TPS40210-Q1, TPS40211-Q1
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SLVS861D –AUGUST 2008–REVISED APRIL 2010
The capacitor on the SS pin (CSS) also plays a role in overcurrent functionality. It is used as the timer between
restart attempts. The SS pin is connected to GND through a resistor, RSS(dchg), when the controller senses an
overcurrent condition. Switching stops and nothing else happens until the SS pin discharges to the soft-start
reset threshold, VSS(rst). At this point, the SS pin capacitor is allowed to charge again through the charging
resistor RSS(chg), and the controller restarts from that point. The shortest time between restart attempts occurs
when the SS pin discharges from VSS(ofst) (approximately 1 V) to VSS(rst) (150 mV) and then back to VSS(ofst) and
switching resumes. In actuality, this is a conservative estimate since switching does not resume until the VSSE
ramp rises to a point where it is commanding more output voltage than exists at the output of the controller. This
occurs at some SS pin voltage greater than VSS(ofst) and depends on the voltage that remains on the output
overvoltage the converter while switching has been halted. The fastest restart time can be calculated by using
Equation 16, Equation 17, and Equation 18.
æ
ö
÷
÷
ø
V
SS(ofst)
t
= R
´ C ´lnç
DCHG
SS(dchg)
SS
ç
V
SS(rst)
è
(16)
æ
ç
ç
è
ö
÷
÷
V
- VSS(rst)
(
(
)
)
BP
tCHG = RSS(chg) ´ CSS ´ln
V
- VSS(ofst)
BP
ø
(17)
(18)
tRSTRT min = tCHG + tDCHG
(
)
V
BP
V
SS
t
RSTR(min)
V
SS(ofst)
V
SS(rst)
T - Time
Figure 26. Soft Start During Overcurrent
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