SNVS015E – FEBRUARY 1998 – REVISED APRIL 2013
HEATSINKING
A heatsink may be required depending on the maximum power dissipation and maximum ambient temperature of
the application. Under all possible operating conditions, the junction temperature must be within the range
specified under Absolute Maximum Ratings.
To determine if a heatsink is required, the power dissipated by the regulator, P
D
, must be calculated.
The figure below shows the voltages and currents which are present in the circuit, as well as the formula for
calculating the power dissipated in the regulator:
I
IN
= I
L
÷ I
G
P
D
= (V
IN
−
V
OUT
) I
L
+ (V
IN
) I
G
Figure 24. Power Dissipation Diagram
The next parameter which must be calculated is the maximum allowable temperature rise, T
R
(max). This is
calculated by using the formula:
T
R
(max) = T
J
(max)
−
T
A
(max)
where:
•
•
T
J
(max) is the maximum allowable junction temperature, which is 125°C for commercial grade parts.
T
A
(max) is the maximum ambient temperature which will be encountered in the application.
Using the calculated values for T
R
(max) and P
D
, the maximum allowable value for the junction-to-ambient
thermal resistance,
θ
(J−A)
, can now be found:
θ
(J−A)
= T
R
(max)/P
D
NOTE
If the maximum allowable value for
θ
(J−A)
is found to be
≥
53°C/W for the TO-220 package,
≥
80°C/W for the DDPAK/TO-263 package, or
≥174°C/W
for the SOT-223 package, no
heatsink is needed since the package alone will dissipate enough heat to satisfy these
requirements.
If the calculated value for
θ
(J−A)
falls below these limits, a heatsink is required.
HEATSINKING TO-220 PACKAGE PARTS
The TO-220 can be attached to a typical heatsink, or secured to a copper plane on a PC board. If a copper plane
is to be used, the values of
θ
(J−A)
will be the same as shown in the next section for the DDPAK/TO-263.
If a manufactured heatsink is to be selected, the value of heatsink-to-ambient thermal resistance,
θ
(H−A)
, must
first be calculated:
θ
(H−A)
=
θ
(J−A)
− θ
(C−H)
− θ
(J−C)
Where:
•
•
θ
(J−C)
is defined as the thermal resistance from the junction to the surface of the case. A value of 3°C/W can be
assumed for
θ
(J−C)
for this calculation.
θ
(C−H)
is defined as the thermal resistance between the case and the surface of the heatsink. The value
of
θ
(C−H)
will vary from about 1.5°C/W to about 2.5°C/W (depending on method of attachment, insulator, etc.). If
the exact value is unknown, 2°C/W should be assumed for
θ
(C−H)
.
8
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