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參數(shù)資料
| 型號(hào)： | LM2485 |
| 廠商： | National Semiconductor Corporation |
| 英文描述： | 220V Triple Bias Clamp with G1 Blank Amplifier |
| 中文描述： | 220三偏置與G1期空白鉗放大器 |
| 文件頁(yè)數(shù)： | 5/8頁(yè) |
| 文件大?。?/td> | 858K |
| 代理商： | LM2485 |
NSC Demonstration Board
Figure 6
shows the recommended application schematic for
the NSC Demonstration Board that can be used to evaluate
the LM2423/LM2485 AC coupled solution in a Direct-View
CRT Digital TV application.
Figure 7
shows the routing and
component placement on the NSC LM2423/LM2485 Dem-
onstration Board. This board provides a good example of a
PCB layout that was designed for robust arc protection and
optimal video performance.
PCB LAYOUT CONSIDERATIONS
Note that the NSC Demonstration Board complies with the
layout guidelines outlined below. For convenience, the com-
ponent names are shown in parenthesis to cross-reference
with the schematic and layout images provided.
Power Supplies and Grounds
0.1uF bypass capacitors (C3 and C6) should be placed
very close to V
and V
of the LM2485. These capaci-
tors should have a short, direct return to the LM2485
ground.
1uF electrolytic capacitors (C1 and C2) should also be
placed reasonably close to both supply pins.
GND Pins 8 and 14 should connect to a solid ground
plane under the LM2485. The LM2485 ground plane
should connect directly to the CRT Driver ground at one
point (above C21). Do not connect the LM2485 ground
plane directly to CRT ground to protect the LM2485 from
arcing.
Inputs
The LM2485 input traces should be routed away from all
output traces of the CRT Driver. This will help protect the
video preamplifier and LM2485 from high frequency,
large amplitude video coupling and potential damage
from arcing at the cathodes.
If the LM2485 inputs are coming from the main board,
0.1uF capacitors (C4, C5, C7) should be placed at the
V
IN
pins of the input connector. If a LM12XX preamplifier
is placed on the neck board, these 0.1uF capacitors
should be placed very close to its DAC output pins to
protect it during arcing. In both cases, these capacitors
should be connected to the LM2485 ground and should
not be connected directly to CRT ground.
1k
series resistors (R1, R3, R4) should be placed close
to the V
pins and a small resistor may also be placed
close to the VB
pin. These resistors will protect the
preamplifier in the event that the LM2485 fails.
Outputs
1.5k
resistors (R6, R7, R8) and 0.1uF capacitors (C21,
C24, C25) should be placed near the V
pins to limit
the current and voltage surges on the outputs during an
arc event. These bypass capacitors should have a short,
direct return to the LM2485 ground.
Because the clamp outputs carry DC, the clamp output
traces routed to each AC coupled video output can be
long. Avoid cutting a ground trace(s) simply to route
clamp output traces and use wire jumpers, if necessary.
Clamp diodes (D3, D5, D6) and pull-up resistors (R20,
R23, R24) should be placed closest to the video output
traces to minimize the size of the video nodes and reduce
parasitic capacitance on the video outputs. The clamp
diodes must be 1SS83 or equivalent diodes; do not use
1N4148 type diodes. These diodes must have a fast
transient response, low shunt capacitance, low series
impedance, and a high peak current rating to clamp the
video output voltage to the V
CC
supply during an arc
event.
0.1uF bypass capacitors (C46, C48, C9) should be
placed very close to the 1SS83 type arc protection diodes
(D1/D2, D4/D7, D8/D9) between V
and ground. The
ground connection should have a short, direct path to
CRT ground. Therefore, arc currents shunted by the pro-
tection diodes can return to the CRT DAG (J6).
300V spark gaps (S1, S2, S3, S4) are strongly recom-
mended and should be placed very close to the cathode
and G1 pins and grounded directly to the CRT ground
plane, so arc current can return to the nearby CRT DAG.
Spark gaps help to minimize voltage stress on the CRT
Driver and LM2485.
If the G1 blanking feature is used, all G1 circuit compo-
nents should be placed as close as possible to the G1 pin
of the CRT connector, with the exception of a small
resistor (R2) placed close to the VB
pin to limit any
current toward the LM2485. The sequence of circuit com-
ponents from the VB
pin to the G1 pin is recom-
mended to protect the LM2485 against arcing. All these
components must be grounded directly to the CRT
ground plane. Therefore, arc currents can easily return to
the CRT DAG. Do not omit any of the components in the
G1 circuit as they are required to protect the LM2485
against arcing and for correct G1 blank operation.
If the G1 blanking feature is not used, simply place a
470pF capacitor from the VB
OUT
pin to LM2485 ground
and do not connect the output to the G1 circuitry.
Pin Options and No Connects
Depending on the pin options required, Pin 12 and Pin 13
can be connected to ground via a ground plane, trace, or
jumper wire. See the
Section Theory of Operation
to
determine the appropriate pin option for the application.
The “No Connect” or N/C pins (pins 16, 18, 20, 22, 24)
are not connected to the die internally in order to achieve
larger spacing between the high voltage V
and V
pins. These N/C pins should not be used to route other
traces through.
L
www.national.com
5
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