L3281 데이터 시트보기 (PDF) - STMicroelectronics

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L3281

LOW VOLTAGE TELEPHONE SPEECH CIRCUITS

STMicroelectronics 

L3281

DC CHARACTERISTIC

The fig.5 shows the equivalent simplified circuit of

the DC regulator that provides to give the oppor-

tune DC impedance Zdc.

⋅ ⋅ VL = IdcRBZ4 (RA + RB) + VD + VR1

⋅ ⋅ VL = (Idc



Z4)

RA

RB

+

1

+

VD

+

VR1

since RA = RB

⋅ ⋅ VL = (Idc Z4 2) + VD + VR1

When IL 18 mA and considering neglectible the

VD + VR1 variation versus line current :

⋅ ZDC

=

∆

∆

VL

Idc

=

2

Z4

At IL = 6.5 mA no current flows through Z4 but

only in the rest of the circuit for internal biasing

(Io;Ia). The bias current Io is fixed by the resistor

R2. The line voltage in this case is :

VL = Ia RA + VR1 = 1.6 V

The Fig.6 shows the DC characteristic (voltage

between pin 12 and pin 3 versus line current).

The device own an equivalent zener voltage at

pin 5 that can be used as supply voltage for elec-

tret microphone (see Block Diagram).

The value of the resistor R2 and the capacitor C2

should be chosen in order to not affect the AC

line inpedance. The Fig.7 shows the zener

Figure 5: Equivalent Simplified Circuit

equivalent.

The zener voltage will be:

⋅ Vz

=

 70K

13.6K

+

1



Vbe

It is possible to supply 1mA to the electrete volt-

age if VL > (1mA + Iz) ⋅ Rz + Vz

Figure 6: Low Voltage Speech Circuit.

Figure 7: Zener Equivalent.

6/10

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