Preliminary Information
When using DTMF signalling only i.e. pulse dialling
is not used, the capacitor, C7, can be permanently
connected to ground and does not require to be
switched out during dialling.
MT91600
From Figure 3 with R1 = R2 = 220Ω
For I
LOOP
= 25mA, V
LC
= 0V, Vbat=-48V
R3
43kΩ
V
LC
C9
100nF
R4
130kΩ
MT91600
Power up Sequence
The circuit should be powered up in the following
order: AGND, VEE, VDD, V
BAT.
6
V
REF
Application
The following Application section is intended to
demonstrate to the user the methods used in
calculating and selecting the external programming
components in implementing the MT91600 as an
analog line interface in a communication system.
The programming component values calculated
below results in the optimum performance of the
device.
Refer to Figure
designation.
5
for
MT91600
components
I
LOOP
= 1.07 * Vs
(R1 +R2)
V
BAT
Figure 3 - Resistor Divider
C9 is inserted to ensure pin 6, Vref, remains at a.c.
ground. 100nF is recommended.
I
LOOP
can also be set by directly driving Vref with a
low impedance voltage source. (See Figure 4). It is
recommended that a small resistor be placed in
series with the Vref pin. In this case:
where, Vs < 0
Component Selection
Feed Resistors (R1, R2)
Vs
2kΩ
6
V
REF
The selection of feed resistors, R1 and R2, can
significantly affect the performance of the MT91600.
It is recommended that their values fall in the range
of:
200Ω <= R1 <= 250Ω
where, R1 = R2
The resistors should have a tolerance of 1% (0.15%
matched) and a power rating of 1 Watt.
Loop Current Setting (R3, R4, C9)
By using a resistive divider network, (Figure 3), it is
possible to maintain the required voltage at Vref to
set I
LOOP
. The loop current programming is based on
the following relationship:
I
LOOP
= - [ F * V
LC
+ G * V
BAT
] * K
o
* H
(R1 +R2)
where,
F = R4 / (R4 + R3)
G = R3 / (R4 +R3)
K
o
= 200000 / (200000 + (R4//R3) )
H = 1.07
I
LOOP
is in Ampere
C9
100nF
MT91600
Figure 4 - Direct Voltage
Calculating Component Values For AC
Transmission
There are five parameters a designer should know
before starting the component calculations. These
five parameters are:
1)
2)
3)
4)
5)
characteristic impedance of the line Z
o
network balance impedance Z
NB
value of the feed resistors (R1 and R2)
2W to 4W transmit gain
4W to 2W receive gain
The following example will outline a step by step
procedure for calculating component values. Given:
5
MITEL [ MITEL NETWORKS CORPORATION ]
