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PDF LM2408 Data sheet ( Hoja de datos )
| Número de pieza | LM2408 | |
| Descripción | Monolithic Triple 4.5 ns CRT Driver | |
| Fabricantes | National Semiconductor | |
| Logotipo |  |
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December 1996
LM2408
Monolithic Triple 4 5 ns CRT Driver
General Description
The LM2408 is an integrated high voltage CRT driver circuit
designed for use in color monitor applications The IC con-
tains three high input impedance wide band amplifiers
which directly drive the RGB cathodes of a CRT The gain of
each channel is internally set at b15 and can drive CRT
capacitive loads as well as resistive loads presented by oth-
er applications limited only by the package’s power dissipa-
tion
The IC is packaged in an industry standard 11-Lead TO-220
molded plastic power package See thermal considerations
on page 5
Features
Y Rise fall times typically 4 5 ns with 8 pF load
Y Output swing capability 50 VPP for VCC e 80
40 VPP for VCC e 70
30 VPP for VCC e 60
Y Pinout designed for easy PCB layout
Y 1V to 7V input range
Y Stable with 0 pF – 20 pF capactive loads
Y Convenient TO-220 staggered lead package style
Applications
Y CRT driver for 1280 c 1024 (Non-interfaced) and XGA
display resolution color monitors
Y Pixel clock frequency up to 160 MHz
Schematic and Connection Diagrams
TL H 12683 – 2
Note Tab is at GND
Top View
TL H 12683 – 1
FIGURE 1 Simplified Schematic Diagram (One Channel)
C1996 National Semiconductor Corporation TL H 12683
RRD-B30M126 Printed in U S A
http www national com
1 page  
EFFECT OF LOAD CAPACITANCE
The output rise and fall times will be slower than specified if
the load capacitance at the output is more than 8 pF as
shown in Figure 8
TL H 12683 – 9
FIGURE 8 Effect of Load Capacitance on
Rise Fall Time
The monitor designer should ensure that stray capacitance
applied to the LM2408 is as low as possible
THERMAL CONSIDERATIONS
Power supply current increases as the input signal increas-
es and consequently power dissipation also increases
The LM2408 cannot be used without heat sinking Typical
‘‘average’’ power dissipation with the device output voltage
at one half the supply voltage is 1 9W per channel for a total
dissipation of 5 7W package dissipation The power dissipa-
tion does not vary much as output voltage varies The
LM2408 case temperature must be maintained below
100 C If the maximum expected ambient temperature is
50 C then a maximum heat sink thermal resistance can be
calculated
100 C b 50 C
Rth e
5 7W
e88C W
This example assumes a typical CRT capacitive load and is
without a resistive load Note that this thermal resistance
must be achieved when the heat sink is operating in the
monitor
PC BOARD LAYOUT CONSIDERATIONS
For optimum performance an adequate ground plane isola-
tion between channels good supply bypassing and minimiz-
ing unwanted feedback are necessary Also the length of
the signal traces from the preamplifier to the LM2408 and
from the LM2408 to the CRT cathode should be as short as
possible The following references are recommended
Ott Henry W ‘‘Noise Reduction Techniques in Electronic
Systems’’ John Wiley and Sons New York 1976
‘‘Guide to CRT Video Design’’ National Semiconductor Ap-
plication Note 861
‘‘Video Amplifier Design for Computer Monitors’’ National
Semiconductor Application Note 1013
Because of its high small signal bandwidth the part may
oscillate when it is used in a typical application with a
preamp in a monitor if feedback occurs around the video
amplifier through the chassis wiring To prevent this leads
to the input circuit should be shielded and input circuit wir-
ing should be spaced as far as possible from output circuit
wiring Power should be removed as quickly as possible
from an amplifier that is oscillating since power dissipation
in the part is very high in this mode and the part may be
damaged if oscillations continue and the power supply can
supply more than 250 mA
Capacitive loading on the output will cause some overshoot
and peaking This can be controlled by placing a resistor in
series with the output of the part Because of differences in
stray capacitance in different pc board layouts the best val-
ue of resistance to use must be determined separately for
each application Typical values between 50X and 200X
provide good performance with the larger values resulting
in less peaking and slower rise and fall times
Driving the output voltage of the part outside of its linear
range will cause distorted signal waveforms and recovery
times that are very much longer than the specified rise and
fall times When the amplifier output voltage is being driven
from positive saturation into the linear range an overshoot
of several volts for up to 50 ns may occur In a typical moni-
tor design this may occur if blanking pulses are applied to
the video signal The output voltage range should be limited
so this does not happen and will be approximately no lower
than 25V and no higher than VCCb5V
TYPICAL APPLICATION
A typical application of the LM2408 is shown in Figure 9
Used in conjunction with an LM1205 a complete video
channel from monitor input to CRT cathode can be
achieved Performance is satisfactory for all applications up
to 1280 c 1024 non-interfaced and pixel clock frequencies
up to 160 MHz
5 http www national com
5 Page | ||
| Páginas | Total 8 Páginas | |
| PDF Descargar | [ Datasheet LM2408.PDF ] | |
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