TDA4887PS datasheet(10/68 Pages) PHILIPS

10 / 68 page

2001 Oct 19

Philips Semiconductors

Product speciﬁcation

160 MHz bus-controlled monitor video

preampliﬁer

TDA4887PS

7.6

Clamping and blanking pulses

There are two pins for clamping and blanking purposes

(pins CLI and HFB):

1. Pin CLI (input clamping, vertical blanking)

The pin CLI of TDA4887PS can be connected directly

to pin CLBL of e.g. TDA4855 sync processor for input

clamping pulses and vertical blanking pulses.

Input clamping pulses and blanking pulses are

completely separated from the sandcastle input, that

means there is normally (outside detected vertical

blanking) no blanking during input clamping and the

clamping pulse is not suppressed during vertical

blanking.

The input pulse is scanned with two thresholds:

a) 1.4 V (typical) for vertical blanking

b) 3 V (typical) for input clamping.

In order to separate the vertical blanking pulse from

the sandcastle pulse it is necessary that the input

clamping pulse has rise/fall times faster than 75 ns/V

during the transition from 1.2 to 3.5 V and vice versa.

The leading edge of the internal vertical blanking pulse

is delayed by typically 270 ns (after the end of an input

clamping pulse or the beginning of a separate blanking

pulse), the trailing edge is delayed by typically 115 ns.

During the vertical blanking pulse signal blanking,

brightness blanking and pedestal blanking will be

activated. In buffered mode, the leading edge of the

internal vertical blanking pulse is used to synchronize

data transmitted via the I2C-bus (see Section 7.10.1).

For correct input clamping the input signals have to be

at black level during the input clamping pulse.

2. Pin HFB (output clamping and blanking)

The input pulse (e.g. horizontal flyback pulse) is

scanned with two thresholds. If the input pulse

exceeds the first threshold (typically 1.4 V) signal

blanking, brightness blanking and pedestal blanking

will be activated. If the input pulse exceeds the second

threshold (typically 3 V) output clamping will be

activated additionally.

Especially for applications with DC-coupled cathodes

(FPOL = 0), it is useful that the leading edge of the

(internal) clamping pulse is slightly delayed with

respect to the leading edge of the (internal) blanking

pulse in order to avoid initial misclamping due to the

delay of the feedback signal from the cathodes.

7.7

On Screen Display insertion and OSD contrast

On Screen Display (OSD) insertion and OSD contrast are

controlled by a 4-bit DAC driven via the I2C-bus.

If the fast blanking input signal at pin FBL exceeds the

threshold (typically 1.4 V) the input signals are blanked

(signal blanking) and OSD signals are enabled. Then, any

signal at pins OSD1, OSD2 or OSD3 exceeding the same

threshold will create an insertion signal with an amplitude

of 100% of the maximum colour signal. The amplitude can

be controlled by OSD contrast (driven via the I2C-bus) with

a range of 12 dB. The OSD signals are inserted at the

same point as the contrast-controlled input signals and will

be treated with brightness and gain control as with normal

input signals.

Identical pulses at OSD signal input pins and FBL have to

be handled very carefully. Each difference in pulse delay

at the inputs will produce glitches at pulse edges at signal

outputs.

When control bit DISO = 1 the OSD signal insertion and

fast blanking (pin FBL) are disabled.

7.8

Subcontrast adjustment, contrast modulation

and beam current limiting

The pin LIM is a linear contrast control pin which allows

subcontrast setting, contrast modulation and beam current

limiting. The maximum contrast is defined by the actual

I2C-bus setting. Input signals at pin LIM act on video and

OSD signals and do not affect the contrast bit resolution.

If the pin is not used it should be decoupled with a

capacitor or tied to the supply voltage.

7.8.1

BEAM CURRENT LIMITING

The open-circuit voltage is approximately 5 V, contrast

reduction starts at input voltages <4.4 V (typical) and

signal amplification will be reduced with descending input

voltages. The input resistance of pin LIM is very high to

make it possible to choose a time constant sufficient for the

open-circuit voltage to recover through the application.

7.8.2

SUBCONTRAST

In order to fit the maximum signal amplification to the post

amplifier gain, an input voltage of <4.4 V can be used.

7.8.3

CONTRAST MODULATION

To achieve brightness uniformity over the screen, scan

dependent contrast modulation is possible. The nominal

input voltage should be <4.4 V having enough margin for

positive and negative modulation.

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