Tips for Reducing Power Dissipation

The following methods can be used to reduce the power

dissipated by the LM2433 in order to optimize heatsink size

and cost:

•

Use a lower V

cient operating range for cutoff, brightness, and drive

adjustments.

•

Lower the maximum V

able picture contrast and brightness.

•

Reduce the input bandwidth to the LM2433 while main-

taining acceptable picture performance.

•

Minimize capacitive load on the LM2433 output by using

good PCB layout practices.

OPTIMIZING TRANSIENT RESPONSE

Referring to Figure 12, there are three components (R1, R2

and L1) that can be adjusted to optimize the transient re-

sponse of the application circuit. Increasing the values of R1

and R2 will slow the circuit down while decreasing over-

shoot. Increasing the value of L1 will speed up the circuit as

well as increase overshoot. It is very important to use induc-

tors with very high self-resonant frequencies, preferably

above 300 MHz. Ferrite core inductors from J.W. Miller

Magnetics (part # 78FR_ _k) were used for optimizing the

performance of the device in the NSC application board. The

values shown in Figure 15 can be used as a good starting

point for the evaluation of the LM2433. Using a variable

resistor for R1 will simplify finding the value needed for

optimum performance in a given application. Once the opti-

mum value is determined, the variable resistor can be re-

placed with a fixed value.

Figure 11 shows a typical cathode pulse response with an

output swing of 110V

provides input speeds with 12 ns rise and fall times. Note:

The RGB processor’s sharpness feature adds emphasis

(pre-shoot and over-shoot) to the rising and falling edges of

the input pulse, which consequently adds emphasis to the

cathode pulse response.

CATHODE CURRENT OUTPUT FOR IK FEEDBACK

SYSTEMS

IK Feedback Systems

IK feedback was developed to accurately bias the CRT and

continuously calibrate it to the correct cut-off and/or drive

levels over the useful life of the CRT. RGB video processors

that use IK feedback to automatically adjust only cut-off, or

black level, are realized by a 1-point calibration system. A

few trade names for this system are Auto Kine Bias (AKB)

and Black Current Stabilization (BCS). RGB processors that

can automatically adjust both cut-off and drive, or white

level, are realized by a 2-point calibration system. This is

commonly known as Continuous Cathode Calibration

(CCC). For convenience, some 2-point RGB processors may

be programmed to 1-point operation if drive calibration is not

required. The LM2433 is compatible with both 1- and 2-point

systems.

To be compatible with various RGB processors, an interface

circuit may be needed in the feedback path between the

LM2433 IK output and the processor’s IK input. This feed-

back circuit depends on the RGB processor and feedback

topology (voltage or current) used. Because each processor

has its own IK input signal and topology requirements, it is

outside the scope of this data sheet to describe each feed-

back circuit in detail. For more information, please refer to

the RGB processor data sheet or contact your local National

Semiconductor Sales Office with your specific application

requirements.

Feedback Topologies

RGB processors that use voltage feedback require the

LM2433 IK current to be converted to voltage via a resistor

(R

input of the RGB processor through an interface circuit,

which will be AC or DC coupled depending on the proces-

sor’s IK input requirement. For proper feedback operation,

some processors may require an emitter follower to isolate

the IK input from the high impedance of the resistor. During

the closed-loop IK measurement interval, the IK input volt-

age will be sampled and compared with the processor’s

internal reference voltage to automatically calibrate the video

levels for the next field. The value of R

establishes the IK voltage and consequently, the operating

point of the CRT. Once a stable operating point is estab-

lished with a properly chosen resistor, this point can be

fine-tuned using the adjustment range of the processor’s

RGB cut-off and/or gain controls via the I

measurement interval (usually at the end of blanking), nor-

mal video will resume and high currents will flow out of the IK

output. These high video currents will produce large IK volt-

ages across the resistor that can exceed the maximum

voltage rating for V

high-speed diode (D

a safe level (preferably V

diode is used instead, it may be necessary for the RGB

processor to have IK leakage compensation for the leakage

current attributed to the zener. Lastly, it is possible to use a

single R

See a simplified voltage feedback interface circuit in Figure

13.

RGB processors that use current feedback do not require

voltage conversion. The LM2433 IK current can be fed back

directly to the IK input of the RGB processor, although some

protection circuitry will be needed to protect the RGB pro-

cessor and LM2433. During the closed-loop IK measure-

ment interval, the voltage of the RGB processor’s IK pin will

be internally clamped, and the IK current will be sampled and

compared with the processor’s internal reference current to

calibrate the video levels. The operating point of the CRT

can be fine-tuned using the adjustment range of the proces-

sor’s RGB cut-off and/or gain controls via the I

20146224

FIGURE 13. Simplified IK Interface for Voltage

Feedback Systems

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