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Output Short Circuit Protection

T − Time − 10 µs / div

(100 mV/ div)

(10 A/ div)

T − Time − 20 ms / div

(1 V/ div)

T − Time − 10 µs / div

TPS40190

SLUS658A–JULY 2005–REVISED AUGUST 2005

APPLICATION INFORMATION (continued)

The short circuit detection in the TPS40190 is done by sensing the voltage drop across the high side FET when

it is on. If the voltage drop across this FET exceeds the selected threshold in any given switching cycle, a

counter counts up one count and the FET is turned off early. If the voltage drop across that FET does not exceed

this threshold, the counter is decremented for that cycle and the FET is allowed to remain on for the normal

pulse width commanded by the internal pulse width modulator. If the counter fills up (a count of 7) a fault

condition is declared and the drivers turn both FETs off. After a timeout of approximately 95 ms, the controller

attempts to restart. If a short circuit is still present at the output, the current ramps quickly up to the short-circuit

threshold and another fault condition is declared. The device then waits 95 ms to attempt to restart again.

Figure 1. Output Short Circuit Detected (Nominal

Figure 2. Output Fault Hiccup Restart Timing

Threshold 25 A)

The TPS40190 provides three selectable short circuit protection thresholds: 160 mV, 320 mV and 460 mV. The

thresholds for corresponding resistors from COMP to GND. Note that since the TPS40190 measures the

resistance from COMP to GND during a 2-ms window, the compensation network from COMP to FB should have

a time constant significantly less than 1 ms or there can be issues detecting the resistance and setting the

correct short circuit threshold. This network should have no DC path from COMP to FB.

The short circuit detection threshold in the TPS40190 has some temperature compensation built in to help offset

the high-side FET rise in resistance as its temperature rises. A typical FET has a resistance temperature

coefficient of about 4500 ppm/°C. The temperature coefficient of the short circuit threshold is approximately

the FET resistance increase. The relative FET resistance change is based on an estimate of a linear

4500 ppm/°C temperature coefficient. The effectiveness of this compensation depends on how tight the thermal

coupling between the TPS40190 and the high-side FET is. Better thermal coupling between the TPS40190 and

the high-side FET gives better compensation effectiveness.

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