NSC DEMONSTRATION BOARD

Figure 15 shows the routing and component placement on

the NSC LM2451 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 compo-

nents:

•

C26—V

and ground pins

•

C27—V

ground

•

C28, C30, C33 — V

and V

The routing of the LM2451 outputs to the CRT is very critical

to achieving optimum performance. Figure 16 shows the

routing and component placement from pin 13 (V

LM2451 to the blue cathode. Note that the components are

placed so that they almost line up from the output pin of the

LM2451 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 pro-

tection diodes. The anode of protection diode D1 is con-

nected directly to a section of the ground plane that has a

short and direct path to the CRT ground and the LM2451

ground pins. The cathode of D8 is connected to V

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 LM2451 during an arc over event.

This demonstration board uses large PCB holes to accom-

modate socket pins, which function to allow for multiple

insertions of the LM2451 in a convenient manner. To benefit

from the enhanced LM2451 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 LM2451 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 LM2451 to a

preamplifier with an ac coupled IK input.

This feedback system consists of the preamp, LM2451, 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 used for AVPs that have a voltage input for

their IK sense input.

The output at pin 8 of the LM2451 is filtered of high fre-

quency 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 Q4

and result in improper operation of the driver.

R35 is essential to convert the IK current to voltage. Choos-

ing 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 imped-

ance 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 com-

plete 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.

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