DN60

Design Note 60
Issue 1 May 2001
Zetex High Voltage MOSFETS in Call Router
Circuits
A router is a device that receives data
from a telephone line and decides which
route the data should be sent. Routers
are used extensively in computer
networks. They are increasingly being
used in devices installed on subscriber’s
premises to reduce the cost of calls.
When used in this application the router
performs the following functions:
1)Stops the dialed signals from reaching
the telephone line.
2)Analyzes the dialed number and
identifies the cheapest provider using
stored data.
3)Regenerates and transmits the dialing
signal with appropriate prefix denoting
the best route.
This note shows the use of Zetex high
voltage MOSFETs in this application.
The diagram below shows the ZVN4525
in a call router circuit. With different
drive circuits the complementary
P-channel device ZVP4525 may be used.
Gyrator
Z1
2
ZVN4525
10M
1
BR
3
Phone
Line
4
10M
Z2
ZVN4525
100K
FMMT458
Gyrator
100K
100K
Controller
100K
FMMT458
Figure 1:
Call router interface circuit
3
Design Note 60
Issue 1 May 2001
The FMMT458 devices are part of a
comprehensive range of Zetex high
voltage transistors that can be used to
interface controllers to telephone lines.
The circuit is symmetrical and will work
when the polarity of the line connections
is reversed. MOSFETs are used as the
pass element to enable a signal path to
be maintained continuously from the
line to the telephone circuit through the
body diode. Signals from the telephone
to the line are controlled by the call
router.
The ZVN4525 and ZVP4525 250V
MOSFET devices indicated in this design
note are available in a variety of package
outlines and full part numbers and short
form details are provided in the table
below.
The Gyrator is a circuit that maintains a
DC path between the telephone and the
line while blocking any AC signals. The
bridge BR ensures the correct polarity of
the MOSFET gate signals. The zener
diode pairs Z1 and Z2 provide gate
protection for the MOSFETs.
Device
Type
BVDSS
ID
PD
V
A
W
VGS(th)
Min
Max
V
V
RDS(on)
@2.4V
Package
ZVN4525G
250
0.31
2.0
0.8
1.8
9.5
SOT223
ZVN4525Z
250
0.24
1.2
0.8
1.8
9.5
SOT89
ZVN4525E6
250
0.23
1.1
0.8
1.8
9.5
SOT23-6
BVDSS
ID
PD
RDS(on)
@2.4V
Package
V
A
W
Device
Type
VGS(th)
Min
Max
V
V
ZVP4525G
250
0.265
2.0
0.8
2.0
18
SOT223
ZVP4525Z
250
0.205
1.2
0.8
2.0
18
SOT89
ZVP4525E6
250
0.197
1.1
0.8
2.0
18
SOT23-6
4
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