Datasheet

SS
SS
REF / EA
27 A T
C
V
+
m ´
=
VSENSE
VREF
COMP
FB/EA-
REF/EA+
SS
UCC28250
+
+
C
SS
C
Z2
R
Z2
C
P1
C
Z3
R
Z3
R
O2
R
O1
R
S2
R
S1
VOUT
UCC28250
SLUSA29C –APRIL 2010– REVISED JULY 2011 www.ti.com
SS (Soft Start Programming Pin) (13/20)
The soft-start circuit gradually increases the converter’s output voltage until steady state operation is reached.
This reduces start-up stresses and current surge.
When the UCC28250 reaches its valid operating threshold, the SS pin capacitor is charged with a 27-µA current
source. The UCC28250’s internal error amplifier non-inverting terminal follows the SS pin voltage on REF/EA+
pin voltage depending on which one is lower. Hence, during soft start, the SS pin voltage is lower than REF/EA+.
The internal error amplifier then uses the SS pin as its reference voltage, until the SS pin voltage rises above the
REF/EA+ level. Once the SS pin voltage is above REF/EA+ voltage, soft-start time is considered finished.
The soft-start implementation scheme and timing is different, depending on the location of the UCC28250 with
respect to the isolation barrier.
For secondary-side control, the internal error amplifier is used to achieve the voltage regulation. The REF/EA+ is
connected to an external reference voltage, FB/EA- is connected to the voltage sensing divider, and the error
amplifier’s output pin (COMP) is connected through a compensation filter back to the FB/EA- pin (Figure 13). In
this case, the primary output’s start-up is a closed loop soft start (soft-start input reference of error amplifier). The
output soft-start time is determined by the external capacitor connected at SS pin based on the internal 27-µA
charging current and the voltage set at REF/EA+ pin.
Based on the soft-start time T
SS
, choose soft start capacitor C
SS
value as:
(9)
Figure 13. Error Amplifier EAMP Connections for secondary-side Control
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