USER-PROGRAMMABLE COMPONENTS
Z
T
is connected between the VTX and RSN pins. The
fuse resistors are R
F
, and Z
2WIN
is the desired 2-wire
AC input impedance. When computing Z
T
, the internal
current amplifier pole and any external stray
capacitance between VTX and RSN must be taken into
account.
Z
RX
is connected from V
RX
to R
SN
. Z
T
is defined
above, and G
42L
is the desired receive gain.
R
DC1
, R
DC2
, and C
DC
form the network connected to
the RDC pin. I
LOOP
is the desired loop current in the
constant-current region.
R
DCR1
, R
DCR2
, and C
DCR
form the network connected
to the RDCR pin.
See Applications Circuit for these components.
Z
T
= 500
(
Z
2WIN
–
2R
F
)
1000
•
Z
T
Z
L
-
-
Z
RX
= ------------
•
---------------------------------------------------
G
42L
Z
T
+ 500
(
Z
L
+ 2R
F
)
2500
R
DC1
+
R
DC2
= ---------------
I
LOOP
2250
R
DCR1
+
R
DCR2
=
----------------------
-
I
RINGLIM
R
DC1
+ R
DC2
-
C
DC
=
19 ms
•
----------------------------------
R
DC1
R
DC2
R
DCR1
+ R
DCR2
-
C
DCR
= ------------------------------------------
•
150
µs
R
DCR1
R
DCR2
Loop-Threshold Detect Equations
C
DCR
sets the ringing time constant, which can be
between 15 µs and 150 µs.
Active and OHT state
R
D
R
LTH
= -------
18
Standby state
V
BAT1
–
8
R
LTH
= --------------------------
•
R
D
–
400
–
2R
F
915
54
R
LTH
=
---------
–
R
D
–
400
–
2R
F
-
875
Ring-Trip Detection Equation
R
D
is the resistor connected from the RD pin to GND
and R
LTH
is the loop-resistance threshold between on-
hook and off-hook detection. R
D
should be greater than
52 kΩ to guarantee detection occurs in the Standby
state. Choose the value of R
D
for high battery state;
then use the equation for R
LTH
to find where the
threshold is for low battery.
This equation shows at what resistance the standby
threshold is; it is actually a current threshold rather than
a resistance threshold, which is shown by the Vbat
dependency.
V
BAT1
>
–
62 V
V
BAT1
>
–
62 V
V
BAT1
–
1
-
I
RTD
=
---------------------------- +
I
OFFSET
•
325
R
RT1
I
OFFSET
= 8 µA for ON transition
I
OFFSET
= –20 µA for OFF transition
12
Le79R101 Data Sheet
ZARLINK [ ZARLINK SEMICONDUCTOR INC ]
