Datasheet

2 25
10
SS SS
. V
t C
Am
æ ö
= ´
ç ÷
è ø
0
3
10
VSENSE
SS SS
V V
t C
Am
æ ö
-
= ´
ç ÷
è ø
V
SS
V
VSENSE
V
SS
if no adaptive current
Time (s)
(V)
PFC externally
disabled due to
AC-line drop-out
AC-Line recovers
and SS pin released
Reduced delay to regulation
UCC28070
www.ti.com SLUS794E –NOVEMBER 2007–REVISED APRIL 2011
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 V
VSENSE
exceeds the 0.75-V enable threshold (V
EN
), 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 pull-up almost immediately pulls the SS
pin to 0.75 V (V
VSENSE
) 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 (I
SS
) takes over. Thus,
through the selection of the soft-start capacitor (C
SS
), the effective soft-start time (t
SS
) may be easily programmed
based on the equation below.
(20)
Often, a system restart is desired following a brief shut-down. In such a case, VSENSE may still have substantial
voltage if V
OUT
has not fully discharged or if high line has peak charged C
OUT
. To eliminate the delay caused by
charging C
SS
from 0 V up to the pre-charged V
VSENSE
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 C
SS
to V
VSENSE
,
after which time the 10-μA source controls the V
SS
accent to the desired soft-start ramp rate. In such a case, t
SS
is estimated as follows:
(21)
where V
VSENSE0
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 V
OUT
, the SS ramp must be slower
than the voltage-loop control response. Choose C
SS
≥ C
VZ
to ensure this.
Figure 21. Soft-Start Ramp Rate
Copyright © 2007–2011, Texas Instruments Incorporated 27