LM1205 datasheet(17/24 Pages) NSC

17 / 24 page

Circuit Description (Continued)

comparators of the three video channels Q103 is added to

help drive the base of Q86 in the clamp comparator in-

creasing the accuracy of the current mirror Q101 drives

R79 and R80 This sets the current thru Q102 thus setting

the current thru Q86 of the clamp comparator

BLANK GATE CIRCUIT

Figure 13 is a simplified schematic of the Blank Gate circuit

With the exception of the simple output stage and the spot

killer circuit this circuit is almost identical to the clamp gate

circuit The only difference is that the output stage is driven

from the opposite side of the differential pair Thus Q111 is

connected as a diode instead of Q109 With the input at pin

13 at a low level Q108 is turned on also turning on Q29 the

output transistor Q29 is part of the blanking circuit in the

output stage shown in

Figure 14 When Q29 is turned on the

output is clamped to a blanking level that is ‘‘blacker than

black’’ allowing blanking to be done on the cathodes of the

CRT

The spot killer circuit is used to force the outputs of the

LM1205LM1207 into blanking when the VCC drops below

106V Forcing the outputs to a blacker-than-black level will

drive the cathode driver stage well above the black level

cutting off the beam current in the CRT This prevents the

bright spot from occurring when the monitor is turned off

preserving the phosphor of the CRT The CRT will also have

its beam current cut off during the time the monitor is first

turned on This is not a critical period for the CRT since the

filaments have not warmed up to generate a current flow

The comparator along with R89 R90 and Q115 all form the

spot killer circuit Q115 acts the same as Q106 When Q115

has a high signal at its base it is turned off and the outputs

of the LM1205LM1207 are in the normal operating mode

A low signal at the base of Q115 turns on this transistor

blanking the outputs of the LM1205LM1207 Q115 is driv-

en by the output of the comparator The inverting input of

the comparator is connected to an internal 12V reference

The non-inverting side is connected to a resistor divider net-

work R89 and R90 When VCC is above 106V the non-in-

verting input is above the 12V reference therefore the out-

put of the comparator is high This high output turns off

Q115 Once the VCC drops below 106V the comparator’s

output goes low turning on Q115 which forces the outputs

into the blanking mode

TLH11881 – 20

FIGURE 12 Simplified Schematic of LM1205LM1207 Clamp Gate Circuit

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