LM3544

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SNVS062C – AUGUST 2000 – REVISED MAY 2013

Figure 24. Thermal Shutdown Characteristics when both

First-Stage and Second-Stage Thermal-Shutdown Modes are Needed

In Figure 23, port 1 is enabled into a short. When this occurs, the MOSFET switch of port 1 repeatedly opens

and closes as the device temperature rises and falls between 145°C and 135°C. In this example, the device

temperature never rises above 160°C. The second stage thermal shutdown is not used and port 2 remains

operational.

When port 1 is enabled into a short in the example illustrated in Figure 24, the device temperature immediately

rises above 160°C. A higher ambient temperature or a larger number of shorted outputs can cause the junction

temperature to increase, resulting in the difference in behavior between the current example and the previous

one. When the junction temperature reaches 160°C, all four ports are disabled (ports 3 and 4 are not shown in

the figure) and all four fault-flag signals are asserted. Just prior to time index 2.5 ms, the device temperature falls

below 135°C, all four ports activate, and all four fault flags are removed. The short condition remains on port 1,

however. For the remainder of the example, the device temperature cycles between 135°C and 145°C, causing

port 1 to repeatedly turn on and off but allowing the un-shorted ports to function normally.

SOFT START

When a power switch is enabled, high levels of current will flow instantaneously through the LM3544 to charge

the large capacitance at the output of the port. This is likely to exceed the over-current threshold of the device, at

which point the LM3544 will enter its current-limit mode. The amount of current used to charge the output

capacitor is then set by the current-limit circuitry. The device will exit the current-limit mode when the current

needed to continue to charge the output capacitor is less than the LM3544 current-limit level.

FAULT FLAG

The fault flags are open-drain outputs, each capable of sinking up to a 10 mA load current to typically 100 mV

above ground.

A parasitic diode exists between the flag pins and VIN pins. Pulling the flag pins to voltages higher than VIN will

forward bias this diode and will cause an increase in supply current. This diode will also clamp the voltage on the

flag pins to a diode drop above VIN.

The fault flag is active (pulled low) when any of the following conditions are present: under-voltage, current-limit,

or thermal-shutdown.

The LM3544 has an internal delay in reporting fault conditions that is typically 7 ms in length. In start-up, the

delay gives the device time to charge the output capacitor(s) and exit the current-limit mode before a flag signal

is set. This delay also prevents flag signal glitches from occurring when brief changes in operating conditions

momentarily place the LM3544 into one of its three error conditions. If an error condition still exists after the delay

interval has elapsed, the appropriate fault flag(s) will be asserted (pulled low) until the error condition is removed.

In most applications, the 7 ms internal flag delay eliminates the need to extend the delay with an external RC

delay network.

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