LA1235 Datasheet, PDF (5/12 Pages) Sanyo Semicon Device – FM IF System Applications

LA1235
• Setting of muting circuit constnats
Drive current to be output at muting drive output pin 12 is 0.75mA typ., but
approximately 0.4mA may be caused by variations in characteristic or
changes in temperature (smaller at higher temperatures). It is desirable to
design the circuit so that the total current to be output from pin 12 at the time
of muting ON does not exceed 0.35mA. When driving the muting circuit of
the LA1235, the muting drive current (input current at pin 5) must be
considered besides this total output current. The muting drive current of the
LA1235 is 0.2mA max.
Thus, the muting constants are obrained as follows. If the muting drive
current is I_R207≥0.2mA, R₂₀₇≤14kΩ occurs and the total current It is shown
by the following expression.
Muting Circuit
It=I_R206+I_R207=
6.2V
R₂₀₆
+
4.8V
R₂₀₆+10kΩ
(10kΩ : Input resistance at pin 5, Refer to the above figure.)
If R₂₀₇=10kΩ is taken with the variation s in voltage V₁₂ at pin 12 and input resistance 10kΩ at pin 5 considered,
R₂₀₆≈56.4kΩ is obtained, and then R₂₀₆=68kΩ and R₂₀₇=10kΩ are obtained.
• Setting of C₁₁₂ (Capacitance between pin 12 and ground)
C₁₁₂ influences S/N and muting attenuation. S/N is improved 0.5 to 2.0dB by changing C₁₁₂ from 1µF to 100µF.
Muting attenuation becomes as shown in Mute (att) – fm (next page) characteristic. This phenomenon occurs
because the output at pin 7 appears at pin 6 through pin 10 and capacitance C₁₁₀ between pin 7 and ground also exerts
influence. The relation between muting attenuation andC₁₁₀ is such that if C₁₁₀=2.2µF and C₁₁₂=220µF, attenuation
at modulation frequency 100Hz becomes –80dB.
AF Output Circuit
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