
• Manufacturer: National Semiconductor

Figure 1 shows a typical test circuit for evaluation of the
Typical Performance
Characteristics
T
Test Circuit
LM2419 The input signal is AC coupled into the input of
LM2419 and is referenced to 3 9V DC using an external
3 9V DC bias through a 390
designed to allow testing of the LM2419 in a 50
ment such as a 50
4950
forms a 100 1 voltage divider when connected to a 50
oscilloscope or network analyzer
Theory of Operation
The LM2419 is a high voltage triple CRT driver suitable for
SVGA IBM 8514 and 1024 x 768 display resolution moni-
tors The device is packaged in the industry standard
11 lead TO-220 molded power package The heat sink is
electrically isolated and may be grounded for ease of manu-
facturing and RFl EMl shielding
The schematic diagram of LM2419 is shown in Figure 2 Q1
and R2 provide a conversion of the input voltage to current
while Q2 acts as a common base amplifier to drive the load
resistor R1 Resistor R4 along with R2 sets up the DC bias
at the base of Q1 Emitter followers Q3 and Q4 isolate R1
from the capacitive load at the output thus making the rise
and fall times relatively insensitive to the load capacitance
The gain of the amplifier is
approximately
limited by the time constant due to R1 and the capacitances
associated with D1 Q2 Q3 and Q4 Diode D1 is used to
provide some bias voltage for Q3 and Q4 so as to reduce
A
e
25 C Test Circuit Figure 1
resistor in series with the output of the LM2419
Pulse Response
b
15 The bandwidth of LM2419 is primarily
oscilloscope or network analyzer The
(Continued)
b
R1 (R2
resistor The test circuit is
ll
R4) and is fixed at
TL H 11442 – 5
environ-
3
small signal cross over distortion Resistor R3 is used to
prevent Q2 from oscillating at high frequencies
Application Hints
POWER SUPPLY BYPASS
Since the LM2419 is a wide-bandwidth amplifier with greater
than 10 000 V
is critical for optimum performance Improper power supply
bypassing can result in large overshoot ringing and oscilla-
tion A 0 01
close to the supply pin as is practical (preferably less than
ceramic capacitor should be as small as is practical In addi-
tion 10
nected from the supply pin to ground The electrolytic ca-
pacitor should be placed reasonably close to the LM2419’s
supply pin
ARC PROTECTION
The LM2419 must be protected from arcing within the CRT
To limit the arcover voltage a 200V spark gap is recom-
mended at the cathode Clamp diodes D1 and D2 (as
shown in Figure 3 ) are used to clamp the voltage at the
output of LM2419 to a safe level The clamp diodes used
should have high current rating low series impedance and
low shunt capacitance Resistor R2 in Figure 3 limits the
arcover current while R1 limits the current into LM2419 and
reduces the power dissipation of the output transistors
when the output is stressed beyond the supply voltage
Having large value resistors for R1 and R2 would be desir-
able but this has the effect of reducing rise and fall times
from the supply pin) The lead length of the 0 01
F – 100
FIGURE 2 Schematic Diagram of
F ceramic capacitor should be connected as
s slew rate proper power supply bypassing
One Section of LM2419
F electrolytic capacitor should be con-
TL H 11442 – 7
F