ISL5571A

FN4920 Rev 5.00

Page 10 of 12

Jul 2004

The DC current limiting response has a negative temperature

coefficient. As the temperature of the device increases the DC

current limit of the switch will decrease. This is illustrated in

Figure 4.

Figure 4 shows the V-I curves of a switch at two different die

temperatures, T1 and T2. In this illustration T2 is greater in

temperature than T1. This shows that when a switch is driven

into current limit and held there, the current limit will decrease

over time as the switch temperature increases. If the power

through the switch is great enough, the temperature of the

switch will continue to increase until the switch goes into

thermal shutdown (Thermal Shutdown is described below).

Dynamic Current Limiting Response

The DC current limit response described above pertains to DC

and AC voltage sources applied across the switches. The

dynamic response is the response of the current limit circuit to

a fast or high dv/dt pulse. The dynamic response would be

seen, for example, during a lightning surge

Figure 5 shows the dynamic response that is observed when

SW1, SW2 or SW3 is surged with a 1000V at 10/1000

s

telecom surge pulse. (Note: This surge test is done with the

switch in the on state and with the appropriate external

secondary protection in place.) The dynamic current limit of

SW1, SW2 or SW3 will limit the current through the switch to

less than 2.0A for 0.5

s as shown in Figure 5. Once the switch

has turned off, the voltage at the TLine and RLine terminals will

increase to a point where the external secondary protection

device will trigger and crowbar the voltage at TLine and RLine

to a low voltage, protecting the ISL5571A against damage.

Since the Line Break switches (SW1, SW2) have this dynamic

current limit feature, the internal over-voltage protection

clamping circuit of the ISL5571A device will need to only

protect the SLIC against a 2.0A

0.5s pulse during a lightning

surge.

Thermal Shutdown (TSD)

The ISL5571A has a built in thermal shutdown protection

circuit. The thermal shutdown protection mechanism is invoked

if a fault condition causes the junction temperature of the die to

exceed about 150°C. Once the thermal limit is exceeded the

thermal shutdown circuitry will force the switches into an All

OFF state, regardless of the logic inputs. While in thermal

shutdown the TSD logic pin (pin 11) will be driven low by the

thermal shutdown circuit. (Note: During normal operation the

TSD pin is high.) The thermal shutdown mechanism was

designed to have a thermal hysteresis of about 12°C. Once in

thermal shutdown the device will begin to cool down, because

all the switches are off and no current flows. When the

temperature of the die cools to about 138°C the ISL5571A will

cycle out of thermal shutdown and the switches will close

again. If the fault condition is still present, the temperature of

the die will again increase and this cycle will be repeated.

Over Voltage Protection Clamping Circuit

The ISL5571A contains an over-voltage clamping circuit on the

SLIC side of the Line Break switches, see Figure 1. This

clamping circuit consists of a diode bridge and SCR. During

lightning surges and power-cross fault conditions this circuit

will clamp the voltage at the TBAT and RBAT terminals of the

SLIC to a safe level and will shunt harmful currents to ground

away from the SLIC.

The clamping circuit is externally connected to ground through

the FGND pin (pin 1) of the device. The battery voltage of the

SLIC is connected to the clamping circuit through the VBAT pin

(pin 16) of the device. The operation of diode bridge and the

SCR circuit is described below.

DIODE BRIDGE WITH SCR (ISL5571A)

During a positive lightning surge or during the positive cycle of

a power-cross / induction fault, the voltage at the TBAT and

RBAT terminals of the SLIC will be clamped to a diode drop

above ground. The fault current will flow harmlessly through

diodes D1 and D2 of the diode bridge to ground (see Figure 1).

During a negative lightning surge or during the negative cycle

of a power-cross / induction fault when the voltage at the TBAT

and RBAT terminals reach 2V to 4V more negative than the

VBAT voltage, the protection SCR will trigger and turn on.

When the SCR turns on and latches, it will crowbar the voltage

at the TBAT and RBAT lines to a low-voltage state,

approximately 3 diode drops below ground. This low-voltage

on state will cause the current resulting from the over voltage

to be safely direct to ground through diodes D3 and D4 of the

diode bridge and the SCR (see Figure 1). Once the fault

current decrease below the protection SCR holding current

(110mA) the SCR will turn off and the SLIC will be able to

return to normal operation.

VBAT Fault Circuit Protection - Loss of Battery

Voltage

The ISL5571A device contains a VBAT fault circuit which

monitors the SLIC battery voltage (VBAT). When this circuit

detects that the VBAT voltage has risen above -10V, it will

cause the ISL5571A to enter the All OFF state. All the switches

will remain off (open) until the circuit detects that the SLIC

battery voltage has dropped below -15V.

Design Considerations

External Protection

Subscriber line card circuits using the ISL5571A require the

use of an external protection circuit on the loop side or phone

side of the device, see Figure 1. This protection is required to

minimize the power stress on the ISL5571A during overvoltage

and overcurrent conditions. When the proper external

protection circuitry is used in conjunction with the integrated

secondary protection, features offered by the ISL5571A, the