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Where (T
J
‐T
A
) is the temperature difference between the RS7212 die junction and the surrounding air,
θ
JC
is the thermal
resistance of the package chosen, and
θ
CA
is the thermal resistance through the printed circuit board, copper traces and
other materials to the surrounding air. For better heat‐sinking, the copper area should be equally shared between the V
IN
,
V
OUT
, and GND pins.
The thermal resistance θ
JA
of SOT‐25 package of RS7212 is 250°C/W. Based on a maximum operating junction temperature
125°C with an ambient of 25°C, the maximum power dissipation will be:
P
MAX
=
(
T
J
−
T
A
) (
125
−
25
)
=
=
0
.
40W
250
θ
JC
+
θ
CA
Thermal characteristics were measured using a double sided board with 1”x2” square inches of copper area connected to the
GND pin for “heat spreading”.
Dropout Voltage
A regulator’s minimum input‐output voltage differential, or dropout voltage, determines the lowest usable supply voltage. In
battery‐powered systems, this will determine the useful end‐of‐life battery voltage. The RS7212 use a P‐ channel MOSFET
pass transistor, its dropout voltage is a function of drain‐to‐source on‐resistance R
DS(ON)
multiplied by the load current.
V
DROPOUT
=
V
IN
−
V
OUT
=
R
DS
(
ON
)
×
I
OUT
DS‐RS7212‐02
September, 2009
www.Orister.com
ORISTER [ ORISTER CORPORATION ]
