LA1781M_09 Datasheet, PDF (13/48 Pages) Sanyo Semicon Device – For Car Radios Single-Chip Tuner IC

Continued from preceding page.
Pin No.
Pin function
39
Keyed AGC
AM stereo buffer
LA1781M
Pin description
The keyed AGC operates when the
voltage created by dividing the pin 24
S-meter output voltage by the 6.4 and
3.6kΩ resistors becomes lower than
the voltage determined by the resistor
between pin 39 and ground.
This pin also is used as the AM stereo
IF buffer pin.
Equivalent circuit
S-meter
Comparator
KEYED
AGC
+
--
1.3V
VCC
24
6.4kΩ
3.6kΩ
39
90μA
AM IF out
50pF _150Ω
41
HCC capacitor
The HCC frequency characteristics
are determined by the external
capacitor connected at this pin.
+
20kΩ
20kΩ
VCC
42
AM L.C. pin
43
Pilot detector
41
2200pF
This pin is used to change the
frequency characteristics of the
unneeded audio band under 100Hz in
AM mode to produce a clear audio
signal.
Note : The LC capacitor must be
connected between this pin
and VCC (pin 40).
This is because the detector
circuit operates referenced to
VCC.
The cutoff frequency fC is determined
by the following formula.
fC = 1/2π × 50kΩ × C
Inserting a 1MΩ resistor between pin
43 and VCC will force the IC to mono
mode.
DET
VCC
VCC
C
42
50kΩ
1kΩ
50kΩ
1kΩ
19kHz∠0°
BIAS
30kΩ
30kΩ
+
30kΩ
VCC
43
1μF
+
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