Alpha Semiconductor Inc. 1031 Serpentine Lane. Pleasanton, CA 94568 Tel: (925) 417-1391 Fax: (925) 417-1390

Rev. 9/14/99

AS1117

APPLICATION NOTES EXTERNAL

CAPACITOR

To ensure the stability of the AS1117 an output capacitor of

at least 10

µF (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 0.5

Ω. The output capacitance could

increase in size to above the minimum value. The larger

value of output capacitance as high as 100

µF can improve

the load transient response.

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.

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:

Maximum Junction Temperature range:

ambient)

Maximum Junction temperature must not exceed the 125

°C.

10uF

+

27K

AS1117-2.85

+

10V

10uF

2.85V

50 X 50 mm

35 X 17 mm

16 X 10 mm

Fig. 1. Circuit Layout, Thermal Experiments.

Fig. 2. Substrate Layout for SOT-223