TL H 11442
LM2419
Triple
65
MHz
CRT
Driver
December 1994
LM2419
Triple 65 MHz CRT Driver
General Description
The LM2419 contains three wide bandwidth large signal
amplifiers designed for large voltage swings The amplifiers
have a gain of
b
15 The device is intended for use in color
CRT monitors and is a low cost solution to designs conform-
ing to 1024 x 768 display resolution
The device is mounted in the industry standard 11-lead
TO-220 molded power package The heat sink is electrically
isolated and may be grounded for ease of manufacturing
and EMI RFI shielding
Features
Y
50 V
PP
output swing at 65 MHz
Y
Rise Fall time 5 ns with 12 pF load
Y
60 V
PP
output swing capability
Y
Pin and function compatible with LM2416
Y
No low frequency tilt compensation required
Applications
Y
CRT driver for SVGA IBM 8514 and 1024 x 768
display resolution RGB monitors
Schematic and Connection Diagrams
One Channel
TL H 11442 1
TL H 11442 2
Order Number LM2419T
See NS Package Number TA11B
C1995 National Semiconductor Corporation
RRD-B30M115 Printed in U S A
Absolute Maximum Ratings
(Note 1)
If Military Aerospace specified devices are required
please contact the National Semiconductor Sales
Office Distributors for availability and specifications
Supply Voltage (V
a
)
a
85V
Storage Temperature (T
STG
)
b
25 C to
a
100 C
Operating Case Temperature T
Case
b
20 C to
a
90 C
Lead Temperature (soldering
k
10 sec )
300 C
ESD Tolerance
2 kV
Electrical Characteristics
Unless otherwise specified the following specifications apply for V
a
e
80V DC input bias V
IN DC
e
3 9V 50 V
PP
output
swing frequency
e
1 MHz V
Bias
e
12V C
L
e
12 pF T
A
e
25 C see test circuit
Figure 1
Symbol
Parameter
Conditions
Min
Typ
Max
Units
(Note 3)
(Note 2)
(Note 3)
(Limit)
I
CC
Supply Current (per Amplifier)
Input Output Open Circuit
27
40
mA
I
B
Bias Current (Pins 2 or 7 or 9)
11
mA
V
OUT
Output Offset Voltage
40
50
60
V
t
r
Rise Time
10% to 90%
5
ns
t
f
Fall Time
90% to 10%
5
ns
A
V
Voltage Gain
b
13
b
15
b
18
V V
OS
Overshoot
V
IN
t
r
t
f
k
2 ns
8
%
LE
Linearlty Error
V
OUT
e
25V to 75V
8
%
D
A
V
match
Gain Matching
0 3
dB
Note 1
Absolute Maximum Ratings indicate limits beyond which damage to the device may occur The guaranteed specifications apply only for the test conditions
listed Some performance characteristics may degrade when the device is not operated under the listed test conditions
Note 2
Typical specifications are at 25 C and represent the most likely parametric norm
Note 3
Min Max limits are guaranteed to National's AOQL (Average Outgoing Quality Level)
Typical Performance
Characteristics
T
A
e
25 C Test Circuit
Figure 1
TL H 11442 3
12 pF is the total load capacitance and includes the test fixture capaci-
tance
FIGURE 1 Test Circuit (One Section)
Frequency Response
TL H 11442 4
2
Typical Performance
Characteristics
(Continued)
T
A
e
25 C Test Circuit
Figure 1
Pulse Response
TL H 11442 5
Test Circuit
Figure 1 shows a typical test circuit for evaluation of the
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 390X resistor The test circuit is
designed to allow testing of the LM2419 in a 50X environ-
ment such as a 50X oscilloscope or network analyzer The
4950X resistor in series with the output of the LM2419
forms a 100 1 voltage divider when connected to a 50X
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
b
R1 (R2
ll
R4) and is fixed at
approximately
b
15 The bandwidth of LM2419 is primarily
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
small signal cross over distortion Resistor R3 is used to
prevent Q2 from oscillating at high frequencies
TL H 11442 7
FIGURE 2 Schematic Diagram of
One Section of LM2419
Application Hints
POWER SUPPLY BYPASS
Since the LM2419 is a wide-bandwidth amplifier with greater
than 10 000 V ms slew rate proper power supply bypassing
is critical for optimum performance Improper power supply
bypassing can result in large overshoot ringing and oscilla-
tion A 0 01 mF ceramic capacitor should be connected as
close to the supply pin as is practical (preferably less than
from the supply pin) The lead length of the 0 01 mF
ceramic capacitor should be as small as is practical In addi-
tion 10 mF 100 mF electrolytic capacitor should be con-
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
3
Application Hints
(Continued)
TL H 11442 8
FIGURE 3 Typical Application Circuit (One Channel)
IMPROVING RISE AND FALL TIMES
Because of an emitter follower output stage the rise and fall
times of the LM2419 are relatively unaffected by capacitive
loading However the series resistors R1 and R2 (see
Fig-
ure 3 ) will reduce the rise and fall times when driving the
CRT's cathode which appears as a capacitive load The ca-
pacitance at the cathode typically ranges from 8 pF to
12 pF
To improve the rise and fall times at the cathode a small
inductor is often used in series with the output of the amplifi-
er The inductor L1 in
Figure 3 peaks the amplifier's fre-
quency response at the cathode thus improving rise and fall
times The inductor value is empirically determined and is
dependent on the load An inductor value of 0 1 mH is a
good starting value Note that peaking the amplifier's fre-
quency response will increase the overshoot
REDUCING OVERSHOOT
LM2419's overshoot is a function of both the input signal
rise and fall times and the capacitive loading The overshoot
is increased by either more capacitive loading or faster rise
and fall times of the input signal
Table I shows the overshoot for a typical device with differ-
ent capacitive loads and different input signal rise and fall
times As can be observed from Table I overshoot is large
for large capacitive loads and faster input signal rise and fall
times In an actual application the LM2419 is driven from a
preamplifier with rise and fall times of 3 ns to 7 ns When
driven from LM1203 preamplifier (see application circuit
Figure 6 ) the overshoot is 10% with 12 pF capacitive load
The overshoot can be reduced by including a resistor in
series with LM2419's output as in
Figure 3 Larger value
resistors for R1 and R2 would reduce overshoot but this
also increases the rise and fall times at the output Frequen-
cy peaking using an inductor in series with the output may
restore the bandwidth
Table I LM2419 Output Overshoot
vs Capacitive Loading for a Typical Device
Input Signal
C
L
t
r
t
f
5 pF
8 pF
11 pF
15 pF
1 2 ns
4%
6%
7%
8%
7 ns
4%
5%
6%
7%
GAIN VS OUTPUT DC LEVEL
Figure 4 shows LM2419's gain versus output DC level A
100 mV
PP
AC signal is applied at the LM2419's input and
the input signal's DC level is swept As can be seen from
Figure 4 the amplifier's gain is constant at approximately
15 4 (V
OUT
e
1 54 V
PP
) for output DC level between 35V
and 65V Thus the amplifier's output response is linear for
output voltage between 35V and 65V If the output voltage
is less than 35V or more than 70V the amplifier's output
response becomes non-linear (note the change in gain
Fig-
ure 4 ) For optimum performance it is recommended that
LM2419's output low voltage be at 25V or above For a
50 V
PP
swing the output high voltage is 75V With an output
signal swing from 25V to 75V LM2419's linearity error is
measured at 8%
TL H 11442 9
FIGURE 4 Gain vs V
OUT
(DC) V
IN
e
100 mV
PP
4
Application Hints
(Continued)
THERMAL CONSIDERATIONS
LM2419's transfer characteristic and power dissipation ver-
sus DC input voltage is shown in
Figure 5 Power supply
current increases as the input voltage increases conse-
quently power dissipation increases For the LM2419 the
worst case power dissipation occurs when a white screen is
displayed on the CRT Considering a 20% black retrace
time in a 1024 x 768 display resolution application the aver-
age power dissipation for continuous white screen is less
than 4W per channel with 50 V
PP
output signal (black level
at 75V and white level at 25V) Although the total power
dissipation is less than 12W for a continuous white screen
the heat sink should be selected for 13W power dissipation
because of the variation in power dissipation from part to
part
For thermal and gain linearity considerations the output low
voltage (white level) should be maintained above 20V If the
device is operated at an output low voltage below 20V the
power dissipation might exceed 4 7W per channel (i e 14W
power dissipation for the device) Note that the device can
be operated at lower power by reducing the peak to peak
video output voltage to less than 50V and clamping the vid-
eo black level close to the supply voltage
TL H 11442 10
FIGURE 5 V
OUT
and Power Dissipation vs V
IN
The LM2419 requires that the package be properly heat
sunk under all operating conditions
Maximum ratings re-
quire that the device case temperature be limited to 90 C
maximum Thus for 50 C maximum ambient temperature
and 13W maximum power dissipation the thermal resist-
ance of the heat sink should be
i
sa
s
(90 50) C 13W
e
3 C W
SHORT CIRCUIT PROTECTION
The output of LM2419 is not short circuit protected Short-
ing the output to either ground or to V
a
will destroy the
device The minimum DC load resistance the LM2419 can
drive without damage is 1 6 kX to ground or V
a
However
driving a 1 6 kX load for an extended period of time is not
recommended because of power dissipation considera-
tions If the LM2419 is used to drive a resistive load then the
load should be 10 kX or greater
RGB Video Application
A complete video section for an RGB CRT monitor is shown
in
Figure 6 The LM1203 video preamplifier and the LM2419
include almost all the circuitry required between the video
input connection and the CRT's cathodes However an ex-
ternally generated back porch clamp signal is required to
accomplish DC restoration of the video signal
Figure 6 's circuit is excellent choice for a non-interlaced
1024 x 768 display resolution application With 50 V
PP
out-
put swing and 12 pF load the rise fall time for
Figure 6 's
circuit was measured at 7 5 ns In this application feedback
is local to the LM1203 For detailed information on the
LM1203 please refer to the LM1203 data sheet
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance adequate ground plane isolation
between channels good supply bypassing and minimizing
unwanted feedback are necessary Moreover the length of
the signal trace from the preamplifier to the LM2419 and
from the LM2419 to the cathode should be as short as is
practical The following book is highly recommended
Ott Henry W
Noise Reduction Techniques in Electronic
Systems John Wiley
Sons New York 1976
5
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