TPS40007

TPS40009

SLUS589B− NOVEMBER 2003 − REVISED FEBRUARY 2005

9

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APPLICATION INFORMATION

Detailed Description

During each switching cycle, a comparator looks at the voltage across the top side MOSFET while it is on. This

blanking time. If the voltage across that MOSFET exceeds the programmed voltage, the current-switching pulse

is terminated and a 3-bit counter is incremented by one count. If, during the switching cycle, the topside

MOSFET voltage does not exceed a preset threshold, then this counter is decremented by one count. (The

counter does not wrap around from 7 to 0 or from 0 to 7). If the counter reaches a full count of 7, the device

declares that a fault condition exists at the output of the converter. In this fault state, HDRV and LDRV are turned

off, and the soft-start capacitor is discharged. LDRV is maintained OFF during fault timeout to effectively support

soft-start capacitor is fully discharged, the discharging circuit is turned off and the capacitor is allowed to charge

up at the nominal charging rate. When the soft-start capacitor reaches approximately 1.3 V, it is discharged

again and the overcurrent counter is decremented by one count. The capacitor is charged and discharged, and

the counter decremented until the count reaches zero (a total of six times). When this happens, the outputs are

again enabled as the soft-start capacitor generates a reference ramp for the converter to follow while attempting

to restart.

During this soft-start interval (whether or not the controller is attempting to do a fault recovery or starting for the

first time), pulse-by-pulse current limiting is in effect, but overcurrent pulses are not counted to declare a fault

until the soft-start cycle has been completed. It is possible to have a supply attempt to bring up a short circuit

for the duration of the soft start period plus seven switching cycles. Power stage designs should take this into

account if it makes a difference thermally. Figure 3 shows the details of the overcurrent operation.

UDG−03165

0V

VTS

Overcurrent

Cycle

Normal

Cycle

Internal PWM

External

Main Drive

Short Circuit Protection

Threshold Voltage

(+)

VTS

(−)

Figure 3. Short Circuit Operation