US1261
select a heat sink that results in lower temperature rise
than the one calculated in previous step. The following
heat sinks from AAVID and Thermaloy meet this crite-
ria.
Layout Consideration
The US1261 like all other high speed linear regulators need
to be properly laid out to insure stable operation. The most
important component is the output capacitor, which
needs to be placed close to the output pin and con-
nected to this pin using a plane connection with a
low inductance path.
Air Flow (LFM)
0
100
200
300
400
Thermalloy
AAVID
7021B 7020B 6021PB 7173D
593101B 551002B 534202B 577102B 576802B
7141D
Note : For further information regarding the above com-
panies and their latest product offering and application
support contact your local representative or the num-
bers listed below:
Thermalloy
AAVID
PH# (214) 243-4321
PH# (603) 528-3400
Example # 2 :
Assuming the following specifications :
VIN = 3.3V
VOUT 2 = 1.5 V
VOUT 1 = 2.5 V
IOUT 2 MAX = 1.5A
IOUT 1 MAX = 0.2 A
TA = 35°C
The steps for selecting a proper heat sink to keep the
junction temperature below 135°C is given as :
1) Calculate the maximum power dissipation using :
PD = IOUT1 ´ VIN - VOUT1 + IOUT2 ´ VIN - VOUT2
PD = 0.2 ´ 3.3 - 2.5 +1.5 ´ 3.3 - 1.5 = 2.86 W
(
)
(
)
2) Assuming a TO-263 surface mount package, the junc-
tion to ambient thermal resistance of the package is:
qJA = 30°C / W for 1" square pad area
3) The maximum junction temperature of the device is cal-
culated using the equation below :
TJ = TA + PD ´ qJA
TJ = 35 + 2.86 30 =121 C
´
°
Since this is lower than our selected 135°C maximum junc-
tion temperature (150°C is the thermal shutdown of the
device), TO-263 package is a suitable package for our ap-
plication.
Rev. 1.7
3/22/99
3-5
UNISEM [ UNISEM ]
