MH88625
Functional Description
The SLIC uses a transformerless electronic 2-wire to
4-wire conversion which can be connected to a
Codec to interface the 2 wire subscriber loops to a
time division multiplexed (TDM) pulse code
modulated (PCM) digital switching network. For
analog applications, the Tx and Rx of the 2-4 wire
converter can be connected directly to an analog
crosspoint switch such as the MT8816. Powering of
the line is provided through precision battery feed
resistors. The MH88625 also contains control,
signalling and status circuitry which combines to
provide a complete functional solution which
simplifies the manufacture of line cards. This
circuitry is illustrated in the functional block diagram
in Fig. 1. The MH88625 is designed to be pin
compatible with Mitel’s MH88632 and MH88628.
This allows a common PCB design with common
gain, input impedance and network balance.
Preliminary Information
Loop Current Setting
The MH88625 SLIC provides a constant current with
constant voltage fallback. This design feature
provides for long loop capability regardless of the
constant current setting. Refer to Graph 1.
The LCA (Loop Current Adjust) pin is an input to an
internal resistor divider network which generates a
bias voltage. The loop current is proportional to this
voltage. The loop current can be set between 20 and
45mA by various connections to the LCA pin as
illustrated in graph 2 and Figure 8. The loop current
during a fault condition will be limited to a safe level.
Primary over-current protection is inherent in the
current limiting feature of the 200Ω battery feed
resistors. Refer to Graph 1.
Receive and Transmit Audio Path
The audio signal of the 2-wire side is sensed
differentially across the external 200Ω feed resistors
and is passed on to a second differential amplifier
stage in the 2W/4W conversion block. This block
sets the transmit gain on the 4-wire side and cancels
signals originating from the receive input before
outputting the signal.
Approvals
FCC part 68, CCITT, DOC CS-03, UL 1459,
CAN/CSA 22.2 No.225-M90 and ANSI/EIA/TIA-464-
A are system level safety standards and
performance requirements. As a component of a
system, the MH88625 is designed to comply with the
applicable requirements of these specifications.
Programmable Transmit and Receive
Gain
Transmit Gain (Tip-Ring to Tx) and Receive Gain (Rx
to Tip-Ring) are programmed by connecting external
resistors (RRX and RRT) from GRXI to AGND and
from GTX1 to AGND as indicated in Figure 3 and
Tables 1 and 2. The programmable gain range is
from -12dB to +6dB; this wide range will
accommodate any loss plan. Alternatively, the
default Receive Gain of 0dB and Transmit Gain of
0dB can be obtained by connecting GRX0 to GRX1
and GTX0 to GTX1. In addition, a Receive gain of
+6dB and Transmit Gain of +6dB can be obtained by
not connecting resistors RRX and RTX. For correct
gain programming, the MH88625’s Tip-Ring
impedance (Z
in
) must match the line termination
impedance.
For optimum performance, resistor RRX should be
physically located as close as possible to the GRX1
input pin, and resistor RTX should be physically
located as close as possible to the GTX1 input pin.
Battery Feed
The loop current for the subscriber equipment is
sourced through a pair of matched 200Ω resistors
connected to the Tip and Ring. The two wire loop is
biased such that the Ring lead is 2V above V
Bat
(typically -46V) and the Tip lead is 2V below LPGD
(typically -2V) during constant voltage, constant
current mode.
The SLIC is designed for a nominal battery voltage
of -48Vdc and can provide the maximum loop current
of 45mA under the condition.
The MH88625 is designed to operate down to a
minimum of 16mA dc, with a battery voltage of -44V.
The Tip and Ring output drivers can operate within
2V of V
Bat
and LGND rails. This permits a maximum
loop range of 2300Ω.
2-188
MITEL [ MITEL NETWORKS CORPORATION ]
