LM2432TE/NOPB National Semiconductor, LM2432TE/NOPB Datasheet - Page 10

• Manufacturer Part Number: LM2432TE/NOPB
• Manufacturer: National Semiconductor

Specifications of LM2432TE/NOPB

• Lead Free Status / RoHS Status: Compliant

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Application Hints
ever, it was decided to preserve the TV’s original RGB
mainboard circuitry (the PNP buffer) and use two NPN tran-
sistors on the neck board.
The input stage from the RGB processor to the LM2432 will
be determined by the system designer for his specific appli-
cation. The input stage required depends mainly on the
following system parameters:
Once the nominal black level input to the LM2432 estab-
lishes a cathode black level near the CRT cut-off voltage, it
can be fine-tuned using the processor’s cut-off adjustment or
calibrated automatically using the IK feedback system, if
applicable. Lastly, some RGB processor video outputs can-
not adequately drive the capacitive load introduced by the
cabling between the RGB mainboard and neck boards. To
prevent loading the processor’s output, a NPN or PNP buffer
stage can be applied close to the output on the mainboard to
sufficiently drive the video signal through cabling to the neck
board. It is important to bias the buffer stage(s) properly to
obtain optimal video performance and maintain the full video
adjustment range of the RGB processor.
Video Output and Arc Protection
The routing of the LM2432 output to the CRT is very critical
to achieve optimal video performance and robustness
against arcover. Figure 17 shows the routing and component
placement from V
pin of the CRT socket. The components are placed so that
there is a short, direct path from the LM2432 output to the
cathode. This is done to reduce the PCB parasitic capaci-
tance on the LM2432 output and minimize EMI. Note also
that L3, D3, D4, and R6 are placed to minimize the size of
the video nodes that they are attached to. This enhances the
effectiveness of the arc protection diodes. The anode of
protection diode D3 is connected directly to a section of the
ground plane that has a short, direct path to ground. The
cathode of D4 is connected to V
capacitor C7, which is connected to the same section of the
ground plane as D3. The diode placement and routing is
very important to shunt arcover current away from the output
and minimize the voltage stress on the LM2432. The internal
CRT socket spark gap is essential to significantly reduce the
initial arcover voltage seen at the cathode. The DAG con-
nector should be connected to CRT ground for arc return
current.
IK Feedback Circuit
The NSC demonstration neck board was made so that no
modifications to the existing TV circuitry were necessary
(except for replacing the original neck boards). Therefore,
• Nominal CRT cut-off voltage
• Nominal black level output voltage of the RGB processor
• V
DC transfer characteristic)
CC
& V
BB
supply voltages of the LM2432 (determines
OUT
(pin 2) of the LM2432 to the cathode
(Continued)
CC
very close to decoupling
10
the video interface and IK feedback circuits are designed to
be compatible with those original TV circuits. Referring to
Figure 16, the LM2432 IK output (pin 5) is connected to
protection circuitry before the IK current signal is routed to
pin 4 of connector J8. Diode D1 protects the LM2432 IK pin
from excessive voltage during normal video by clamping to
the V
other channels during the active IK measurement interval.
Resistors R17 and R12 limit the current through D1 and D2,
and C12 is used for filtering. From connector J8, the IK
current signal is passed through cabling and combined with
the other two IK signals on the RGB mainboard.
Note: The following paragraph describes circuitry that is not
part of the NSC demonstration neck board. The RGB pro-
cessor, which operates with a voltage feedback IK topology,
uses a single “IK resistor” on the RGB mainboard to convert
all three IK currents into a voltage signal. The IK voltage
signal is then buffered through a PNP transistor before it is
filtered and AC coupled to the RGB processor’s IK input.
This is one implementation of the IK feedback circuit based
on this TV’s specific RGB processor. The system designer
should refer to the RGB processor data sheet to determine
the appropriate feedback circuit implementation for his ap-
plication.
Supply Decoupling
Note the location of the following components:
• C5 — V
• C7 — V
• C9 — V
• C10 and C11 — V
Other Items
Connector J1 and switches JP2 & JP4 can be used to
bypass the TV’s internal 200V and 12V supplies and evalu-
ate the LM2432 with external V
this demonstration board uses medium-sized PCB holes to
accommodate socket pins, which function to allow for mul-
tiple insertions of the LM2432 in a convenient manner. To
benefit from the enhanced LM2432 package with thin leads,
the device should be secured with solder in small PCB holes
to optimize the metal-to-metal spacing between the leads.
and GND pins of LM2432.
arc protection diode and ground. This capacitor is very
important for arc protection.
and GND pins.
placed near supply pins of LM2432.
BB
supply. Diode D2 isolates the IK output from the
CC
CC
BB
bypass capacitor with short traces to the V
bypass capacitor with short traces to the V
bypass capacitor with short traces to the V
CC
and V
CC
BB
and V
electrolytic capacitors
BB
supplies. Also,
CC
CC
BB