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Soft-Start Function
7
Soft-Start Function
V
REF
will rise at an RC rate defined by the internal resistance of the SS pin (r
SS
), and the external capacitor
(C
SS
) connected to the SS pin. This allows the output voltage to rise in a controlled manner until steady-
state regulation is achieved. Typically, five time constants are recommended to assure that the output
voltage is sufficiently close to the final steady-state value. During the soft-start time the output current can
rise to the built-in current limit.
The LP38853S-ADJ Evaluation board is assembled with a 0.01
μ
F capacitor for C
SS
. This sets the soft-
start time to approximately 750
μ
s.
Figure 4. V
OUT
Soft-Start
8
Power Dissipation
The TO-263 package alone has a junction to ambient thermal resistance (
θ
JA
) rating of 60°C/W. When
mounted on the LP38853S evaluation board the
θ
JA
rating is approximately 40°C/W.
Although there is only approximately 0.30 square inches of 1 ounce copper area immediately under the
tab, the top copper surface area is extended to additional copper area on the bottom of the board by nine
thermal vias.
With the 40°C/W thermal rating the LP38853S-ADJ evaluation board will dissipate a maximum of 2.5W
with T
A
= 25°C.
Figure 5. Maximum Power Dissipation vs Ambient Temperature
3
SNVA178C – November 2006 – Revised April 2013
AN-1504 LP38853S-ADJ Evaluation Board
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