AAT4674 Datasheet, PDF(10/13 Page) Advanced Analogic Technologies

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AAT4674 Datasheet, PDF (10/13 Pages) Advanced Analogic Technologies – Power Supply Selector Switch

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In Equation 1, IMAX is the maximum current required

by the load. RDS(ON)(MAX) is the maximum rated

RDS(ON) of the AAT4674/4674-1 at high tempera-

tures (consult the âRDSON vs. Temperatureâ perform-

ance graph in the âTypical Characteristicsâ section

of this datasheet). For estimating the RDS(ON)(MAX)

use the data on the âRDSON vs Temperatureâ per-

formance graph and increase the value from the

performance graph by 50%. RÎ¸JA is the thermal

resistance between the AAT4674/4674-1 and the

printed circuit board (PCB) onto which it is mount-

ed; RÎ¸JA is the thermal resistance of the TSOPJW-

12 package. TA(MAX) is the maximum ambient tem-

perature that the PCB under the AAT4674/4674-1

would be if the AAT4674/4674-1 were not dissipat-

ing power. Equation 1 can be rearranged to solve

for IMAX, into Equation 2 (Eq. 2).

Eq. 2: IMAX =

TSD(MIN) - TA(MAX)

R Â· R DS(ON)(MAX)

ÎJA

TSD(MIN) is the minimum temperature required to

activate the AAT4674/4674-1 over-temperature

protection (thermal shutdown). With typical speci-

fication of 140Â°C, 125Â°C is a safe minimum value to

use.

For example, for a 2.5V input power supply appli-

cation that is specified to operate in 50Â°C environ-

ments where the PCB operates at temperatures as

AAT4674

Power Supply Selector Switch

high as 85Â°C. The application is sealed and its PCB

is small, causing RÎ¸JA to be approximately

160Â°C/W. The RDS(ON)(MAX) is estimate to be

300mÎ© (from the âRDS(ON) vs. Temperatureâ per-

formance graph, where VIN = 2.5V at 85Â°C plus

50%). To find the maximum current (IMAX) for this

application use Equation 2:

IMAX =

125Â°C - 85Â°C

PCB Layout Recommendations

For proper thermal management, to minimize PCB

trace resistance, and to take advantage of the low

RDS(ON) values of the two internal power switches in

the AAT4674/4674-1, certain circuit board layout

rules should be followed: IN1, IN2, and OUT should

be routed using wider than normal traces. The two

IN1 pins (1 and 2) and two IN2 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, the input capacitors

(CIN1/CIN2) and output capacitors (COUT) should be

placed as close to the package pins as possible.

The AAT4674/4674-1 evaluation board layout fol-

lows the printed circuit board layout recommenda-

tions and can be used as an example of an optimal

board layout.

4674.2007.09.1.0

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