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DC Feed Characteristics
As shown in figure 8, the SH POTS
chip-set implements a constant-current
feed. The limit current and the residual
resistance (slope of the characteristic)
are both programmable by the user.
The DC characteristic falls into three
regions. When the combination of line
and subset result in a current less than
the programmed limit current, the sys-
tem behaves like a battery with a fixed
feed resistance of 120Ω, and a voltage
equivalent to the speech supply voltage
(VbatS) minus the bias voltage on both
lines (6V nominal in total). Should line
conditions permit a current which
exceeds the programmed limit current,
the system enters the constant-current
feed mode described above. In order to
protect the output stage in the transition
region at higher line currents (in excess
of 50mA), a third region is defined,
where the system synthesises a fixed
feed resistance of 200Ω. The slope of
the voltage/current characteristic in the
constant-current mode can be user pro-
grammed to select the effective feed-
resistance.
Note: The SH LIC device includes an
over-temperature protection, which acti-
vates in case of overheating of the
device.
Table 8: DC Feed Characteristics (Rfeed = 60Ω total (50Ω+10Ω protection) x2)
Parameter
Conditions
VbiasH
Bias voltage, a wire (Iline=0)
VbiasL
Bias voltage, b wire (Iline=0)
TOLIcl
Current limit tolerance
TRfeedcl
Tolerance on programmed Rfeed
when in current-limit
Icl
Current-limit, useful programmed range
Battery Voltage and Rever-
sal
The open-line voltage (i.e. the voltage
seen on the line when on-hook) is user-
selectable for each channel via an inter-
nal register. It can be either the ringing
battery supply (most common use) or
the speech battery supply. The speech
battery supply is automatically selected
when an off-hook condition is detected,
independently of these control bits. The
selected supply voltage is maintained
when the on-hook signalling function
(ADSI) is enabled.
Iline (mA)
(Programmable)
Rfeed
80
60
40
20
Fig 8: DC Feed characteristics.
Min
2.5
2.5
20
17
MTK-40131
The polarity of the line feed can be
dynamically controlled by the user. In
the 'normal' condition, the A-Wire is the
most positive. Reversal thus makes the B-
Wire the most positive. Battery reversal
is fast (not soft), is controlled by pro-
gramming an internal register and is
independent for both channels. The
selected polarity is used in all states (on-
hook, off-hook, ringing etc...) except on-
hook signalling which is normal battery
mode.
Rfeed=200Ω
Rfeed=120Ω
VfN
Vbats
Max
Units
3.5
V
3.5
V
±15
%
±15
%
70
mA
Vline (V)
Note
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