CPC7691 Datasheet, PDF (13/16 Pages) Clare, Inc. – Line Card Access Switch

INTEGRATED CIRCUITS DIVISION
CPC7691
Ringing to Talk Logic Sequence: Break-Before-Make
State IN_RINGING Latch
Ringing
1
All-Off
1
0
All-Off
0
Talk
0
Ringing
T_SD
Timing
Break Return
Switches Switch
(SW3)
Z
-
Off
On
Hold this state for one-half of the ringing cycle.
0 ^{SW4 waiting for zero current to turn off.}
Off
Off
Zero current has occurred. SW4 has opened
Off
Off
Z
Close break switches
On
Off
Logic states and explanations are provided in the “Introduction” on page 9.
Ringing
Switch
(SW4)
On
On
Off
Off
2.8 Ringing Switch Zero-Cross Current Turn Off
After the application of a logic change to cease
ringing, the ringing switch (SW4), designed to delay
deactivation until a zero current event, will remain
active until a current zero-crossing occurs. Once on
(active) the ringing switch requires a zero current
event to turn off and therefore should not be used to
switch a DC-only signal. This zero current switching
characteristic will reduce and possibly eliminate
overall system impulse noise normally associated with
ringing switches. See IXYS Integrated Circuits
Division application note AN-144, Impulse Noise Benefits
of Line Card Access Switches for more information. The
attributes of ringing switch SW4 may make it possible
to eliminate the need for a zero-cross switching
scheme. A minimum impedance of 300 in series with
the ringing generator is recommended.
2.9 Power Supplies
Both a +5V supply and battery voltage are connected
to the CPC7691. Switch state control is powered
exclusively by the +5V supply. As a result, the
CPC7691 exhibits extremely low power consumption
during active and idle states.
2.10 Internal Protection
The CPC7691 uses a combination of current limited
break switches, a diode bridge/SCR clamping circuit,
and a thermal shutdown mechanism to protect the
SLIC device or other associated circuitry from damage
during line transient events such as lightning and fault
conditions such as power induction and power-cross.
2.10.1 Current Limiting Function
If a lightning strike transient occurs when the device is
in the talk state, the current passed from the line to the
internal integrated protection circuitry is restricted by
the dynamic current limit response of the active break
switches. For instance, during the talk state when a
1000V 10x1000 s lightning pulse (GR-1089-CORE)
is applied to the line though a properly clamped
external protector, the current seen at T_LINE or R_LINE
will be a pulse having a typical magnitude of 2.5A with
a duration less than 0.5 s.
Whenever a power-cross fault occurs with the device
in the talk state, the current passed though the break
switches SW1 and SW2 on to the integrated
protection circuit is limited by the DC current limit
response of the break switches. The DC current limit
specified over temperature ranges from 80mA to
425mA and has a negative temperature coefficient. As
a result, if the device is subjected to extended heating
due to a power-cross fault condition, the measured
current into T_LINE or R_LINE will decrease as the device
temperature increases. If the device temperature rises
sufficiently, the thermal shutdown mechanism will
activate, and the device will enter the All-Off state.
2.10.2 Diode Bridge/SCR
During a positive potential transient or other fault
condition, the fault currents from T_LINE or R_LINE,
limited by the break switches, are conducted through
switching power diodes from T_BAT and R_BAT to ground
via F_GND. Both versions of the CPC7691 utilize a pair
R01
www.ixysic.com
13