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Panasonic Z-421V Technical Manual
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3DQDVRQLF35The three video amplifiers have a beam current output I black, used by the TDA8844 black currentloop to control the black level on the cathodes. The outputs can be connected together because tileblack current loop sequentially controls the black level far each cathode.During leakage measurement (LO) the RGB outputs are blanked implying no current flowing in CRT.Any remaining current (i. e. due to external leakage from CRT and/or video amplifiers) that flows in/outof the black current pin wall be compensated fur by the infernal leakage measurement circuitry. Theleakage value is stored internally so that the subsequent black current measuring current are notInfluenced by the leakage current. The maximum leakage current that can be accounted for by theInternal leakage measurement circuital is +/- 100uADuring the black current measurement periods (3L), the current of each channel flowing Into the blackcurrent input pin, is independently and sequentially stabilized. The measurement occurs for the RGBchannel during LR, LG, LB respectively. During measurement, RGB (internal or external) is blockedand a DC level is supplied to a summing stage.The internal black level clamps ensure that the DC level at RGB outputs changes if order that 10uAflows for each channel during the LR, LG and LB periods. The correct stabilized RGB DC levels arestored by the internal black level clamps and maintained until the heft stabilization occurs (i.e.following 3L period).1.5.3 Beam current limiterThe beam current limiter circuit functions as an average white as well as peak white limiter. Theaverage white limiter needs external circuitry to function, the peak white limiter is a TDA8844 internaldetection circuit ; the function average/peak white limiter reduces the contrast and brightness of RGBsignals .For average limiting. the difference in external discharge current (I BCL) and an internal charge current(ICHARGE) is integrated with the capacitors C4l8 & C553. If l BCL>ICHARGE then the voltage (V BCL) on theBCL pin decreasesThe peak white limiter comes into action if the difference between measurement DC level and whitelevel exceeds approximately 2.5V (at nominal white point adjustments). Then an internal peak whitelimiter discharge current (IPWL) of 200uA will flow; I PWL >ICHARGE so VBCL pin22 decreases; if no capacitoris present at pin22 (no integration), then V BCL will fall immediately after peak signal detection.For both average/peak white limiter :- contrast reduction begins when VBCL < 3.5V ;- brightness reduction begins when V BCL < 2 5V.VBCL is normally 4V when average/peak limiter is not active.The contrast and/or brightness reduction of the RGB OUT is proportional to the voltage decrease on theBCL pin.FBTABL7C4150.1μFR4171K22Beam currentTDA 8374R514100KD406R4138VC4240.47μFFig 34 : Beam current limiter circuit |
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