ISL5571A Datasheet, PDF (9/12 Pages) Intersil Corporation – Access High Voltage Switch

ISL5571A
TSD Pin
The TSD pin (pin 8) can be used as a logic level input or
output. Reference Table 9 for logic state table. The TSD pin
overrides all other logic pins.
As an input, if this pin is driven low, either by external logic
applied to it or by the internal thermal shutdown circuitry, the
ISL5571A device will enter the All OFF state. In the All OFF
state all switches of the ISL5571A are open (off).
As an output, it is capable of driving a TTL input (2.8V at
200µA). The TSD pin will read +5V when the device is in
normal operating mode and 0V when the device is in thermal
shutdown. This pin can be monitored on an oscilloscope to
determine if the ISL5571A device has enter thermal
shutdown. (Thermal Shutdown is described in the Auxiliary
Functions and Features section below.)
Connecting the TSD pin to 5V will have no effect on the
performance of the ISL5571A device and will not disable the
thermal shutdown circuitry.
Auxiliary Functions and Features
In addition to the ISL5571A main function of momentarily
connecting and disconnecting an external ring generator to
ring the phone, the ISL5571A device also provides surge
and power-cross protection to the SLIC and CODEC. This
fault protection is provided by a combination of current-
limiting circuitry, a thermal shutdown mechanism and an
over-voltage clamping circuit. Another feature the device
offers is a V_BAT fault detection circuit. The following
describes each in detail.
Current Limiting
The Line Break switches (SW1 and SW2) and the Ring
Return switch (SW3) are all current-limited. These switches
have a DC current limiting response and a dynamic current
limiting response which were built into the device to provide
protection during lightning and power-cross faults. Each of
these current limiting responses are explained below.
DC CURRENT LIMITING RESPONSE
The ON state V-I Graph for SW1, SW2, and SW3 is shown
in Figure 3. It represents the DC current limiting response of
the switches. The graph shows that over a certain range of
positive and negative voltages, the current and voltage
relationship is linear and behaves according to Ohms law
(V = IR). Note: At around ±1.5V an inflection point occurs
decreasing the on resistance by 2/3. The on resistance
specified in the data sheet is measured in the region prior to
the inflection point (between ±1.5V).
When current through the switch reaches the current limit of
the switch, the current is clamped and held at a constant
value. The switch then operates as a constant current
source. Increasing the voltage beyond this point will not
change the value of the current.
+I
I_LIMIT
CURRENT
LIMITING
2/3 r_ON
-1.5
-V
r_ON
r_ON
+1.5
+V
2/3 r_ON
CURRENT
LIMITING
I_LIMIT
-I
FIGURE 3. ON STATE V-I GRAPH OF SW1, SW2 AND SW3
+I
T1
T2
-V
T2
T1
+V
T2 > T1
-I
FIGURE 4. EFFECT OF TEMPERATURE ON DC CURRENT
LIMIT
+I
2A
0
0.5µs
TIME
FIGURE 5. DYNAMIC CURRENT LIMIT RESPONSE
9