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LM2450 Datenblatt(PDF) 9 Page - Texas Instruments

Teilenummer LM2450
Bauteilbeschribung  Monolithic Triple Channel 7 MHz DC Coupled CRT DTV Driver
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OBSOLETE
15A§LM2450
www.ti.com
SNOSAN6D – SEPTEMBER 2005 – REVISED APRIL 2013
DEMONSTRATION BOARD
Figure 15 shows the routing and component placement on the LM2450 demonstration board. This board
provides a good example of a layout that can be used as a guide for future layouts. Note the location of the
following components:
C26—VCC bypass capacitor, located very close to pin 12 and ground pins
C27—VBB bypass capacitor, located close to pin 7 and ground
C28, C30, C33—VCC bypass capacitors, near LM2450 and VCC clamp diodes. Very important for arc
protection.
The routing of the LM2450 outputs to the CRT is very critical to achieving optimum performance. Figure 16
shows the routing and component placement from pin 13 (VOUT3) of the LM2450 to the blue cathode. Note that
the components are placed so that they almost line up from the output pin of the LM2450 to the blue cathode pin
of the CRT connector. This is done to minimize the length of the video path between these two components.
Note also that D1, D8 and R36 are placed to minimize the size of the video nodes that they are attached to. This
minimizes parasitic capacitance in the video path and also enhances the effectiveness of the protection diodes.
The anode of protection diode D1 is connected directly to a section of the ground plane that has a short and
direct path to the heater ground and the LM2450 ground pins. The cathode of D8 is connected to VCC very close
to decoupling capacitor C28 which is connected to the same area of the ground trace as D1. The diode
placement and routing is very important for minimizing the voltage stress on the LM2450 during an arc over
event.
This demonstration board uses large PCB holes to accommodate socket pins, which function to allow for multiple
insertions of the LM2450 in a convenient manner. To benefit from the enhanced LM2450 package with thin
leads, the device should be secured in small PCB holes to optimize the metal-to-metal spacing between the
leads.
CURRENT OUTPUT FOR IK FEEDBACK SYSTEMS
The LM2450 can be used in DTV applications that use an IK feedback system. Figure 14 shows an example of
an interface circuit used to feed back the IK output of LM2450 to a preamplifier with an ac coupled IK input.
This feedback system consists of the preamp, LM2450, and interface circuit, forming a closed loop to
automatically adjust the black level of the drive signals to the cutoff point of the RGB cathodes. Following is a
description of the interface circuit operation used for AVPs that have a voltage input for their IK sense input.
The output at pin 8 of the LM2450 is filtered of high frequency noise by C14. D7 is used to limit the peak voltage
at pin 8. Without this clamp diode the voltage would easily exceed 12V during active video when the cathode
currents are much greater than the small currents being detected during vertical blanking. Exceeding 12V could
damage Q1 and result in improper operation of the driver.
R35 is essential to convert the IK current to voltage. Choosing the value of R35 sets the gain of the feedback
voltage, and consequently, the operating point of the tube. Once a 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 R35 will change the cutoff voltage at the cathode. A smaller value of R35 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 set lower to meet higher
current requirement. A higher value of R35 will do the opposite, raising the cathode voltage because less IK
current is needed to maintain the same voltage at R35.
The emitter follower, Q7, isolates R35 from the input impedance of the preamp. R21 and R39 bias the emitter of
Q7 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. C9 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.
Some AVPs do have a direct current input for their IK sense input. For interfacing to these AVPs the only
components to be used in the IK sense section are D7 and R41. To complete the signal path a jumper must be
used to replace R34, C9 and the base-emitter junction of Q7. C14 can still be used for high frequency filtering.
Copyright © 2005–2013, Texas Instruments Incorporated
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