CPC75282 Datasheet, PDF (15/19 Pages) Clare, Inc. – Line Card Access Switch

tied to V_CC, the channel just ignores this input, and still
enters the thermal shutdown state at high
temperature.
2.6 Ringing Switch Zero-Cross Current Turn Off
After the application of a logic input to turn SW4 off,
the ringing switch is designed to delay the change in
state until the next zero-crossing. Once on, the switch
requires a zero-current cross to turn off, and therefore
should not be used to switch a pure DC signal. The
switch will remain in the on state no matter the logic
input until the next zero crossing. These switching
characteristics will reduce and possibly eliminate
overall system impulse noise normally associated with
ringing switches. See Clare’s 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 300Ω in series with the ringing
generator is recommended.
2.7 Power Supplies
Both a +5V supply and battery voltage are connected
to the CPC75282. Switch state control is powered
exclusively by the +5V supply. As a result, the
CPC75282 exhibits extremely low power consumption
during active and idle states. Although battery power
is not used for switch control, it is required to supply
current during negative overvoltage fault conditions at
tip and ring.
2.8 Battery Voltage Monitor
The CPC75282 also uses the V_BAT voltage to monitor
battery voltage. If system battery voltage is lost, both
channels of the CPC75282 immediately enter the
All-Off state. It remains in this state until the battery
voltage is restored. The device also enters the All-Off
state if the battery voltage rises more positive than
about –10V with respect to ground and remains in the
All-Off state until the battery voltage drops below
approximately –15V with respect to ground. This
battery monitor feature draws a small current from the
battery (less than 1μA typical) and will add slightly to
the device’s overall power dissipation.
CPC75282
2.9 Protection
2.9.1 Diode Bridge
Both channels of the CPC75282 use a combination of
current limited break switches, a diode bridge, and a
thermal shutdown mechanism to protect the SLIC
device or other associated circuitry from damage
during line transient events, such as lightning. During
a positive transient condition, the fault current is
conducted through the diode bridge to ground via
F_GND. Voltage is clamped to a diode drop above
ground. Negative lightning is directed to battery via
steering diodes in the diode bridge. For power
induction or power-cross fault conditions, the positive
cycle of the transient is clamped to a diode drop above
ground and the fault current directed to ground. The
negative cycle of the transient is steered to battery.
Fault currents are limited by the current-limit circuit.
2.9.2 Current Limiting function
If a lightning strike transient occurs when the device is
in the Idle/Talk state, the current is passed along the
line to the integrated protection circuitry, and restricted
by the dynamic current limit response of the active
switches. During the Idle/Talk state, when a 1000V
10x1000μ s lightning pulse (GR-1089-CORE
lightning) is applied to the line though a properly
clamped external protector, the current seen at T_LINE
and R_LINE will be a pulse with a typical magnitude of
2.5A and a duration less than 0.5μs.
If a power-cross fault occurs with the device in the
Idle/Talk state, the current is passed though break
switches, SW1 and SW2, on to the integrated
protection circuit, but is limited by the DC current limit
response of the two break switches. The DC current
limit is dependent on the switch differential voltage, as
shown in “Figure 2: Switches 1-3” on page 17.
Note that the current limit circuitry 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 at T_LINE and
R_LINE will decrease as the device temperature
increases. If the device temperature rises sufficiently,
the temperature shutdown mechanism will activate
and the device will enter the All-Off state.
R00D
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