TDA4887 データシートの表示（PDF） - Philips Electronics

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TDA4887

160 MHz bus-controlled monitor video preamplifier

Philips Electronics 

Philips Semiconductors

160 MHz bus-controlled monitor video

preamplifier

Product specification

TDA4887PS

7.4 Pedestal blanking

The pedestal blanking is driven by a 2-bit DAC via the

I2C-bus. Pedestal blanking inserts a negative output level

related to the reference black level (should always

correspond to the ‘extended cut-off voltage’ at the

cathode) during blanking and output clamping. In this way

retrace lines during vertical flyback are suppressed

(blanking to spot cut-off). The depth of pedestal blanking

(voltage difference between reference black level and

pedestal black level) is bus-controlled (2 bits, 0 to 13.5%

of the maximum colour signal) and does not change with

any other control or adjustment. The pedestal blanking

level is used for output clamping instead of the reference

black level (see Section 7.5). If the pedestal blanking level

is the most negative output signal and if the application is

for AC-coupled cathodes, a very simple black level

restoration with a DC diode clamp can be used.

7.5 Output clamping and feedback references

The aim of the output clamping is to set the reference black

level of the signal outputs to a value which corresponds to

the ‘extended cut-off voltage’ of the CRT cathodes. With

missing output clamping pulses the integrated storage

capacitors will be discharged resulting in output signals

going to switch-off voltage. If using pedestal blanking, the

pedestal black level will be controlled by output clamping

(see Fig.5). It is therefore not allowed to change the

pedestal depth after black level adjustment of the monitor.

Feedback references are driven via the I2C-bus and

controlled by an 8-bit DAC for DC feedback references or

by a 3-bit DAC for AC feedback references:

1. DC-coupled cathodes (control bit FPOL = 0)

The cathode voltage is divided by a voltage divider and

fed back to the IC (pins FB/R1, FB/R2 and FB/R3).

During the output clamping pulse it is compared with a

bus-controlled feedback reference voltage with a

range of approximately 5.75 to 3.95 V. Any difference

will lead to a reference black level correction

(subaddress 0BH = 00H) or pedestal black level

correction (subaddress 0BH ≠ 00H) by charging or

discharging the integrated capacitors that store the

black level information between the output clamping

pulses. The DC voltages of the output stages should

be designed in such a way that the reference black

level/pedestal black level is within the range of

0.5 to 2.4 V at the preamplifier output.

For correct operation it is necessary that there is

enough headroom for ultra black signals (negative

brightness setting and pedestal blanking). Any clipping

with the video supply voltage at the cathode can

disturb the signal rise/fall times or the black level

stabilization.

After power-on, the control bit FPOL is set to logic 1

and all alignment registers are set to logic 0 resulting

in the reference black level at its lowest level (0.53 V)

with no output signal. Normal operation starts after all

data registers have been refreshed via the I2C-bus.

Brightness control with grey scale tracking (control bit

BRI = 0) can be used as well as brightness control

without grey scale tracking (control bit BRI = 1) using

the mixing function of bus-controlled brightness offset

(0 to −1.4 V) to feedback reference voltages

(see Section 7.2).

2. AC-coupled cathodes (control bit FPOL = 1)

For applications with AC-coupled cathodes the signal

outputs are fed back internally. During the output

clamping pulse they are compared with a bus

controlled feedback reference voltage (0.5 to 1.9 V).

These values ensure a good adaptability to both

discrete and integrated post amplifiers.

For black level restoration, the DAC outputs (FB/R1,

FB/R2 and FB/R3) with a range of approximately

3.95 to 5.75 V can be used. Pedestal blanking is

recommended because it allows use of a simple

restoration circuit. After power-on, the DAC outputs

will be at maximum output voltage (register value

logic 0), so when using a non-inverting amplifier for the

reference voltages the monitor will start with black.

Brightness control with grey scale tracking (control bit

BRI = 0) can be used as well as simple brightness

control without grey scale tracking (control bit BRI = 1)

using the mixing function of bus controlled brightness

offset (0 to −1.4 V) to DAC output voltages

(see Section 7.2).

2001 Oct 19

9

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