UCC28070-Q1
SLUSA71A –JULY 2010–REVISED JUNE 2011
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Adaptive Soft Start
In order to maintain a controlled power up, the UCC28070 has been designed with an adaptive soft-start function
that overrides the internal reference voltage with a controlled voltage ramp during power up. On initial power up,
once VVSENSE exceeds the 0.75-V enable threshold (VEN), the internal pull down on the SS pin is released, and
the 1.5-mA adaptive soft-start current source is activated. This 1.5-mA pullup almost immediately pulls the SS pin
to 0.75 V (VVSENSE) to bypass the initial 25% of dead time during a traditional 0 V to Vregulation SS ramp. Once
the SS pin has reached the voltage on VSENSE, the 10-μA soft-start current (ISS) takes over. Thus, through the
selection of the soft-start capacitor (CSS), the effective soft-start time (tSS) may be easily programmed based on
the equation below.
æ
ç
è
ö
÷
ø
2.25V
10mA
tSS = CSS ´
(20)
Often, a system restart is desired following a brief shut-down. In such a case, VSENSE may still have substantial
voltage if VOUT has not fully discharged or if high line has peak charged COUT. To eliminate the delay caused by
charging CSS from 0 V up to the pre-charged VVSENSE with only the 10-μA current source and minimize any
further output voltage sag, the adaptive soft start uses a 1.5-mA current source to rapidly charge CSS to VVSENSE
,
after which time the 10-μA source controls the VSS accent to the desired soft-start ramp rate. In such a case, tSS
is estimated as follows:
æ 3V -VVSENSE0
ç
ö
÷
ø
tSS = CSS ´
10mA
è
(21)
where VVSENSE0 is the voltage at VSENSE at the moment a soft start or restart is initiated.
NOTE
For soft start to be effective and avoid overshoot on VOUT, the SS ramp must be slower
than the voltage-loop control response. Choose CSS ≥ CVZ to ensure this.
(V)
VSS
VVSENSE
VSS if no adaptive current
Time (s)
PFC externally
disabled due to
Reduced delay to regulation
AC-Line recovers
AC-line drop-out
and SS pin released
Figure 21. Soft-Start Ramp Rate
26
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