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Current-Limit Resistor Selection

TPS40200-EP

SGLS371 – JANUARY 2007

GENERAL INFORMATION (continued)

As shown in Figure 27, a resistor in series with the power MOSFET sets the overcurrent protection level. Use a

low-inductance resistor to avoid problems with ringing signals and nuisance tripping. When the FET is on and

the controller senses 100 mV or more drop from the VDD pin to the ISNS pin, an overcurrent condition is

declared. When this happens, the FET is turned off and, as shown in Figure 28, the soft-start capacitor is

discharged. When the soft-start capacitor reaches a level below 150 mV, the converter clears the overcurrent

condition flag and attempts to restart. If the condition that caused the overcurrent event to occur is still present

on the output of the converter (see Figure 27), another overcurrent condition is declared and the process

repeats indefinitely. Figure 27 shows the soft-start capacitor voltage during an extended output fault condition.

The overall duty cycle of current conduction during a persistent fault is approximately 2%.

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