Toshiba TLP411E, Technical Training Manual

Page: 17 / 84

4-2
1. OUTLINE
This circuit is described using G process as an example
and composed of level shifter, gamma (
g
), black limiter,
inverted signal amplifier, sample & hold circuit and liquid
crystal panel.
A block diagram of the drive circuit is shown in Fig. 4-1.
3.6V
2.3V
Q949
R976
Q945
Q946
Q947
Q948
Q950
4.8 mA 4.8 mA
Q952
Q953
Q951
R978
220
R979
15
5.36V
4.06V
1.83V
DC
1
4
2
3
5 13
Q945 – Q953 Q954 – Q963 Q965 – Q970
Q971 – Q973
Q974 – Q980
SW circuit
Q982
PD74HC4066A
Q983
CXA2504N
Level
shifter
Gamma
circuit
Level shifter
black limiter
Normal
signal drive
Inverted
signal drive
Sample
and
hold circuit
Sample and hold pulse
Liquid
crystal
panel
Fig. 4-1
2. CIRCUIT DESCRIPTION
The following description will be given assuming that V
BE
of transistor is 0.7V.
2-1. Level Shifter (Q945 – Q953)
This circuit is composed of the emitter follower Q945, full
feedback unit gain amplifier Q946 – Q950, and the current
source circuit of sub bright for Q951 – Q953. The circuit
operates to vary only the DC level of the input signal and
develops the signal with only the DC level shifted from the
input signal at Q949. The shift level is determined by the
current flowing into R976.
When a triangular waveform of 2.3V – 3.6V shown in Fig.
4-2 is input, a triangular waveform of 3.0 – 4.3V appears at
the base of Q946. At the same time, a triangular waveform
of approx. 3.0V – 4.3V also appears at the base of Q948.
Assuming the sub brightness adjustment voltage at Q953
is 1.83 V
DC
, a current of (1.83V – 0.7V)/(220 + 15) = 4.97
mA flows into Q951 and Q950, and this current also flows
into R976. Therefore, the emitter voltage of Q949 devel-
ops 1.06V higher than the base voltage of Q948 as shown
in the equation; 4.8 mA x 220
W
= 1.06V. And the triangu-
lar waveform of 4.06V – 5.36V appears at the emitter of
Q949.
Fig. 4-2 Level shifter circuit