AN1500A
15 Off-Hook conditions & DC mask
15.1 Line current path
When going off-hook, SW1/1 short
circuits the leakage supply resistor
R1 and enables a low-ohmic path
from line into the telephone. At the
same time SW1/2 supplies the base
of Q2 via R7 and R8 and signals a
“off-hook” state to the IC´s
HS/DP-pin (#10). The base/gate of
Q1 is pulled down by Q2 and the
SA2532K is started up. VLI (pin #27)
is regulated to the specified voltage
by shunt regulation from Q3.
Line current flows through the
following path:
La -- RB1 -- Q1 -- R11 -- Q3 -- VSS --
RB1 -- Lb
Fig. 8: DC mask & line current path
15.2 DC mask path
The DC characteristics (DC loop resistance) of the application is determined by the following conditions:
Va,b = VLI + VR11 + VCE,Q1 + 2x(Vf,RB1
)
•
•
VLI :
VR11
The voltage at LI (#27) is shunt-regulated to 4.5V by Q3.
VR11 = Iline * R11,
where: R11=30Ω
:
changing the value of R11 is not recommended, since it affects the following parameters:
AC impedance
Tx and Rx gains
DTMF level
AGC switching positions
•
•
VCE,Q1
:
This parameter is mostly affected by the type of hook transistor configuration used, see pt.16
for details
Generally, the rectifier bridge must be installed to meet the specification for independence of
polarity. Vf is typ. 0,7....0,8V (20...100mA). If lower forward voltages are required, an active
(MOSFET) bridge must be used.
2x(Vf,RB1):
15.2.1 Setting for low DC mask:
The standard application regulates the voltage at LI to ≈4.5VDC for line currents >10mA, determining the absolute
level of the DC mask. Lower line currents cause a slope-down of VLI, enabling operation down to 5mA . The
resulting DC mask fits into most countries´ DC mask specs. Some countries however, like Denmark or Norway
require a very low DC mask at line currents below ≈20mA line current. The constant voltage level at LI can be
lowered to about 4.3V by connecting a 16kΩ-resistor from CI (#7) to Vss (#26). With this adaption, the DC mask is
15/27
sa m e s
SAMES [ SAMES ]
