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TDA4886A Scheda tecnica(PDF) 7 Page - NXP Semiconductors |
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TDA4886A Scheda tecnica(HTML) 7 Page - NXP Semiconductors |
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7 / 52 page  1998 Dec 04 7 Philips Semiconductors Product specification 140 MHz video controller with I2C-bus TDA4886A 7 FUNCTIONAL DESCRIPTION See block diagram (Fig.1) and definition of levels and output signals (see Chapter “Characteristics” notes 1 to 3; Figs 3 to 6). 7.1 Signal input stage (input clamping, blanking and clipping) The RGB input signals with nominal signal amplitude of 0.7 V are capacitively coupled into the TDA4886A from a low-ohmic source (75 Ω recommended) and actively clamped to an internal DC voltage during signal black level. Because of the high-ohmic input impedance of the TDA4886A the coupling capacitor (which also functions as a storage capacitor between clamping pulses) can be relatively small (10 nF recommended). Very small input currents will discharge the coupling capacitor resulting in black output signals for missing input clamping pulses. Composite signals will not disturb normal operation because a clipping circuit cuts all signal parts below black level. A fast signal blanking stage belongs to the input stage which is driven by several blanking pulses (see Section “Clamping and blanking pulses”) and control bit DISV = 1. During the off condition the internal reference black level will be inserted instead of the input signals. 7.2 Electronic potentiometer stages 7.2.1 CONTRAST CONTROL (DRIVEN BY I2C-BUS, 6-BIT DAC) The input signals related to the internal reference black level can be simultaneously adjusted by contrast control with a control range of typically 32 dB. The nominal contrast setting is defined for 26H (4.2 dB below maximum). 7.2.2 BRIGHTNESS CONTROL (DRIVEN BY I2C-BUS, 6-BIT DAC) With brightness control the video black level will be shifted in relation to the reference black level simultaneously for all three channels. With a negative setting (maximum 10% of nominal signal amplitude) dark signal parts will be lost in ultra black while for positive settings (maximum 30% of nominal signal amplitude) the background will alter from black to grey. The nominal brightness setting (10H) is no shift. The brightness setting is also valid for OSD signals. During blanking and output clamping the video black level will be blanked to reference black level (brightness blanking). 7.2.3 GAIN CONTROL (DRIVEN BY I2C-BUS,6-BIT DAC) AND GREY SCALE TRACKING Gain control is used for white point adjustment (correction for different voltage to light amplification of the three colour channels) and therefore individual for the three channels. The video signals related to the reference black level can be gain controlled within a range of typically 7.3 dB. The nominal setting is maximum gain. The video signal is the addition of the contrast controlled input signal and the brightness shift. The gain setting is also valid for OSD signals, thus the complete ‘grey scale’ is effected by gain control. 7.3 Output stage In the output stage the nominal input signal will be amplified to 2.8 V output colour signal at nominal contrast and maximum gain. The maximum input to output amplification at maximum contrast and gain settings is 16.2 dB. By output clamping the reference black level can be adjusted. In order to achieve fast rise and fall times of the output signals with minimum crosstalk between the channels, each output stage has its own supply voltage pin. 7.4 Pedestal blanking For the video portion the reference black level should correspond to the ‘extended cut-off voltage’ at the cathode. Nevertheless during vertical flyback retrace lines may be visible, though blanking to spot cut-off is useful. With control bit PEDST = 1 the pedestal black level will be adjusted by output clamping instead of the reference black level (see Fig.5). The pedestal black level is more negative than the video black level at minimum brightness setting and the voltage difference to reference black level is fixed. 7.5 Output clamping, feedback references and DAC outputs The aim of the output clamping (pins FB/R1, FB/R2 and FB/R3 with control bit FPOL = 0, internal feedback with control bit FPOL = 1) 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 a lack of output clamping pulses the integrated storage capacitors will be discharged resulting in output signals going to switch-off voltage. Feedback references are driven by the I2C-bus. |
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