AAT4672 Datasheet, PDF (22/24 Pages) Advanced Analogic Technologies – Power Supply Selector Switch

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Eq. 2: I_MAX =
T_SD(MIN) - T_A(MAX)
R · R _DS(ON)(MAX)
ΘJA
T_SD(MIN) is the minimum temperature required to activate
the AAT4672 over-temperature protection (thermal
shutdown). With typical specification of 140°C, 125°C is
a safe minimum value to use.
For example, for a 2.5V input power supply application
that is specified to operate in 50°C environments where
the PCB operates at temperatures as high as 85°C. The
application is sealed and its PCB is small, causing R_θJA to
be approximately 160°C/W. The R_DS(ON)(MAX) is estimated
to be 300mΩ (from the “R_DS(ON) vs. Temperature” perfor-
mance graph, where V_IN = 2.5V at 85°C plus 50%). To
find the maximum current (I_MAX) for this application, use
Equation 2:
I_MAX ⁼
125°C - 85°C
300mΩ · 160°C/W ^{= 0.913A}
PRODUCT DATASHEET
AAT4672
Power Supply Selector Switch
PCB Layout Recommendations
For proper thermal management, to minimize PCB trace
resistance, and to take advantage of the low R_DS(ON) val-
ues of the two internal power switches in the AAT4672,
certain circuit board layout rules should be followed:
MAIN, AUX, and OUT should be routed using wider than
normal traces. The two MAIN pins (1 and 2) and two
AUX pins (3 and 4) should be connected to the same
wide PCB trace; and GND should be connected to a
ground plane. For best performance, input capacitors
(C_MAIN, C_AUX) and output capacitor (C_OUT) should be placed
as close to the package pins as possible. The AAT4672
evaluation board layout follows the printed circuit board
layout recommendations and can be used as an example
of an optimal board layout.
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