L3281 STMicroelectronics, L3281 Datasheet - Page 6

• Manufacturer Part Number: L3281
• Description: LOW VOLTAGE TELEPHONE SPEECH CIRCUITS
• Manufacturer: 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.
since RA = RB
When IL 18 mA and considering neglectible the
VD + VR1 variation versus line current :
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 :
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
6/10
VL
VL
VL
Idc
VL = Ia RA + VR1 = 1.6 V
Idc Z4
RB
ZDC
Idc Z4 2
Z4
RA
RA
RB
Idc
VL
RB
1
2 Z4
VD
VD VR1
VD VR1
VR1
equivalent.
The zener voltage will be:
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.
Vz
13.6K
70K
1
Vbe