LM1117I National Semiconductor Corporation, LM1117I Datasheet - Page 10

• Manufacturer Part Number: LM1117I
• Description: 800ma Low-dropout Linear Regulator
• Manufacturer: National Semiconductor Corporation

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APPLICATION NOTE
into the output of the regulator. The discharge current de-
pends on the value of the capacitor, the output voltage of the
regulator, and rate of decrease of V
lators, the internal diode between the output and input pins
can withstand microsecond surge currents of 10A to 20A.
With an extremely large output capacitor ( 1000 µF), and
with input instantaneously shorted to ground, the regulator
could be damaged.
In this case, an external diode is recommended between the
output and input pins to protect the regulator, as shown in
Figure 4 .
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 sum-
ming the individual parts consisting of a series of tempera-
ture rises from the semiconductor junction to the operating
environment. A one-dimensional steady-state model of con-
duction 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.
Leadframe Size &
Material
No. of Conduction Pins
Die Size
Die Attach Material
Molding Compound Size
and Material
R
JC
FIGURE 4. Regulator with Protection Diode
(Component Vari-
ables)
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
R
CA
(Continued)
IN
(Application Vari-
. In the LM1117 regu-
ables)
DS100919-15
10
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 deter-
mine if a heatsink is needed, the power dissipated by the
regulator, P
Figure 6 shows the voltages and currents which are present
in the circuit.
Mounted on a printed circuit board. Note that the case
temperature is measured at the point where the leads
FIGURE 5. Cross-sectional view of Integrated Circuit
I
P
IN
D
= I
= (V
L
contact with the mounting pad surface
FIGURE 6. Power Dissipation Diagram
+ I
IN
D
-V
G
, must be calculated:
OUT
)I
L
+ V
IN
I
G
DS100919-16
DS100919-37