10

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

µs

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

µs as shown in Figure 5. Once the

terminals will increase to a point where the external

secondary protection device will trigger and crowbar 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

≤0.5µs 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

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

SLIC to a safe level and will shunt harmful currents to

ground away from the SLIC.

The clamping circuit is externally connected to ground

voltage of the SLIC is connected to the clamping circuit

diode bridge and the SCR circuit is described below.

DIODE BRIDGE WITH SCR (ISL5571A)

During a positive lightning surge or during the positive cycle

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

turn on. When the SCR turns on and latches, it will crowbar

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.

Voltage

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

ISL5571A