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TDA4887PS датащи(PDF) 9 Page - NXP Semiconductors |
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TDA4887PS датащи(HTML) 9 Page - NXP Semiconductors |
9 / 68 page 2001 Oct 19 9 Philips Semiconductors Product specification 160 MHz bus-controlled monitor video preamplifier 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). |
Аналогичный номер детали - TDA4887PS |
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Аналогичное описание - TDA4887PS |
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