LM1207AN National Semiconductor, LM1207AN Datasheet - Page 18

• Manufacturer Part Number: LM1207AN
• Manufacturer: National Semiconductor

Specifications of LM1207AN

• Power Supply Requirement: Single
• Dual Supply Voltage (typ): Not RequiredV
• Dual Supply Voltage (min): Not RequiredV
• Dual Supply Voltage (max): Not RequiredV
• Mounting: Through Hole
• Lead Free Status / RoHS Status: Not Compliant

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Circuit Description
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 out-
put 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
LM1205A/LM1207A into blanking when the V
10.6V. Forcing the outputs to a blacker-than-black level will
drive the cathode driver stage well above the black level, cut-
ting off the beam current in the CRT. This prevents the bright
FIGURE 12. Simplified Schematic of LM1205A/LM1207A Clamp Gate Circuit
(Continued)
CC
drops below
18
spot from occurring when the monitor is turned off, preserv-
ing 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 fila-
ments 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 LM1205A/LM1207A are in the normal operating mode. A
low signal at the base of Q115 turns on this transistor, blank-
ing the outputs of the LM1205A/LM1207A. Q115 is driven by
the output of the comparator. The inverting input of the com-
parator is connected to an internal 1.2V reference. The
non-inverting side is connected to a resistor divider network,
R89 and R90. When V
input is above the 1.2V reference, therefore the output of the
comparator is high. This high output turns off Q115. Once the
V
turning on Q115 which forces the outputs into the blanking
mode.
CC
drops below 10.6V the comparator’s output goes low,
CC
is above 10.6V the non-inverting
DS012866-20