AS1117
APPLICATION NOTES EXTERNAL
CAPACITOR
To ensure the stability of the AS1117 an output capacitor of
at least 10µF (tantalum)or 50µF (aluminum) is required.
The value may change based on the application
requirements on the output load or temperature range. The
capacitor equivalent series resistance (ESR) will effect the
AS1117 stability. The value of ESR can vary from the type
of capacitor used in the applications. The recommended
value for ESR is 0.5Ω. The output capacitance could
increase in size to above the minimum value. The larger
value of output capacitance as high as 100µF can improve
the load transient response.
Maximum Junction Temperature range:
T
J
= T
ambient
(max) + P
D
* thermal resistance (Junction-to-
ambient)
Maximum Junction temperature must not exceed the 125°C.
10V
2.85V
AS1117-2.85
+
+
SOLDERING METHODS
The AS1117 SOT-223 package is designed to be compatible
with infrared reflow or vapor-phase reflow soldering
techniques. During soldering the non-active or mildly
active fluxes may be used. The AS1117 die is attached to
the heatsink lead which exits opposite the input, output, and
ground pins.
Hand soldering and wave soldering should be avoided since
these methods can cause damage to the device with
excessive thermal gradients on the package. The SOT-223
recommended soldering method are as follows: vapor phase
reflow and infrared reflow with the component preheated to
within 65°C of the soldering temperature range.
10uF
10uF
27K
Ω
..
P
O
= (10V - 2.85)(105mA) = (7.15)(105mA) = 750mW
Fig. 1. Circuit Layout, Thermal Experiments.
THERMAL CHARACTERISTICS
The thermal resistance of AS1117 is 15°C/W from junction
to tab and 31
°C/W
from tab to ambient for a total of 46
°C/W
from junction to ambient. The AS1117 features the
internal thermal limiting to protect the device during
overload conditions. Special care needs to be taken during
continuos load conditions the maximum junction
temperature does not exceed 125
°C.
Taking the FR-4 printed circuit board and 1/16 thick with 1
ounce copper foil as an experiment (fig.1 & fig.2), the PCB
material is effective at transmitting heat with the tab
attached to the pad area and a ground plane layer on the
backside of the substrate. Refer to table 1 for the results of
the experiment.
The thermal interaction from other components in the
application can effect the thermal resistance of the AS1117.
The actual thermal resistance can be determined with
experimentation. AS1117 power dissipation is calculated as
follows:
P
D
= (V
IN
- V
OUT
)(I
OUT
)
50 X 50 mm
35 X 17 mm
16 X 10 mm
Fig. 2. Substrate Layout for SOT-223
Alpha Semiconductor Inc. 1031 Serpentine Lane. Pleasanton, CA 94568 Tel: (925) 417-1391 Fax: (925) 417-1390
Rev. 9/14/99
ALPHA [ ALPHA INDUSTRIES ]
