Diodes AP1186 - Manual (Page 6)

AP1186
1.5A ULTRA LOW DROPOUT POSITIVE ADJUSTABLE OR
FIXED-MODE REGULATOR
AP1186 Rev. 1 - 0 6 of 9 MARCH 2007
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Stability
The AP1186-XXX requires the use of an output capacitor as part
of the frequency compensation in order to make the regulator
stable. Typical designs for the microprocessor applications use
standard electrolytic capacitors with typical ESR in the range of
50 to 100mΩ and an output capacitance of 100uF to 1000uF. Fortunately as the capacitance increases, the ESR decreases
resulting in a fixed RC time constant. The AP1186-XXX takes
advantage of the phenomena in making the overall regulator loop
stable.
For most applications a minimum of 100uF aluminum electrolytic
capacitor insures both stability and good transient response.
Thermal Design
The AP1186-XXX incorporates an internal thermal shutdown that
protects the device when the junction temperature exceeds the
allowable maximum junction temperature. Although this device
can operate with junction temperatures in the range of 150
o C, it is
recommended that the selected heat sink be chosen such that
during maximum continuous load operation the junction
temperature is kept below this number. The example below
shows the steps in selecting the proper surface mount package.
Assuming, the following conditions:
Vout = 2.5V
Vin = 3.3V
Vctrl = 5V
Iout = 1.5A DC Avg.
Calculate the maximum power dissipation using the
following equation:
Pd = Iout*(Vin-Vout)+(Iout/60)*(Vctrl-Vout)
Pd = 2*(3.3-2.5)+(2/60)*(5-2.5) = 1.68W
Using table below select the proper package and the
amount of copper board needed.
Note: Above table is based on the maximum junction temperature of 135 o C. As shown in the above table, any of the two packages will do
the job.
Function Descriptions
(Continued)
Package Copper Area Θ
JA
( o C/W) Max Pd ( Ta = 25 o C ) Max Pd ( Ta = 45 o C )
TO263 Pad Size-1.4”X1.4” 25-45 2.4W-4.4W 2.0W-3.6W