Datasheet
SLUS507D − JANUARY 2002 − REVISED NOVEMBER 2005
11
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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
comparator is enabled after the SW node reaches a voltage greater than (V
DD
−1.2 V) followed by a 100-ns
blanking time. If the voltage across that MOSFET exceeds a programmable threshold 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 is
turned off and LDRV is turned on and the soft-start capacitor is discharged. The counter is decremented by one
by the soft start capacitor (C
SS
) discharge. When the 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 about 700 mV, 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−01145
0V
V
TS
Overcurrent
Cycle
Normal
Cycle
Internal PWM
External
Main Drive
Overcurrent
Threshold
Voltage
(+) V
TS
(−)
Figure 3. Switch Node Waveforms for Synchronous Buck Converter