Datasheet

» ´
RS SS
t 2.28 C
=
SS
SS
t
C
0.09
UDG-09204
t – Time
t
RS
OC_FAULT
V
SS
Persistent FAULT
FAULT SetFAULT Reset
2.5 V
300 mV
TPS40170
www.ti.com SLUS970A –MARCH 2011–REVISED NOVEMBER 2013
Soft-Start During Overcurrent Fault
The soft-start block also has a role to controls the fault-logic timing. If an overcurrent fault (OC_FAULT) is
declared, the soft-start capacitor is discharged internally through the device by a small current I
SS(sink)
(1.05 µA,
typ.). Once the SS pin capacitor is discharged to below V
SS(flt,low)
(300 mV, typ.), the soft-start capacitor begins
charging again. If the fault is persistent, a fault is declared which is determined by the overcurrent protection
state machine. If the soft-start capacitor is below V
SS(flt,high)
(2.5 V, typ.), then the soft-start capacitor continues to
charge until it reaches V
SS(flt,high)
before a discharge cycle is initiated. This ensures that the re-start time-interval
is always constant. Figure 22 shows the re-start timing.
Figure 22. Overcurrent Fault Restart Timing
NOTE
For the feedback to be regulated to the SS_EAMP voltage, the TRK pin must be pulled up
high directly or through a resistor to VDD.
Equations for Soft-Start and Restart Time
The soft-start time (t
SS
) is defined as the time taken for the internal SS_EAMP node to go from 0 V to the 0.6 V
V
REF
voltage. The SS_EAMP starts rising as the SS pin goes beyond 0.65 V. The offset voltage between the SS
and the SS_EAMP starts increasing as the SS pin voltage starts rising. Figure 21, shows that the SS time can be
defined as the time taken for the SS pin voltage to change by 1.05 V (see Equation 10).
The restart time (t
RS
) is defined in Equation 11 as the time taken for the soft-start capacitor (C
SS
) to discharge
from 2.5 V to 0.3 V and to then recharge up to 2.5 V.
(10)
where
• C
SS
is the soft-start capacitance in nF
• t
SS
is the soft-start time in ms
• t
RS
is the re-start time in ms (11)
NOTE
During soft-start (V
SS
< 2.5 V), the overcurrent protection limit is 1.5 times normal
overcurrent protection limit. This allows higher output capacitance to fully charge without
activating overcurrent protection.
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