®
AN1220
APPLICATION NOTE
LINECARD PROTECTION BY OVERVOLTAGE
AND OVERCURRENT WITH A CLP270M
P.Merceron / A.Bremond
A.S.D
ä
INTRODUCTION
This device includes a primary protection level and is suitable for main distribution frames and line cards.
This protection concept is explained and, in addition, the CLP270M performances are analysed when
facing different surges as described in the BELLCORE GR 1089 recommendations.
Figure 1 is a simplified block diagram of a subscriber line protection that is commonly used.
Fig. 1:
Subscriber line protection topology
“PRIMARY PROTECTION”
“SECONDARY PROTECTION”
Telecommunication
line
CLP270M
MDF
SLIC
EXCHANGE
Telecommunication
line
This shows two different topologies :
n
n
O
so
b
A “primary protection” located on the Main Distribution Frame (MDF) eliminates coarsely the high energy
environmental disturbances (lightning transients and AC power mains disturbances)
A “secondary protection” located on the line card includes a primary protection level (first stage) and a
residual protection (second stage) which eliminates finely the remaining transients that have not been totally
suppressed by the first stage.
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MDF
so
b
-O
CLP270M
EXCHANGE
P
te
le
LINE CARD
THDTxx
or
LCP1511D
or
LCDP1511D
LINE CARD
od
r
s)
t(
uc
“SECONDARY PROTECTION”
SLIC
January 2000
1/13
APPLICATION NOTE
STMICROELECTRONICS CLP270M CONCEPT
1. Evolution of the SLIC protection
Fig. 2:
Line card protection
Over the years, the silicon protection
Programmable
performances have considerably changed.
I
thanks to an
Programmable
The first generation of products like SMTHBTxx
external resistor
thanks to any
and SMTHDTxx offered fixed overvoltage
+I
SWON
external voltage
protection against surges on either TIP or RING line
reference
in four packages.
V
The following generation like THBTxx and
THDTxx still offered fixed overvoltage protection
against surges on both TIP and RING lines in two
-I
SWON
packages.
The next step was the introduction of the LCP
Line card
operating
devices which brought the advantage of full
conditions
programmable voltage.
Today, the CLP270M combines the features of all
the previous generations. In addition to that, it offers an overcurrent detection when operating in speech
mode and also a Failure Status output signal.
The figure 2 summarizes the firing modes of the CLP270M which basically holds the SLIC inside its correct
voltage and current values.
Fig. 3 :
CLP270M in line card
I
Fuse
TIP
Rsense
TIPL
TIPS
Overcurrent
detector
OR
SW3 SW1
FS
SW4 SW2
O
so
b
Fuse
te
le
ro
P
OR
uc
d
Overvoltage
detector
Overvoltage
detector
s)
t(
Over voltage
reference
(+/- 270 V)
Overvoltage
reference
(+/- 270 V)
so
b
-O
Rp
GND
P
te
le
1
2
od
r
s)
t(
uc
-Vbat
TIP
External
voltage
reference
-Vbat
(*)
SLIC
1
Rp
2
RING
Overcurrent
detector
Ring
Generator
RINGL
RINGS
RING
Rsense
(*) LCP1511D or THDT series or LCDP1511D
2/13
APPLICATION NOTE
2. Application circuit: CLP270M in line card
Figure 3 shows the topology of a protected analog subscriber line at the exchange side. The CLP270M is
connected to the ring relay via two balanced Rp resistors, and to the Subscriber Line Interface Circuit. A
second device is located near the SLIC : it can be either a LCP1511D, a THDT series or a LCDP1511D.
These two devices are complementary and their functions are explained below :
n
The first stage based on CLP270M manages the high power issued from the external surges. When
used in ringing mode, the CLP270M operates in voltage mode and provides a symmetrical and
bidirectional overvoltage protection at +/- 270 V on both TIP and RING lines. When used in speech
mode, the CLP270M operates in current mode and the activation current of the CLP270M is adjusted
by R
sense
.
n
The second stage is the external voltage reference device which defines the firing threshold voltage
during the speech mode and also assumes a residual power overvoltage suppression. This protection
stage can be either a fixed or programmable breakover device. The THDTxx family acts as a fixed
breakover device while the LCP1511D or the LCDP1511D operates as a programmable protection.
Thanks to this topology, the surge current in the line is reduced after the CLP270M. Because the
remaining surge energy is low, the power ratings of Rp, the ring relay contacts and the external voltage
reference circuit can be downsized. This results in a significant cost reduction.
Fig. 4:
Switching by voltage during ringing mode.
Fuse
TIP
I
LG
Rsense
TIPL
TIPS
1/2 CLP270M
1
Rp
Overcurrent
detector
2
Overvoltage
detector
Overvoltage
reference
(+/- 270 V)
V
LG
OR
SW3 SW1
FS
GND
O
3. Ringing mode
In ringing mode (Ring relay in position 2), the only protection device involved is the CLP270M.
In normal conditions, the CLP270M operates in region 1 of
A1
curve, and is idle.
If an overvoltage occuring between TIP (or RING) and GND reaches the internal overvoltage reference
(+/- 270 V), the CLP270M acts and the line is short-circuited to GND. At this time the operating point
moves to region 2 for positive surges (region 3 for negative surges). Once the surge current falls below
the switch off current I
SWOFF
, the device returns to its initial state (region 1).
For surges occuring between TIP and RING, the CLP270M acts in the same way. This means that the
CLP270M ensures a tripolar protection.
so
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t(
so
b
-O
P
te
le
-270
od
r
3
I
LG
s)
t(
uc
A1
2
1
+270
V
When used alone, the CLP270M acts at the internal overvoltage reference level (+/- 270 V). Furthermore, it
is possible to adjust this threshold level to a lower voltage by using:
n
up to 4 fixed external voltage reference (V
Z1
to V
Z4
) (see fig. 5a, here-below).
n
external reference supplies, V
b1
and V
b2
(see fig. 5b, on next page).
3/13
APPLICATION NOTE
Fig. 5a:
Method to adjust the reference voltage.
1
Fuse
TIP
Rsense
TIPL
TIPS
Rp
2
VZ1
Overcurrent
detector
OR
SW3 SW1
Overvoltage
detector
Overvoltage
reference
(+/- 270 V)
VZ2
FS
SW4 SW2
OR
Overvoltage
detector
Overvoltage
reference
(+/- 270 V)
GND
VZ3
Overcurrent
detector
VZ4
RINGL
RINGS
1
RING
Fuse
Rsense
Rp
Fig. 5b:
Method to adjust the reference voltage.
Fuse
TIP
Rsense
TIPL
TIPS
Rp
Overcurrent
detector
SW3 SW1
FS
O
so
b
RING
te
le
Fuse
ro
P
SW4 SW2
uc
d
OR
OR
s)
t(
Overvoltage
detector
b
-O
Overvoltage
reference
(+/- 270 V)
Overvoltage
reference
(+/- 270 V)
so
GND
P
te
le
od
r
s)
t(
uc
2
1
2
VB1
Overvoltage
detector
VB2
Overcurrent
detector
RINGL
RINGS
1
Rsense
Rp
2
4/13
APPLICATION NOTE
4. Speech mode
In speech mode (Ring relay in position 1), the protection is provided by the combination of both CLP270M
and the external voltage reference device (see figure 6)
Fig. 6:
Switching by current during speech mode.
Fuse
TIP
I
LG
Rsense
TIPL
TIPS
1
I
LG
A2
-Vbat
Rp
Overcurrent
detector
2
Overvoltage
detector
Overvoltage
reference
(+/- 270 V)
5
4
OR
SW3 SW1
V
LG
External
voltage
reference
-V
REF2
V
REF1
6
V
LG
FS
GND
In normal conditions, the working point of this circuit is located in region 4 of A2 curve : the CLP270M is idle.
When a surge occurs on the line, the external voltage reference device clamps at GND or -V
bat
respectively
for positive and negative surges.
This generates a current which is detected by R
sense
and causes the protection to act : the line is
short-circuited to GND.
The operating point moves to region 5 for positive surges or region 6 for negative surges.
Once the surge current falls below the switching-off current I
SWOFF
, the CLP270M returns to its initial state
(region 4).
The choice of the switching-on currents is function of the R
sense
resistors.
In normal operating condition the current (typically below -100 mA) should not activate the protection
device CLP270M. Therefore the level of activation has to be chosen just above this limit (-200 mA). This
level is adjusted through R
sense
.
Figures 7a and 7b enable the designers to choose the right R
sense
value.
Example:
The choice of R
sense
= 4
Ω
ensures a negative triggering of -190 mA min and -320 mA max. In this case, the
positive triggering will be 150mA min and 280 mA max.
Fig. 7a and 7b:
Switching-on current versus R
sense
.
2
b
O
1
so
ISWON (T, Rsense) / ISWON (25°C, 4
Ω
)
@-20°C @25°C
@75°C
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t(
so
b
-O
P
te
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s)
t(
uc
Iswon @ 25°C (mA)
500
300
200
100
50
Iswon min Iswon max Iswon min Iswon max
negative
negative
positive
positive
0.5
0.3
0.2
3
3
5
7
Rsense (
Ω
)
9
11
13
5
7
Rsense (
Ω
)
9
11
5/13
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