2

8

5

+

+

+

S Q

Q

R

Latched

Fault

150mV

V

IN

SS

GND

+

Fault

TPS40200

100mV

Reset

Fault

7

ISNS

Enable

EAMP

+ 300mV

EAMP

SSRef

V

S-S

100kW

300

kW

TPS40200-Q1

www.ti.com

SLUS739E – SEPTEMBER 2006 – REVISED JULY 2013

Current-Limit Resistor Selection

As shown in Figure 31, 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 30, 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 29), another overcurrent condition is declared and the process repeats indefinitely.

Figure 29 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%.

Figure 30. Current-Limit Reset

Copyright © 2006–2013, Texas Instruments Incorporated

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