TPS2010A, TPS2011A, TPS2012A, TPS2013A

POWER-DISTRIBUTION SWITCHES

SLVS189A – DECEMBER 1998 – REVISED NOVEMBER 1999

18

POST OFFICE BOX 655303

APPLICATION INFORMATION

overcurrent (continued)

Three possible overload conditions can occur. In the first condition, the output has been shorted before the

immediately switches into a constant-current output.

In the second condition, the excessive load occurs while the device is enabled. At the instant the excessive load

occurs, very high currents may flow for a short time before the current-limit circuit can react (see Figures 12–19).

After the current-limit circuit has tripped (reached the overcurrent trip threshhold) the device switches into

constant-current mode.

In the third condition, the load has been gradually increased beyond the recommended operating current. The

current is permitted to rise until the current-limit threshold is reached or until the thermal limit of the device is

exceeded (see Figures 7–10). The TPS201xA is capable of delivering current up to the current-limit threshold

without damaging the device. Once the threshold has been reached, the device switches into its

constant-current mode.

power dissipation and junction temperature

The low on-resistance on the n-channel MOSFET allows small surface-mount packages, such as SOIC, to pass

large currents. The thermal resistances of these packages are high compared to those of power packages; it

the input voltage and operating temperature. As an initial estimate, use the highest operating ambient

P

D +

r

DS(on)

I2

Finally, calculate the junction temperature:

T

J +

P

D

RqJA ) TA

Where:

Compare the calculated junction temperature with the initial estimate. If they do not agree within a few degrees,

repeat the calculation, using the calculated value as the new estimate. Two or three iterations are generally

sufficient to get an acceptable answer.

thermal protection

Thermal protection prevents damage to the IC when heavy-overload or short-circuit faults are present for

extended periods of time. The faults force the TPS201xA into constant current mode, which causes the voltage

across the high-side switch to increase; under short-circuit conditions, the voltage across the switch is equal

to the input voltage. The increased dissipation causes the junction temperature to rise to high levels. The

protection circuit senses the junction temperature of the switch and shuts it off. Hysteresis is built into the thermal

sense circuit, and after the device has cooled approximately 20 degrees, the switch turns back on. The switch

continues to cycle in this manner until the load fault or input power is removed.