LM2423TE/NOPB National Semiconductor, LM2423TE/NOPB Datasheet - Page 9

• Manufacturer Part Number: LM2423TE/NOPB
• Description: IC CRT DTV DRIVER 15MHZ TO220-11
• Manufacturer: National Semiconductor

Specifications of LM2423TE/NOPB

• Display Type: CRT
• Current - Supply: 21mA
• Voltage - Supply: 100 V ~ 230 V
• Mounting Type: Through Hole
• Package / Case: TO-220-11 (Bent and Staggered Leads)
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Operating Temperature: -
• Interface: -
• Configuration: -
• Digits Or Characters: -
• Other names: LM2423TE

Application Hints
CURRENT OUTPUT FOR IK FEEDBACK SYSTEMS
The LM2423 can be used in DTV applications that use an IK
feedback system. Figure 14 shows an example of an inter-
face circuit used to feed back the IK output of LM2423 to a
preamplifier with an ac coupled IK input.
This feedback system consists of the preamp, LM2423, and
interface circuit, forming a closed loop to automatically ad-
just the black level of the drive signals to the cutoff point of
the RGB cathodes. Following is a description of the interface
circuit operation.
The output at pin 9 of the LM2423 is filtered of high fre-
quency noise by C12. D7 is used to limit the peak voltage at
pin 9. Without this clamp this voltage would easily exceed
12V during active video, in which the cathode currents are
much greater than the small currents being detected during
vertical blanking. Exceeding 12V could damage Q4 and
result in improper operation of the driver.
R21 is essential to convert the IK current to voltage. Choos-
ing the value of R21 sets the gain of the feedback voltage,
and consequently, the operating point of the tube. Once a
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stable operating point is established, this point can be fine-
tuned using the adjustment range of the feedback system or
standard preamp controls. Changing the value of R21 will
change the cutoff voltage at the cathode. A smaller value of
R21 requires more IK current to maintain the feedback loop.
The cutoff voltage set at the cathode will be lower to adjust to
the higher IK current. This additional current must come from
the cathode; therefore, the cathode voltage is lower to meet
higher current requirement. A higher value of R21 will do the
opposite, raising the cathode voltage because less IK cur-
rent is needed to maintain the same voltage at R21.
The emitter follower, Q4, isolates R21 from the input imped-
ance of the preamp. R18 and R19 bias the emitter of Q4 to
limit the maximum voltage to the preamp. These resistor
values should be chosen to limit the maximum voltage at the
emitter and protect the preamp from any large voltages that
would otherwise occur during active video. R2 and C8 are
used for further filtering of the IK signal. C11 is used to AC
couple the IK signal to the preamp. The advantage of AC
coupling is that any DC component (leakage current from the
driver) of the IK signal is not detected by the IK sense input
of the preamp.
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