TDA4887 datasheet(44/68 Pages) PHILIPS

44 / 68 page

2001 Oct 19

Philips Semiconductors

Product speciﬁcation

160 MHz bus-controlled monitor video

preampliﬁer

TDA4887PS

12.3

Building the application board

12.3.1

GENERAL

• Double-sided board

• Short HF loops by large ground plane on the rear

• SMD components with minimum parasitics.

12.3.2

VOLTAGE OUTPUTS

• Capacitive loads as small as possible

• Be aware of internal output resistance (typically 75 Ω).

12.3.3

SUPPLY VOLTAGES

• Capacitors as near as possible to the pins

• Use electrolytic capacitors with small serial resistance

and inductance.

12.3.4

FLASHOVER

High electric field strength is present between the gun

electrodes of picture tubes. In case of a flashover large

transient currents and voltages may damage electronic

components. It is therefore important to provide protective

circuits with spark gaps, series resistors and protection

diodes. Be aware that not only electronic components that

are directly connected to the tube socket are endangered

if interconnection lines on the application board are

unfavourably routed.

12.4

Application hints

12.4.1

ALIGNMENT RECOMMENDATIONS USING BRIGHTNESS

CONTROL WITH GREY SCALE TRACKING

12.4.1.1

Introduction (philosophy of TDA4887PS)

With the TDA4887PS the user may change contrast,

brightness and even colour temperature (R, G, B gains) or

any combination at will. The ‘x,y’ colour point will remain

stable for the full grey scale. This feature is achieved in the

following way:

A change of brightness will cause a change of black

level which is proportional to the actual gain setting

Conversely, a change of gain setting will cause a

change of black level that is proportional to the deviation

of brightness from its nominal setting.

To benefit fully from this colour tracking feature, the

reference black levels of the video amplifiers must match

exactly to the cut-off points of the cathodes.

Re-adjustments of black level settings by the end user

should be avoided, because this will upset the tracking

feature.

12.4.1.2

Difﬁculty during monitor production

The factory cut-off alignment is done at a quite high level

(e.g. 2.4 cd/m2). As a consequence it is not certain that the

reference black level will match the cut-off exactly after a

first black level adjustment. If then the R, G, B gains are

adjusted for the (x, y) white point at e.g. 102.8 cd/m2, the

white balance at 2.4 cd/m2 will have changed. So two or

more alignment cycles may be needed to achieve good

results.

12.4.1.3

Considerations for a single-pass factory

alignment

The nominal brightness setting is 40H. In this condition the

black level equals reference black level and must match to

the CRT cut-off.

For a better understanding, discrete values for luminance,

video and feedback gain have been taken (these values

should be regarded as examples). For special applications

actual values have to be taken instead.

White point must be aligned at maximum luminance

(e.g. at 102.8 cd/m2) with maximum contrast and nominal

brightness. It is recommended to use only a small white

square for white point alignment, to prevent variations of

the voltage at grid G1 (Vg1) and grid G2 (Vg2) and to

prevent unwanted activation of the automatic beam limiter

(ABL).

For practical reasons, alignment of the R, G, B reference

black levels must be done with a small amount of drive for

obtaining a luminance level of approximately 2.4 cd/m2.

This drive can be simulated by setting the brightness to a

certain value. Assuming 102.8 cd/m2 luminance with full

white video (100% drive) and a cathode characteristic with

gamma = 2.25, the drive for black level adjustment can be

shown as:

which corresponds to a brightness setting of B8H.

After black level adjustment for L = 2.4 cd/m2, the cathode

voltages are fixed and the cut-off voltages are set with

equal gain condition in all channels. During white point

adjustment the gains will be changed. In the factory

procedure for single pass adjustment, the luminance level

for black level alignment (2.4 cd/m2) is kept constant while

adjusting the gain settings. To achieve this, the black level

references are compensated by software and an

alignment computer (this compensation is for factory

alignment only and is not needed for any user change of

R, G, B gain).

2.4

102.8

1/2.25

---------------------------

100 = 18.8% drive

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