iru1260 International Rectifier Corp., iru1260 Datasheet - Page 5

• Manufacturer Part Number: iru1260
• Description: Dual 6a And 1a Low Dropout Positive Adjustable Regulator -
• Manufacturer: International Rectifier Corp.

Stability
The IRU1260 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 100mV and the output
capacitance of 500 to 1000mF. Fortunately as the ca-
pacitance increases, the ESR decreases resulting in a
fixed RC time constant. The IRU1260 takes advantage
of this phenomena in making the overall regulator loop
stable. For most applications a minimum of 100mF alu-
minum electrolytic capacitor with the maximum ESR of
0.3V such as Sanyo, MVGX series, Panasonic FA se-
ries as well as the Nichicon PL series insures both sta-
bility and good transient response. The IRU1260 also
requires a 1mF ceramic capacitor connected from V
V
in order to further insure stability.
Thermal Design
The IRU1260 incorporates an internal thermal shutdown
that protects the device when the junction temperature
exceeds the maximum allowable junction temperature.
Although this device can operate with junction tempera-
tures in the range of 1508C, it is recommended that the
selected heat sink be chosen such that during maxi-
mum continuous load operation the junction tempera-
ture is kept below this number. The example given shows
the steps in selecting the proper regulator heat sink for
driving the Pentium II
resistors and the Clock IC using the IRU1260 TO-263
package.
Rev. 2.1
09/19/02
CTRL
and a 10V, 0.1W resistor in series with V
TM
processor GTL+ termination
CTRL
www.irf.com
IN
pin
to
Example:
Assuming the following specifications:
The steps for selecting a proper heat sink to keep the
junction temperature below 1358C is given as:
1) Calculate the maximum power dissipation using:
2) Assuming a TO-263 surface mount package, the junc-
3) The maximum junction temperature of the device is
tion to ambient thermal resistance of the package is:
calculated using the equation below:
Since this is lower than our selected 1358C maxi-
mum junction temperature (1508C is the thermal shut-
down of the device), TO-263 package is a suitable
package for our application.
V
V
V
I
I
T
P
P
u
T
T
OUT1(MAX)
OUT2(MAX)
JA
A
J
J
IN
OUT1
OUT2
D
D
= 358C
= T
= 35 + 2.86330 = 1218C
= I
= 0.23(3.3 - 2.5) + 1.53(3.3 - 1.5) = 2.86W
= 308C/W for 1" square pad area
= 3.3V
= 2.5V
= 1.5V
OUT1
A
+ P
= 0.2A
= 1.5A
3(V
D
3u
IN
JA
- V
OUT1
) + I
OUT2
3(V
IRU1260
IN
- V
OUT2
)
5