Datasheet

VCC
To FB
R
FB1
VOUT
R
2
R
1
R
FB2
C
1
D
B
R
FB1
x R
FB2
R
FB1
+ R
FB2
t
S
= C
1
x (R
2
+
)
LM5018
SNVS787E –JANUARY 2012–REVISED DECEMBER 2013 www.ti.com
To achieve the desired soft-start, the following design guidance is recommended:
(1) R
2
is selected so that V
FB
is higher than 1.225 V for a V
CC
of 4.5 V, but is lower than 5 V when V
CC
is 8.55 V.
If an external V
CC
is used, V
FB
should not exceed 5 V at maximum V
CC
.
(2) C
1
is selected to achieve the desired start-up time that can be determined as:
(3) R
1
is used to maintain the node B voltage at zero after the soft-start is finished. A value larger than the
feedback resistor divider is preferred.
Based on the schematic shown in Figure 18, selecting C
1
= 1 uF, R
2
= 1 kΩ, R
1
= 30 kΩ results in a soft-start
time of about 2 ms.
Figure 18. Soft-Start Circuit
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