SNOS412M – FEBRUARY 2000 – REVISED MARCH 2013
Figure 22. Regulator with Protection Diode
HEATSINK REQUIREMENTS
When an integrated circuit operates with an appreciable current, its junction temperature is elevated. It is
important to quantify its thermal limits in order to achieve acceptable performance and reliability. This limit is
determined by summing the individual parts consisting of a series of temperature rises from the semiconductor
junction to the operating environment. A one-dimensional steady-state model of conduction heat transfer is
demonstrated in
The heat generated at the device junction flows through the die to the die attach pad,
through the lead frame to the surrounding case material, to the printed circuit board, and eventually to the
ambient environment. Below is a list of variables that may affect the thermal resistance and in turn the need for a
heatsink.
Rθ
JC
(Component Variables)
Leadframe Size & Material
No. of Conduction Pins
Die Size
Die Attach Material
Molding Compound Size and Material
Rθ
CA
(Application Variables)
Mounting Pad Size, Material, & Location
Placement of Mounting Pad
PCB Size & Material
Traces Length & Width
Adjacent Heat Sources
Volume of Air
Ambient Temperatue
Shape of Mounting Pad
The case temperature is measured at the point where the leads contact with the mounting pad surface
Figure 23. Cross-Sectional View of Integrated Circuit Mounted on a Printed Circuit Board
The LM1117-N regulators have internal thermal shutdown to protect the device from over-heating. Under all
possible operating conditions, the junction temperature of the LM1117-N must be within the range of 0°C to
125°C. A heatsink may be required depending on the maximum power dissipation and maximum ambient
temperature of the application. To determine if a heatsink is needed, the power dissipated by the regulator, P
D
,
must be calculated:
I
IN
= I
L
+ I
G
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