ISL5571A_04 Datasheet, PDF (11/12 Pages) Intersil Corporation – Access High Voltage Switch

ISL5571A
with the integrated secondary protection, features offered by
the ISL5571A, the application circuit will pass the AC power-
cross and lightning immunity tests of the following regulatory
requirements:
• GR 1089-CORE
• ITU-T K.20
This section will discuss the issues that must be considered
when designing an external protection circuit for use with the
ISL5571A.
The external protection circuitry should be designed to limit
the peak voltages on the T_Line and R_Line terminals of the
ISL5571A. The most potentially stressful condition concerning
the ISL5571A is low level power-cross when the ISL5571A
switches are closed. Under this condition, the external
protection circuitry limits the voltage and corresponding power
dissipation until the ISL5571A thermal shutdown circuitry
opens the switches.
The external protector chosen for the T_Line terminal
must limit at a maximum of 130V thereby limiting the
power stress on the Ring Return switch (SW3).
The protector chosen for the R_Line terminal must limit at a
maximum of 220V thereby limiting the power stress on the
Line Break switch (SW2). The 220V break-over voltage of the
protector on RLine is large enough to not interfere with the AC
ring signal during ringing.
Texas Instruments and Teccor Electronics have designed
specific parts to protect solid state line card access switches.
The following protectors are recommended:
Texas Instruments Part Number TISPL758LF3D(T_Line
and R_Line)
Teccor Electronics Part Number P1200SC (T_Line) and
P2000SC (R_Line)
Refer to the above company’s data sheets for information on
their parts and reference designs for protection of solid state
line card access switches, see Related Literature section on
Page One.
Break-Before-Make Operation
The ISL5571A device inherently has a Break-before-Make
condition between the following switches:
a. Between the Line Break switch SW1 and the Ring
Return switch SW3 during the transition from the Idle
state to the Power Ringing state.
b. Between the Line Break switch SW2 and the Ring
Access switch SW4 during the transition from the
Idle state to the Power Ringing state.
Make-Before-Break Operation
The ISL5571A device could exhibit a Make-before-Break
condition between the Line Break switch SW2 and the Ring
Access switch SW4 during the transition from the Power
Ringing state to the Idle / Talk state. There is a period of time
that can be as much as 25ms (1/2 cycle of the 20Hz ring
signal) when both SW2 and SW4 will both be on (closed).
This occurs because SW4 is an SCR and requires a zero
current crossing to turn off.
Protection SRC Latch-Up
In the Make-before-Break condition, when transiting from the
Power Ringing state to the Idle/ Talk state, during the negative
cycle of the ring generator it is possible for enough current to
flow that the protection SCR will turn on. This will result in
shorting the ring generator and the Ring terminal of the SLIC
to ground. When either SW2 or SW4 turns off, the Ring
terminal of the SLIC will remain shorted to ground unless the
output current limit of the SLIC is less than the holding current
(110mA) of the protection SCR. The current limit of most
SLICs are set well below the 110mA minimum holding current
of the ISL5571A protection SCR and therefore should not be a
concern. Another method to prevent latch-up of the protection
SCR would be to strobe the TSD pin of the ISL5571A. as
discussed in Break-Before-Make section.
Ring Access Switch Quiescent Current During
Ringing
The Ring Access switch (SW4) is a silicon control rectifier
type switch (SCR). During power ringing, the Ring Access
switch will draw a nominal 2mA of current from the ring
generator. This current is called I_{RING QUIESCENT} and is
equal to:
I_{RING QUIESCENT} = I_{RING GEN} - I_{RING LOAD}.
System designers need to ensure that this additional current
can be provided by the ring generator.
Glossary of Acronyms
AC = Alternating Current
BIMOS = Bipolar Metal-Oxide Semiconductor
CO = Central Office
CODEC = CODer-DECoder
DC = Direct Current
DLC = Digital Loop Carrier
DAML = Digitally Added Main Line
EMI = Electromagnetic Interference
ESD = Electrostatic Discharge
FITL = Fiber in the Loop
HFC = Hybrid Fiber Coax
ICs = Integrated Circuits
LCAS = Line Card Access Switch
PBX = Private Branch Exchange
REN = Ring Equivalency Number
MOSFET = Metal-Oxide Semiconductor Field-Effect Transistor
SCR = Silicon Control Rectifier
SLIC = Subscriber Line Interface Circuit
SMDs = Surface Mount Devices
SOIC = Small Outline Integrated Circuit
TSD = Thermal ShutDown
TTL = Transistor-Transistor Logic
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