lm2433 National Semiconductor Corporation, lm2433 Datasheet - Page 10

• Manufacturer Part Number: lm2433
• Description: 220v Monolithic Single Channel 16 Mhz Edtv Crt Driver
• Manufacturer: National Semiconductor Corporation

www.national.com
Application Hints
The input stage from the RGB processor to the LM2433 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 LM2433 estab-
lishes a cathode black level near the CRT cut-off voltage, it
can be fine-tuned using the processor’s cut-off adjustment
and 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 LM2433 output to the CRT is very critical
to achieve optimal video performance and robustness
against arcover. Figure 16 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 LM2433 output to the
cathode. This is done to reduce the PCB parasitic capaci-
tance on the LM2433 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 the LM2433
ground plane. The cathode of D4 is connected to V
close to decoupling capacitor C7, which is connected to the
same section of the ground plane as D3. The diode place-
ment and routing is very important to shunt arcover current
away from the output and minimize the voltage stress on the
LM2433. The internal CRT socket spark gap will serve to
significantly reduce the initial arcover voltage seen at the
cathode. The DAG connector 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
• Nominal CRT cut-off voltage
• Nominal black level output voltage of the RGB processor
• V
CC
& V
BB
supply voltages of the LM2433
OUT
(pin 2) of the LM2433 to the cathode
(Continued)
CC
very
10
(except for replacing the original neck boards). Therefore,
the video interface and IK feedback circuits are designed to
be compatible with those original TV circuits. Referring to
Figure 15 , the LM2433 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 LM2433 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 LM2433 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 LM2433 in a convenient manner. To
benefit from the enhanced LM2433 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 LM2433.
arc protection diode and ground. This capacitor is very
important for arc protection.
and GND pins.
placed near supply pins of LM2433.
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