鈭?/div>
s
P
Selecting a Heatsink
1. On the chart for the device that you are planning to
Where:
use, cross reference the maximum current required and
(2)
the system鈥檚 maximum ambient temperature.
胃
s-a
= thermal impedance sink-to-ambient (擄C/W)
2. Read the required heatsink thermal impedance (sink- T
j
to-ambient) off the chart. If your plotted point is between
two lines, use the
lower
thermal impedance value.
T
a
3. Look up your required thermal impedance value in
Table 1: Suggested Heatsink Selection, ensuring that
胃
j-c
you use the column relevant to the air flow in your appli-
cation. If the air flow in your application is between two
of the values, then the
lower
flow rate column must be
胃
c-s
used. From this table you can obtain suggested part
numbers for three different manufacturers.
Notes
1. These charts apply to heatsinking TO-220 regulators
converting from 5V to 3.3V only. Please refer to the
next column for information on how to calculate the
heatsink requirements for other applications.
2. Note that if the device is mounted with no insulation,
the heatsink will be smaller, less hardware will be used,
and therefore the cost will be lower! Remember that in
this case, the heatsink will be at the same potential as
the output of the regulator. In cases with very high
power dissipation, using insulation may not be an op-
tion. In extreme cases, consider using one of Semtech鈥檚
switching solutions, or the EZ1900 Load Balance Con-
troller to enable the use of two smaller heatsinks.
= maximum junction temperature (擄C)
(obtained from the device data sheet)
= maximum ambient temperature (擄C)
= thermal impedance junction-to-case (擄C/W)
(obtained from the device data sheet)
= thermal impedance case-to-sink (擄C/W)
(look up in your thermal management hardware
supplier鈥檚 data - for the TO-220 package, typi-
cally 1.25擄C/W using silicone pads, and
0.5擄C/W using thermal grease with no
insulation )
= power dissipated in the device (W)
(where P = I
OUT(max)
(V
IN(max)
-V
OUT(min)
) for worst
case calculation)
P
1
漏 1998 SEMTECH CORP.
652 MITCHELL ROAD NEWBURY PARK CA 91320
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