3DQDVRQLF
35
The three video amplifiers have a beam current output I black, used by the TDA8844 black current
loop to control the black level on the cathodes. The outputs can be connected together because tile
black 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/out
of the black current pin wall be compensated fur by the infernal leakage measurement circuitry. The
leakage value is stored internally so that the subsequent black current measuring current are not
Influenced by the leakage current. The maximum leakage current that can be accounted for by the
Internal leakage measurement circuital is +/- 100uA
During the black current measurement periods (3L), the current of each channel flowing Into the black
current input pin, is independently and sequentially stabilized. The measurement occurs for the RGB
channel during LR, LG, LB respectively. During measurement, RGB (internal or external) is blocked
and 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 10uA
flows for each channel during the LR, LG and LB periods. The correct stabilized RGB DC levels are
stored by the internal black level clamps and maintained until the heft stabilization occurs (i.e.
following 3L period).
1.5.3 Beam current limiter
The beam current limiter circuit functions as an average white as well as peak white limiter. The
average white limiter needs external circuitry to function, the peak white limiter is a TDA8844 internal
detection circuit ; the function average/peak white limiter reduces the contrast and brightness of RGB
signals .
For average limiting. the difference in external discharge current (I
BCL
) and an internal charge current
(I
CHARGE
) is integrated with the capacitors C4l8 & C553. If l
BCL
>I
CHARGE
then the voltage (V
BCL
) on the
BCL pin decreases
The peak white limiter comes into action if the difference between measurement DC level and white
level exceeds approximately 2.5V (at nominal white point adjustments). Then an internal peak white
limiter discharge current (I
PWL
) of 200uA will flow; I
PWL
>I
CHARGE
so V
BCL
pin22 decreases; if no capacitor
is 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 V
BCL
< 3.5V ;
- brightness reduction begins when V
BCL
< 2 5V.
V
BCL
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 the
BCL pin.
FBT
ABL
7
C415
0.1µF
R417
1K
22
Beam current
TDA 8374
R514
100K
D406
R413
8V
C424
0.47µF
Fig 34 : Beam current limiter circuit
