MH88437-P
40
Advance Information
35
30
Iloop=15mA
25
V(t-r)
Iloop=20mA
Iloop=26mA
20
15
10
5
0
50
150
250
350
450
550
650
750
850
950
R2(kOhms)
Figure 3 - DC Programming Capability
applying a logic 0 to Pin 17, CL, the loop current will
be limited to below 60mA as required in France and
the European TBR21 specification. For all other
countries where current limiting is not required, CL
should be set to 1.
The AC input impedance should be set by the user to
match the line impedance.
To find the external programming components for
configuration 4, the following formula should be
used:
Zext = [(10 x R1)-1k3]+[(10 x R2)//(C1/10)]
e.g. If the required input impedance = 220Ω + (820Ω/
/115nF), the external network to be connected to ZA
will be:
ZA = 900Ω
+
(8k2Ω//12nF)
Where the input impedance (Z) = 600R the equation
can be simplified to:
ZA = (10 x Z) - 1k3Ω
For complex impedances the configuration shown in
Figure 4 (below) is most commonly found.
ZA = 4k7Ω
Note: A table of commonly used impedances can be
found in the DAA Application’s document MSAN-154.
Where Zext = external network connected between
ZA and AGND and Zint = 1.3kΩ (internal resistance)
between NB1 and NB2. For countries where the
balance impedance matches the line impedance, a
16kΩ resistor should be added between NB1 and
NB2.
Input Impedance
The MH88437 has a programmable input impedance
set by fitting external components between the ZA
pin and AGND.
R1
R2
C1
Figure 4 - Complex Impedances
4
MITEL [ MITEL NETWORKS CORPORATION ]
