Datasheet

ADP1821
Rev. C | Page 18 of 24
SETTING THE SOFT START PERIOD
The ADP1821 uses an adjustable soft start to limit the output
voltage ramp-up period, limiting the input inrush current. The
soft start is set by selecting the capacitor, C
SS
, from SS to GND.
The ADP1821 charges C
SS
to 0.8 V through an internal 100 kΩ
resistor. The voltage on C
SS
while it is charging is
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−=
SS
RC
t
CSS
eV 1V8.0
(48)
where
R is the internal 100 k resistor.
The soft start period, t
SS
, is achieved when V
CSS
= 0.6 V or
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−=
Ω )(k100
1V8.0V6.0
SS
SS
C
t
e
(49)
or
()
386.1
V8.0
V6.0
1n1
k100
=
⎟
⎟
⎠
⎞
⎜
⎜
⎝
⎛
−−=
SS
SS
C
t
(50)
Solve for C
SS
by
C
SS
= t
SS
× 7.21 µF/sec (51)