LM1117/LM1117I
10091915
FIGURE 4. Regulator with Protection Diode
5.0 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 Figure 5. 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.
θ
θ
R JC (Component
R CA (Application
Variables)
Variables)
Leadframe Size & Material Mounting Pad Size,
Material, & Location
No. of Conduction Pins
Die Size
Placement of Mounting Pad
PCB Size & Material
Traces Length & Width
Adjacent Heat Sources
Die Attach Material
Molding Compound Size
and Material
Volume of Air
Ambient Temperatue
Shape of Mounting Pad
10091937
FIGURE 5. Cross-sectional view of Integrated Circuit Mounted on a printed circuit board. Note that the case temperature
is measured at the point where the leads contact with the mounting pad surface
The LM1117 regulators have internal thermal shutdown to protect the device from over-heating. Under all possible operating
conditions, the junction temperature of the LM1117 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, PD , must be calculated:
IIN = IL + IG
PD = (VIN-VOUT)I L + VINIG
Figure 6 shows the voltages and currents which are present in the circuit.
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