CPC5604
Figure 4 On-Hook DC Resistance Tip/Ring Setup
TIP
A
+
-
LITELINKTM DAA Circuit
100VDC
RING
CTR-21 Compliance
On-Hook Resistance
CTR-21 is the standard for connection of data commu-
nications equipment to the European telephone net-
work. The maximum current limit requirement in
CTR-21 (Section 4.7.1) is 60mA and can be selected by
the following equation:
Figure 4 shows the test setup for on-hook DC resist-
ance. The battery is set to 100VDC and an ammeter is
placed in series with the battery connection. When the
DAA is in the on-hook state, the leakage current is
obtained and then the battery voltage is divided by this
current yielding the on-hook resistance. The LiteLinkTM
is guaranteed to have a leakage current < 10uA at 100V
which is equivalent to an on-hook resistance > 10MΩ
thus meeting regulatory approvals.
1V
ILM =
+ 8mA
RZDC
Clare recommends current limit be set to 53mA using an
RZDC value of 22Ω. Since VDD is regulated to +3.5V,
excess power is dissipated in the external MOSFET
package. Since the maximum off-hook line voltage and
current in CTR-21 is 40V and 53mA respectively, the
maximum power dissipated by the MOSFET is approxi-
mately 2.1W.
Current Limiting
The LiteLinkTM includes a current limiting feature that is
selectable via resistor RZDC (R16). The current limit
value is set by the equation:
1V
10
12
RZDC
AC Characteristics
For US/Canada/Japan the recommended value for
RZDC (R16) is 8Ω which yields a current limit value of
133mA. The current limiting feature is especially useful
in the case where the host system is inadvertently con-
nected to a digital PBX telephone port which usually has
a very high current limit value. The current limiting capa-
bility will prevent damage to the LiteLinkTM in this sce-
nario.
In a similar manner to the DC characteristics, AC termi-
nation impedance is set via RZNT (R18). For all applica-
tions, a 604W resistor for RZNT (R18) is required to
reflect 600W to the CO.
XXX
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19
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